PACTOR II

Part 1: Basic Technical Information

The new Dimension in Data Transmission Technology

By Dr. Tom Rink, DL2FAK, and Hans-Peter Helfert, DL6MAA

Titre : PACTOR-II Basics

I. Introduction

PACTOR was designed more than five years ago in Germany, in order to overcome

the known disadvantages of AMTOR and Packet Radio. PACTOR is a cheap and re-

liable means of fast, robust and error-free data transfer over short wave

links, and which does not exceed the usual 500 Hz bandwidth limit for digital

modes.

For the first time, not only the complete ASCII character set, but any given

binary information could be transferred over short wave, even in very poor pro-

pagation conditions. Another aim of the system development was the utilizing of

inexpensive and widespread hardware. Since 8-bit controllers without Digital

Signal Processor chips (DSP's) were state of the art at that time, Frequency

Shift Keying (FSK) had to be chosen as a modulation method. Up to now, PACTOR

with analog Memory-ARQ is still the most robust digital mode used in Amateur

Radio. It is also still the fastest FSK mode that fits into a 500 Hz channel.

These may be some of the reasons that have made PACTOR a standard, which is not

only included in virtually all short wave modems in the Amateur Radio market,

but is also widespread in the commercial world.

In the meantime however, more powerful CPU's and DSP's have been developed.

Processing power that greatly exceeded the financial limits of the average

Radio Amateur a few years ago, has dropped to an acceptable price. Some of the

current high-end modems for short wave operation already include a DSP, and in

a few years you can expect all new modems to contain these chips. The through-

put within a 500 Hz channel, as well as the robustness of a system can still

be dramatically improved, by using modulation methods different to FSK, com-

bined with powerful error control coding algorithms. A new standard that takes

advantage of the forthcoming hardware generation is thus required.

These considerations have led to the development of PACTOR-II. This is not

'another new mode', but a fully backwards compatible improvement to the current

PACTOR protocol, with automatic switching. As there are already several compa-

nies interested in licensing PACTOR-II, the German development group made sure

that an implementation in units different from the original PTC-II will also be

possible. However, using a less powerful hardware means sacrificing at least a

considerable part of the weak signal performance of the system.

This is the first of a series of three parts that describes the PACTOR-II sys-

tem and the ideas behind it. In this first chapter, we will just explain some

important technical fundamentals that everybody has to be aware of, to under-

stand the advantages of the new protocol. The second part describes the PTC-II

hardware. The third part deals with details on the PACTOR-II protocol.

II. Modulation Methods

1. Frequency Shift Keying (FSK)

FSK was the first teletype modulation method used, and is still by far the most

widespread one. The information is encoded using rectangular pulses, represen-

ted by an interleaved ON/OFF keying of two carrier frequencies. The symbol rate

is defined as fraction 1/T, the so-called baud rate. T represents the symbol

duration. In FSK systems, the typical wide spectrum that could be expected when

modulating a rectangular baseband signal on a carrier, is cancelled out by a-

voiding phase discontinuities between successive pulses. Thus only the main

lobes around the two carrier frequencies are dominant. The amplitude of a 200

baud signal at 500 Hz bandwidth is about 30 dB smaller than the amplitude in

the center of the spectrum. If however, the baudrate is increased, the band-

width of the main lobes will naturally also increase. A 300 Baud signal, for

example, clearly exceeds the 500 Hz bandwidth, hence an ordinary CW filter

cannot be applied without distorting the pulses. This greatly deteriorates the

performance and thus also the Bit Error Rate (BER) of the system. Additionally,

signals with higher baud rates suffer from a significant loss of immunity a-

gainst time smearing (see below). For these reasons, 200 baud is commonly con-

sidered to be the maximum useful symbol rate of 2-tone FSK systems, operating

over short wave links. Additionally, with regard to the over-crowded frequen-

cies, all new systems should generally require not just a narrow bandwidth,

but provide an improved spectral efficiency to obtain a higher throughput. We

have therefore to look for a different approach, instead of using FSK.

2. Phase Shift Keying (PSK)

In PSK systems, the phase of a signal is used as a means of the information

transfer. However, as the HF propagation conditions sometimes change quite

rapidly, it is very difficult to track the absolute phase of a signal. There-

fore it has proven to be much more efficient to utilize the phase difference

between successive pulses. This requires a slightly higher Signal to Noise

Ratio (SNR), but in return, the resistance against multipath effects is drama-

tically increased. Modulation that employs phase differences between succes-

sive pulses to encode the information is called Differential Phase Shift Key-

ing (DPSK). If there are only two possible phase differences, signalling logi-

cal one and zero, the modulation is called Differential Binary Phase Shift

Keying (DBPSK). If there are four possible phase differences, signalling logi-

cal dibits, it is called Differential Quadrature Phase Shift Keying (DQPSK).

For example, with an one-tone 100 baud DQPSK signal, 200 bit/sec. can be trans-

ferred. If more phase differences are distinguished, the corresponding systems

are called 8-DPSK, 16-DPSK, etc.

As phase shift keying naturally implies phase discontinuities between succes-

sive pulses, the spectrum of a DPSK system with hard keying shows the typical

strong side-lobes of a rectangular pulse spectrum. The amplitude of a hard-

keyed 100 baud DPSK signal is only around 15 dB smaller at a cut-off frequency

of 500 Hz than in the center of the spectrum. Thus hard keying must not be used

on short wave due to the resulting large bandwidth. To avoid this problem, the

baseband signal of a PSK system must be specifically prepared before it gets as

far as being modulated on the carrier. This is done by transforming the rect-

angular pulses containing the binary information into suitable wave forms,

using special shaping algorithms. H. Nyquist has designed a pulse with very

useful properties, the so-called raised-cosine pulse, which does not produce

any spectral spillover. These pulses do not produce any intersymbol interfer-

ence either, since their amplitude is zero at the sampling time of successive

pulses. Thus they can be overlapped without any interference between them, even

if the pulses are four times longer than the corresponding rectangular pulses.

This is the reason why a very high information density and a good spectral ef-

ficiency can be obtained using raised-cosine pulses. Since a raised-cosine DPSK

signal with a symbol rate of 100 baud only occupies a bandwidth of around 200Hz

at -45 dB, it is obvious that two of these signals can be placed together into

a 500Hz channel. The resulting system is called two-tone-DPSK. It can robustly

transfer 400 bit/sec. within a bandwidth of less than 500Hz, if DQPSK is ap-

plied or up to 600 bit/sec. within the same bandwidth using 8-DPSK.

III. Robustness

The simplest test of the robustness of a modulation system is the measurement

of its behaviour in presence of Arbitrary White Gaussian Noise (AWGN). DQPSK is

known to be more rubust than FSK, though it also has a better spectral effi-

ciency. For example, to obtain a BER of 10E-4, the required SNR per bit is

10.7dB when using DQPSK, but 12.3dB when using FSK. DBPSK requires an even

smaller SNR of 9.3dB in that case, and is thus the most robust mode mentioned.

It is also important to remember that signals with many levels, e. g. 16-DPSK,

require more energy per bit than DBPSK or DQPSK. Generally, a compromise has

to be found between the symbol duration (i. e. the baud rate) and the number

of bits that each symbol has to carry. Short symbols do require a greater band-

width, but a high throughput can be achieved with only a few levels per symbol.

As a result, the signal is more robust against AWGN than a system with the same

throughput using longer symbols and more levels. On the other hand, short sym-

bols are very susceptible to time smearing (see below) and require a higher

bandwidth. DQPSK with 100 baud has proven to be a very good compromise between

robustness against AWGN and time dispersion, especially if it is combined with

powerful error control coding.

Another very important point for a short wave communications system is the

restistance against multipath effects, which occur if there is more than one

path between transmitter and receiver. Due to the various delays at the re-

ceiving end, the combination of the different received signals does not result

in a copy of the original signal, but in a more or less distorted wave form.

Three major multipath effects can be distinguished: time dispersion or 'time

smearing', frequency dispersion and selective fading. These three effects are

closely related to each other. They are strong if the frequency used is much

below the maximum usable frequency, and if the distance is long. A single hop

path on the 20m band, for example, normally does not suffer from severe multi-

path effects. However, a DX link on the 80m band at night often provides con-

siderably strong multipath problems.

The short term time jitter has a magnitude of up to 5 msec. Larger time smea-

ring can only be observed under very special conditions of the ionosphere. A

baud rate of 100 symbols per second has proven to be low enough for almost all

possible propagation conditions, especially if powerful error control coding

is applied.

Frequency dispersion means that the frequency of the original signal is shifted

on the path between transmitter and receiver. It is the same effect as the so-

called Doppler shift, which can be observed on signals from low orbit satel-

lites. The magnitude of the Doppler shift on normal short wave paths is only a

few Hertz, thus it does not influence ordinary FSK systems. However, the demo-

dulator of a PSK signal needs a very accurate information on the carrier fre-

quency in order to work properly. A DQPSK system with a symbol rate of 100 bit/

sec. can only deliver a correct output, if the frequency error is less than +/-

12.5Hz. Automatic frequency tracking must therefore be applied, which can easi-

ly be done on a DSP. The PACTOR-II signal, for example, can automatically be

tracked by the PTC-II within an offset range of +/- 100 Hz. Longer symbols and

more levels of a DPSK signal require a much higher frequency accuracy. For ex-

ample, a 32 baud 16-DPSK signal, as used in CLOVER-II, needs an accuracy of

better than 1Hz. Thus even small Doppler shifts deteriorate the demodulation

process, because it is not possible to track the frequency fast enough at such

a high accuracy.

Selective fading, the third multipath effect, mainly influences FSK systems,

as the channel with lower SNR determines the BER of the whole system, if the

converter cannot switch to space-only mode. The symmetrical property of a bi-

nary FSK signal is destroyed by selective fading. PSK modulation is quite ro-

bust against this effect.

IV. Intermodulation Products and Crest Factor

Whenever a signal, consisting of two or more parallel carriers or 'tones', has

to pass through a non-linear stage, intermodulation products are generated.

Special attention has to be payed to the third order products, because they are

located relatively close to the original signal components. The final RF power

stage(s) of the average Amateur Radio transmitter, are capable of a third order

intermodulation performance of about -30dB. A two-tone signal with a shift of

200Hz thus produces third order intermodulation products that are located vir-

tually within the original bandwidth of the signal. This means that there will

not be any interference on adjacent channels due to intermodulation effects.

However, a signal consisting of four tones that are spaced at 125Hz, will be

broadened to around 1100Hz of bandwidth at -30dB when passing through the same

stage.

Another item that has to be considered is the Crest Factor, which is defined as

the ratio of maximum signal amplitude to average signal amplitude. Modulation

systems designed for radio frequencies should always have a low Crest Factor,

so that the peaks of the signal wave form do not over-drive the final RF sta-

ges. Among other considerations, the Crest Factor is influenced by the number

of tones used by a system. The more tones that are used, the more difficult it

is to get a low Crest Factor. It is in addition, also influenced by the modula-

tion method. PSK, for example, leads to a lower Crest Factor than Amplitude

Shift Keying (ASK).

V. Error Control Coding

The use of a reliable modulation system is only one of the essential steps

towards the goal of optimum data transmission over the difficult paths encoun-

tered on HF radio. Dramatic improvements can additionally be obtained by cor-

rect preparation of the data before it gets as far as being transmitted by the

modem. To be effective, this process, known as Error Control Coding (ECC), im-

poses very high computing demands on the system processor. Actually the final

limit of achievable transmission reliability depends solely on the processing

power used for the ECC. The more power available, the closer the theoretical

throughput limit, the so-called Shannon Boundary, can be approached.

The basic idea behind this coding is quite simple: A certain number of redun-

dant bits is appended to the original information that has to be transmitted

through a noisy channel. The redundant bits are generated from the original

data by applying special rules, depending on the chosen code. Data and redun-

dancy then form a new string of bits, which is called a code word. The ratio

of the number of information bits and the whole length of the code word is

called code rate. The number of valid code words is obviously less than the

number of possible code words. A good code and the corresponding encoding pro-

cess must produce only those valid code words which have a maximum mutual ham-

ming distance, that means a maximum number of different bits. This maximum mu-

tual distance then allows code words to be recognized and distinguished, even

in the presence of received errors. For example, if the valid code words of a

specific code have a minimum mutual hamming distance of three, each code word

containing a single error can be corrected, as the only valid code word with

the greatest similarity then represents the correct one.

Two main approaches of ECC can be distinguished: block codes and convolutional

codes. Both always require data interleaving to be effective on channels with

burst errors. When applying block codes, the message or packet is devided into

short blocks containing only a few bits. Each short block is then encoded se-

parately and forms a relatively short code word. Popular block codes are the

Golay code, the Hamming codes and the Reed-Solomon code. Block codes can easily

be implemented as they often show a cyclic property and thus do not require

much processing power. However, they have proven to be relatively weak, espe-

cially if the BER is high. They are only able to correct a few errors in each

code word and thus do not provide any benefits in very noisy channels or poor

propagation conditions. Additionally, it is very difficult to utilize soft de-

cision when using block codes. Soft decision means that the decoder does not

only use binary decisions for the error finding process, but also the analog

values provided from the demodulator section. It works similar to the analog

Memory-ARQ of PACTOR and requires an ADC or DSP.

If convolutional codes are applied, the entire message or packet is encoded

and the resulting code words are longer than the original packet. These codes

are very powerful, and their efficiency is only limited by processing speed.

The complexity of a convolutional code mainly depends on the length of the

tapped shift registers, which work as binary convolver and represent the heart

of the convolutional encoder. This specific number is called constraint length.

It provides an upper boundary of the coding gain that can be achieved by a con-

volutional code. Several methods have been developed for the decoding process

of these codes. The optimum decoder, which allows maximum likelihood decoding,

is called the Viterbi-Decoder. Unfortunately, the relationship between con-

straint length and complexity of a Viterbi-Decoder is not a linear one, but it

increases exponentially. Real-time applications of Viterbi-Decoders were limi-

ted to quite short constraint lengths for a long time due to slow processor

speeds. Nowadays, using the new generation of DSP's, it has become possible to

apply constraint length 9 or even higher convolutional codes. As a major advan-

tage of convolutional codes and the Viterbi-Decoder, soft decision may be im-

plemented and only slightly increases the complexity of the system. PACTOR-II,

as implemented in the original German PTC-II, applies a convolutional code with

contraint length 9 and soft decision.

Table 1: Comparision of some Error Control Codes

Type of Code Coding Gain (dB)

at BER=10E-5 / BER=10E-2

------------------------------------------------------------------------

Convolutional Code (rate/2, k=3) 3.76 1.72

Convolutional Code (rate/2, k=9) 6.77 3.82

Convolutional Code (rate/2, k=13) 8.02 4.91

Golay Code (24,12) 1.92 0.02

Reed-Solomon Code (rate/2, m=8) 3.66 -1.18

------------------------------------------------------------------------

Note 1: k=constraint length, m=code dimension.

Note 2: Negative coding gain means that the code worsens the performance

in comparison with the uncoded system.

Part 2: Description of the PTC-II

The new Dimension in Data Transmission Technology

By Dr. Tom Rink, DL2FAK, and Dipl.-Ing. Martin Clas, DL1ZAM

Titre : PACTOR-II Hardware

I. Introduction

This second part of the PACTOR-II series explains the final version of the

PTC-II hardware. As already indicated in the first part, the DPSK modulation

system makes it mandatory to use a DSP as the interface to the short wave

transceiver. Additionally, the convolutional coding with Viterbi decoder re-

quires very high processing power, more than available in any existing modem

on the Amateur Radio market.

However, the many wishes, ideas and suggestions from the ranks of the Radio

Amateurs, have finally led to the development of an even more complex and

flexible unit than necessary to implement the PACTOR-II system. For example,

it has been the wish of many for a long time, to be able to operate not only

with usual digital HF modes, but also to cover VHF and UHF Packet Radio at

the same time, with one unit. Unlike currently available in existing units,

a comfortable built-in mailbox should allow simultaneous access from all com-

munications channels with the same priority, for example in Packet Radio on

23 cms with 9600 baud and on 70 cms with 1200 baud. Naturally, during this

time no PACTOR or AMTOR connect on HF should be lost or ignored. This forces

the use of a RISC (Reduced Instruction Set) processor for fast processing of

the HDLC Packet protocols. As the majority of modern HF transceivers can be

programmed via a serial interface, another wish that has slowly formed, is to

use this feature for remote control of the transceiver. To set up a mailbox

that scans various frequencies on HF, external computer and software is no

longer required. Additionally, you could utilize your home based PTC-II whilst

on the move, or from a remote location using Packet Radio. Instruct the HF

transceiver to tune to a specific frequency, then to make a connect in PACTOR

to a particular mailbox and read the messages.

These and many more ideas have led to hardware of very high performance. Due

to the use of signal processing, the PTC-II is very flexible, and allows a

great variety of different applications to be implemented. These range far

wider than those written of up to now. Those technically talented users can

make their own programs and modules, which may be loaded via the serial inter-

face into the flash memory of the PTC-II. The possible uses are almost unlimi-

ted, as the PTC-II provides a complete, very high performance development plat-

form for DSP and 68020 applications. From simple external control functions

(e. g. automatically turning the antenna to the connecting station), or audio

processing functions (e. g. super-steep sided FIR filters 400th order for CW,

automatic N-times notch filters, etc.) right up to complex functions, like an

audio spectrum analyser or extra operating modes, whether known or still to be

invented, may be implemented.

The PTC-II comes on the market in February 1995. However, as some of the com-

ponents used, among them the new DSP chip, are still difficult to get and due

to the great demand for the PTC-II, the time of delivery for a unit will ini-

tially be about 6 weeks.

II. The Processor Section

As has been made clear in the introduction, to achieve simultaneous processing

of three communications channels along with the signal coding of PACTOR-II, a

powerful computer is essential. A 32-bit processor system has been designed,

based on the new communications processor 68360 (QUICC) from Motorola. This

processor contains an expanded 32-bit version of the well known 68020 CPU, as

used in many powerful computers, together with four separate programmable se-

rial communications ports, the so-called SCC's, implemented as a RISC proces-

sor. Two of these SCC's serve as the interfaces to the Packet Radio modems.

The third SCC is used as RS-232 interface to the terminal. A buffer chip

MAX207A provides the correct RS-232 voltage levels. The baud rate to the ter-

minal is detected automatically, but can also be determined by software com-

mand. The last remaining SCC serves as interface to the HF transceiver for re-

mote control purposes. Four RAM chips with 8 bits each are required, to cover

the 32 bit wide data bus. These can vary between 4 times 32k x 8 up to 4 times

512k x 8. The PTC-II can therefore have a maximum of 2 MB of static RAM, which

plays a large part in running the mailbox and internal administration as well

as external programs, that may be loaded into the PTC-II in addition to the

operating software. In order to further expand the possible applications, the

PTC-II may also contain additional dynamic RAM in the form of a 72 pin SIMM

module of up to 32 MB. The operating system is to be found in a flash memory

of up to 512k*8. Additionally, it is possible to load programs over the serial

interface from the terminal, which would enable the PTC-II to do a totally

different job, as already mentioned in the introduction. Operating parameters

for the PTC-II that should be resistant even to a deep reset are also stored

in the flash memory. Data in this kind of memory remains stored even when no

voltage is applied, but contrary to an EPROM, it may be electrically erased

and re-written whilst in circuit. That makes it very easy updating the system

or running different programs on the PTC-II. A battery-backed-up real-time

clock and other features of the previous PTC are of course still included.

III. The HF Modem with Signal-Processor

The 50 MHz version of the XC56156 DSP from Motorola forms the interface to the

HF transceiver. As the clock frequency is programmable, it is automatically ad-

justed to suit the work of the moment. For easy tasks, such as FSK, the proces-

sing speed can be reduced, yielding a corresponding saving in energy. The DSP

contains a built-in 16-bit digital to analog converter, with the help of which

the audio output signal to the transceiver is generated, be it simple AFSK or

the complex phase modulation of PACTOR-II. The output amplitude is also pro-

grammable and may be set in the range between 10 and 4000 mV by software com-

mand. The normally required 'Mic Gain' potentiometer is thus missing. It is

also possible for the PTC-II to control the output power of the transceiver,

so that the power to maintain the link may automatically be adjusted to an

optimum value. No more power than needed being used, which not only saves on

the electricity bills, but can considerably extend the life of transmitting

components and additionally causes less interference to other stations.

For the signal input, the DSP uses a Sigma/Delta analog to digital converter

with a 16-bit dynamic range (14 bit effective), which enables the normally

necessary Anti-Aliasing filter to be dispensed with. With the exception of the

decoupling OP-AMP at the input and output of the DSP, no further components in

the signal path are required. The DSP contains some built-in static RAM, which

in the PTC-II, is further expanded with 4 additional, very fast, static RAM's.

The size of this RAM is 64k-words (16 bit) and is not variable. This enables

difficult algorithms, for example 4096 point FFT, to be used. As the DSP has

direct access to the main processor data bus, it does not tie up an SCC. The

exact receive frequency of PACTOR-II is very quickly and reliably adjusted by

software, using a newly-developed tracking method. In addition, the DSP is

able, through pulsing the up/down function (which almost every modern trans-

ceiver has as microphone buttons) to automatically change the tuning for op-

timum results. The up/down keys are simulated by transistors, which pull the

respective connection to ground.

IV. The PTC-II Power Supply

The PTC-II contains two power supply input options. Either it may be supplied

via a special DC input connector, or directly from the HF transceiver via the

connecting cable and socket. The two options are decoupled via diodes and feed

a switching regulator. This has a high efficiency, and generates the 5V supply

for the digital section. The supply voltage can vary between 8 and 20 volts.

The current requirement, due to the use of the switching regulator, is depen-

dant upon the supply voltage and the Packet Radio modems used. The higher the

supply voltage, the lower the current consumption. This reverse proportionality

is due to the fact that the power consumed is a product of voltage and current,

and must be virtually the same before and after the regulator. The efficiency

of the regulator is almost unaffected by the value of the supply voltage. The

power supply input of the PTC-II contains special filtering, so that the

switching harmonics from the regulator cannot reach the outside world. The

operating voltage is internally fused with a 5x20 mm fuse. Of course an extra

fuse is delivered with each unit to prevent problems in countries with dif-

ferent standards.

V. The Display and Indicator Unit

The display and indicator unit is built on a separate circuit board, and sits

at right angles to the main board, connected by soldering pads. It carries a

tuning indicator of 15 LED's, 15 further LED's to display the various operating

parameters and a 10-character 5x5 dot matrix LED display. Most of the LED's,

including the tuning indicator, are dual colour types, to increase the informa-

tion densitiy and the ease of reading the display. The tuning indicator, for

example, changes from red to green as soon as the tuning is optimum for the

chosen operating mode. The 10-character LED display shows the operating mode

and thus eases the display of any possible future update modes, as the PTC-II

is more than sufficient for modes such as FAX, CLOVER-II, etc. Additionally,

various status information as well as the call of connecting stations is also

given on this display, thus in many cases there will be no need to switch on

a terminal. The display is readable from a distance and from unsuitable view-

ing angles. The brightness is programmable and can be adjusted by a software

command.

VI. The Packet Radio Modems

The ability to operate Packet Radio with the PTC-II is an integral part of the

operating system, and therefore available on all units. The PTC-II is, however,

mainly an HF controller, and thus the Packet Radio modems are implemented as

modules, to be plugged in if the need arises. This enables a certain amount of

mechanical compactness (the entire PTC-II is approximately the size of a modern

VHF mobile transceiver). It also prevents those who only need an HF system from

being forced to pay for an unwanted Packet Radio modem. The PTC-II contains

connectors in the form of double PCB strip headers, on to which the modems can

be easily plugged as required. There is space provided for two modems, which

are automatically sensed when present. Two types of Packet Radio modems will be

offered. A simple and cheaper version using the well-known modem chip TCM3105

for 1200/2400 baud, as well as a version with the XC56156 DSP chip. The DSP

version is able to accomodate all baud rates from 300 to 9600 baud. The switch-

ing is accomplished by a software command. DSP programming and the clock are

supplied from the main board, so that, apart from the DSP chip itself, virtual-

ly no extra components are required for the signal processing. Additionally,

the DSP modem board has space for an EPROM and its own clock generator, as well

as baud rate switching. These enable the modem board also to be used either as

a stand-alone modem, or together with other equipment. It delivers the usual

Packet Radio signals of RxD, RxClock, TxD, TxClock and DCD. It is thus compa-

tible with all other Packet Radio systems. Even the connections to the double

PCB strip header are compatible with the usual Packet Radio system standards.

The Packet Radio boards are not initially available however. They will come on

the market in the middle of 1995, together with the corresponding firmware up-

grade for the PTC-II.

VII. The PTC-II Construction

The PTC-II is made up of two printed boards, a main board of 147x170 mm, and a

front board which, as described above, contains the displays and is mounted at

right angles to the main board. The main board is a 6-level multi-layer con-

struction and contains internal ground and supply voltage areas. On the back

is the DC input connector, an on/off switch, an 8 pin DIN connector for the HF

transceiver, two 5 pin DIN connectors for Packet Radio, an 8 pin Mini-DIN sock-

et for the transceiver control as well as a 9 pin SUB-D socket for the terminal

connector. All DIN plugs are delivered together with the unit. Every single pin

of every socket has its own filter in order to improve the HF rejection in

strong RF fields, as well as to prevent unwanted radiation of electromagnetic

energy. On the front is a row of 52 finger-pads for solder connection with the

front board, a mounting method used on all SCS controllers. The construction is

largely SMD. The flash memory and four static RAM's are socketed to enable easy

system upgrades. The RS-232 interface chip is also in a socket to allow easy

replacement in event of damage. The SIMM module, due to its construction, needs

a holder without which it cannot be mounted. An 8 way DIL switch is also in-

cluded so that various parameters may be set that should not be changed via

software. The whole is enclosed in an aluminium profile case, well known from

previous SCS-PTC's and many TNC's. Both front and rear of the case are silk-

screen printed, at the front using three colours. The green and red lettering

of the dual colour LED's explain their meaning when lighting green or red,

respectively.

PACTOR-II Questions

The new Dimension in Data Transmission Technology

By Dr. Tom Rink, DL2FAK, and Hans-Peter Helfert, DL6MAA

Part 4: Questions and Answers related to PACTOR-II

I. Introduction

As previously mentioned, we conclude this series by answering frequent ques-

tions from the readers related to the items described in the previous parts.

If you have any additional questions, or if you just want to discuss some of

the items mentioned, please feel free to attend our PACTOR forum at the Dayton

Ham Vention '95, which will be held on Saturday, April 29, between 09.00 and

10.45 a.m. in room no. 7.

II. Frequently asked Questions

Will SCS continue PACTOR-I after the release of PACTOR-II?

For Sure. PACTOR-II is not intended to supersede the current FSK-PACTOR stand-

ard. The new level was developed in order to take advantage of the capabilities

of new DSP modems, providing a maximum of throughput and robustness within a

minimum of bandwidth. It may thus become the new standard mode of the forth-

coming DSP hardware generation. As PACTOR Level I already provides virtually

optimum attributes obtainable with an FSK system, it will be used as long as

FSK modems are used. Therefore, SCS will of course continue producing the

PTCplus.

Which companies will license PACTOR-II?

AEA and PacComm have already decided to license PACTOR-II. Additionally, seve-

ral other companies have indicated their interest in a license. Some of them

however, will have to wait until they have developed new units that provide the

required processing power to run the PACTOR-II protocol.

Can a PTC-II still contact an older PACTOR-I or AMTOR station?

Yes. The philosophy of the whole development of PACTOR-II is compatibility with

older systems. All connects in PACTOR are initiated in the standard PACTOR-I

format. Only if both stations are equipped with PACTOR-II, and the 'MYLEVEL'

command of both stations is set to '2' will an automatic change to PACTOR-II

take place. Otherwise, the contact remains in PACTOR-I. 'MYLEVEL' is the com-

mand, that defines the maximum allowed PACTOR system level, the default value

is '2'. The PTC-II also contains AMTOR, RTTY, and CW software as in the PTCplus

and with exception to CW, the earlier Z80-PTC. The new PTC-II owner will there-

fore have no problems in finding a contact, even if there are not many PAC-

TOR-II equipped stations at first.

What are the operational differences when passing PACTOR-II traffic?

There are hardly any differences in comparison with PACTOR Level I. To set up

a PACTOR-II link, you just proceed in the usual way: Enter 'C <CALLSIGN>' to

establish a regular PACTOR-II QSO or 'C !<CALLSIGN>' to initiate a long path

link. If the connected station is also capable of PACTOR-II, an automatic

switching to the higher level is performed. To control the link (i.e. perform

a break-in, change-over, QRT, etc.), you may also still use the existing,

freely definable characters. For an Unproto transmission, you may now not only

chose between two arguments, representing 100 and 200 Baud FSK. Additionally,

the values 3-6 represent 100, 200, 400 and 800 Baud DPSK with the corresponding

error control coding in short cycles, and the values 7-10 represent the same

speed and coding levels using data cycles.

Will I be able to use PACTOR-II with my Transceiver?

All modern (and virtually all older) SSB radios will quite happily work with

PACTOR-II. The switching times are identical to PACTOR-I, and as the data sig-

nals are tailored for an audio bandwidth of 500 Hz, there should be no problems

within the audio sections of any radio system. The RF side of the transceiver

should have low intermodulation distortion, as the signal is more complex than

FSK systems, but providing the transceiver is not overdriven, this should not

be a problem. If the radio performed well with AMTOR or PACTOR-I there should

be no problems with PACTOR-II. All that is required from the transceiver is an

audio input, an audio output, and PTT line. As mentioned before, the frequency

stability and accuracy requirements are no higher than PACTOR-I or AMTOR.

Will I get interference from the PTC-II internal computer on my radio?

A good deal of effort has gone into designing the hardware so that it not only

does not cause interference, but also is not easily interfered with. All supply

lines are filtered, as are the RS-232 and signal inputs and outputs. The cir-

cuit board has also been especially designed with short data and address lines

and large earth planes to prevent radiation. In any event, it will cause con-

siderably less noise than the computer you use it with.

Do I need a powerful computer for PACTOR-II?

No. All the high-tech, powerful computing jobs take place inside the PTC-II.

The interface to the outside world is such that any computer with a standard

RS-232 interface can be used to transmit and receive PACTOR-II. Software is

supplied for an IBM compatible PC. Almost any computer can be used with the

PTC-II using standard terminal software for that machine. A pure software so-

lution for PACTOR-II, using simple interface hardware as in Packet Radio is

however very unlikely. For that you WOULD need a very powerful computer.

Does the PACTOR-II Listen Mode require external software?

No, the PTC-II firmware also provides full listen mode capability. Be it simple

FSK, or any of the complex convolutional coded DPSK frames, the PTC-II Listen

mode checks simultaneously all possible modulation forms of PACTOR-I and -II.

Therefore, any plain terminal software on any computer may be used to exploit

all the features of PACTOR-II.

Can the PTC-II also be operated in FSK mode?

Similar to the previous PTCs, the PTC-II also provides an FSK output line.

However, as the PACTOR-II system utilizes DPSK modulation, you have to use the

audio line when proposing to operate PACTOR-II. If you plan to restrict to the

FSK modes (such as PACTOR-I, AMTOR, RTTY, CW, etc.) in order to use the FSK

line of your transceiver, you have to set the PTC-II command 'MYLEVEL' to '1',

so as to prevent the PTC-II from an automatic switching to PACTOR Level II when

connecting with another station capable of PACTOR-II.

Is the required frequency accuracy to set up a PACTOR-II link similar

to CLOVER?

CLOVER is considered to allow a maximum frequency deviation of around 20 Hz

between two stations intending to set up a link. As all PACTOR-II links ini-

tially start in PACTOR-I, the required frequency accuracy is only about +/- 80

Hz. When switching to PACTOR-II, a newly developed tracking method automatical-

ly adjusts the DSP filters in order to compensate the deviation. Additionally,

in the PACTOR-II mode, the tuning indicator of the PTC-II does not only show

two LED's representing the two signal tones, but a third LED indicates the ab-

solute frequency offset. If the LED in the middle of the display lights up, you

are exactly on zero beat. Each LED deviation in either direction signals a cor-

responding frequency deviation of 10 Hz. As this indicator is not influenced

by the automatic tracking, you are still able to adjust the correct frequency

after the link setup.

Can the current PTC mailbox software also be used with the PTC-II?

Most mailbox software designed for the Z80-PTC, manufactured by PacComm and

previously by SCS, as well as for the modern PTCplus may be used with the

PTC-II without any modifications. This specially refers to the modified GPLX-

BBS by JA3FJ, the KCQ-MBX by W8KCQ, and AMTBOX by DL7AMW. The Dwell-Time of

the PACTOR-II system, i. e. the period required to detect a connect request,

is still about one second. This is identical to PACTOR-I and thus faster than

in CLOVER and G-TOR. This Dwell-Time plays a large part for the fast scan stop

signal, as required for scanning BBS systems. Since the system does not have

to stay too long on one frequency in this case, significantly shorter response

times can be obtained.

Will the PTC-II provide a Host Mode for more comfortable terminal and

BBS operation?

Yes, a Host Mode will be added in a firmware update. However, in the early

stage one has to make do with an expanded status word. In addition to the cur-

rent information, it shows all link parameters including the PACTOR-II speed

levels as well as the kind of on-line data compression used.

Does an upgrade of the PTC-II require an exchange of any memory chips?

The operating system is stored as a compressed file in the flash memory of the

PTC-II. It is automatically unpacked and loaded into the static RAM when the

system is started. To update the unit, a special program ('UPDATE.EXE') is pro-

vided, which electrically erases and re-writes the data field in the flash mem-

ory that contains the PTC-II operating system. For this reason, a PTC-II update

does not require any modification of the hardware, but can simply be done via

the RS-232 interface. The PTC-BIOS is not influenced by 'UPDATE.EXE', therefore

any home-made software can continue to be used as before.

Does PACTOR-II build on the novelties of CLOVER-II and G-TOR?

There are some basic attributes that have been adopted from CLOVER, e.g. the

use of pulse shaped DPSK modulation combined with error control coding and the

employment of modern DSP technology. Even G-TOR encouraged us to do a little

protocol change, i.e. adding run-length encoding. Some other features, like

the hybrid-ARQ, the obligatory data interleaving, or the simple tolerating of

a few bit errors within the CS ('fuzzy' evaluation) are not really new, but

have partly been used in a much more sophisticated way before. For example,

instead of a 'fuzzy' CS check, PACTOR-I as well as PACTOR-II employ the cross

correlation method, which is considered to be the optimum, but requires fine

detail analog information from the demodulator section and high processing

speed. Taken generally, PACTOR-II builds mainly on PACTOR-I, despite some

similarities to other systems.

On the other hand, we should not try to compare apples with pears, as PACTOR-II

is very different to all previous narrow band digital modes. The Nyquist DPSK

modulation and the high performance Viterbi decoding with Soft Decision provide

system properties that have been out of reach before. At the moment, PACTOR-II

is the most adaptive and most robust narrow band ARQ system you can buy. It of

course also provides the best bandwidth efficiency, as it only occupies 450 Hz

of bandwidth at minus 50 dB, even if the actual throughput exceeds 1000 bits

per second. For the on-line data compression, not only upper and lower case

Huffman and run-length encoding are implemented. Additionally, upper and lower

case Pseudo Markov Compression (PMC) for English as well as German texts can

be chosen. This means, the best out of six different compression methods will

automatically and reliably be applied. An overall compression factor of around

2 is achievable with PACTOR-II.

How long until even the PTC-II becomes obsolete?

This is a question that bothers many people as computer systems sometimes be-

come obsolete almost within months. Whilst it is impossible to guarantee, it is

not envisaged that the PTC-II will change in the near future. It is such a high

performance and flexible unit, that it is considered able to master all tasks

applicable to Amateur Radio for a considerable time. Hardware updates have been

allowed for in the initial design, and software updates can be easily made

using the RS-232 interface into the FLASH memory. There are certain boundaries

in physics, beyond which it is impossible to go. PACTOR-II with the PTC-II are

pushing very close to these limits. All other present modes are well within the

hardware capabilities. It is hoped that some of these other modes will be of-

fered in the future as updates. Future, as yet undiscovered or developed modes

are possible, though unlikely to provide a decided advantage over PACTOR-II on

HF links. These too should be workable using the PTC-II hardware. Other exotic

uses are a matter of programming and imagination. Your PTC-II should not be ob-

solete for a long time.

Why should I buy 'yet another digital mode'?

The previous sections were perhaps a bit technical. In short, the major advan-

tages of PACTOR-II are as follows:

- Full compatibility with previous PACTOR systems.

- Much greater immunity to interference.

- Greater transmission speed (over 1000 bits per second under reasonable

conditions).

- Totally automatic in operation.

- A system gain of around 7 dB compared with present PACTOR systems including

analog Memory-ARQ (less power needed).

- Narrow bandwidth (max. 450 Hz at minus 50 dB), i.e. less interference

Part 3: Short Description of the PACTOR-II Protocol

The new Dimension in Data Transmission Technology

By Dr. Tom Rink, DL2FAK, and Hans-Peter Helfert, DL6MAA

I. Introduction

All new modes should provide significant improvements over existing systems,

which must not only refer to the maximum throughput and the robustness. Other

basic attributes, like signal bandwidth, required frequency accuracy and com-

patibility to existing standards also have to be taken into consideration.

Modulation and encoding methods that supply high throughput rates, e.g. 16-

DPSK, normally suffer from a lack of robustness. On the other hand, systems

distinguished by a high robustness, e.g. DBPSK combined with a rate 1/2 con-

volutional code, only provide a low maximum throughput. Therefore adaptive

digital modes have to be used, that apply different modulation and encoding

methods depending on the channel quality. Just changing the symbol rate how-

ever, leads to only little adaptivity and additionally results in variations

of the bandwidth. In order to prevent any spillover in adjacent channels, the

bandwidth should ideally always remain the same, regardless of whether a ro-

bust or a fast data transmission is performed. As 500 Hz CW filters are very

commonly used and due to the usual spacing of the mailbox frequencies on short

wave, a maximum bandwidth of 500 Hz should not be exceeded. In addition, there

should not be too high a demand placed on the transceiver used, regarding its

frequency adjustment and stability. For optimum results, maximum frequency de-

viations similar to FSK modes should be tolerated. This forces the use of po-

werful tracking methods, which allow a link also to be established if the de-

viation is up to +/- 80 Hz. Further, a new mode should be backwards compatible

to an existing standard, preferably with automatic switching, in order to pre-

vent a deficiency of QSO partners in the early stage.

PACTOR-II meets all the above mentioned requirements. It is fully backwards

compatible with the current PACTOR standard, as the initial link setup is still

done in FSK. If both stations are capable of Level II, an automatic switching

is performed. The PACTOR-II protocol basically uses a two-tone DPSK system with

raised cosine pulse shaping, which reduces the required bandwidth to less than

500 Hz. The maximum absolute transfer rate is 800 bits per second. Due to the

improved on-line data compression, maximum effective throughput rates of more

than 1200 bits per second can be obtained. PACTOR-II is thus the fastest short

wave digital mode. Very efficient error control coding using a convolutional

code with a constraint length of 9 and a real Viterbi decoder with soft deci-

sion is applied at all speed levels, in addition to analog Memory-ARQ. PAC-

TOR-II is also therefore, by far the most robust digital mode, which allows a

link to be established and achieve a reasonable throughput in such poor propa-

gation conditions that all other modes fail. In comparison with the current FSK

PACTOR standard including analog Memory-ARQ, which had been the most robust

digital mode until the release of PACTOR-II, a further gain of robustness of

around 7 dB could be obtained. The following chapters describe some details of

the PACTOR-II protocol.

II. Structure and Timing of the PACTOR-II Frames

Similar to the current FSK PACTOR standard, PACTOR-II is also a half-duplex

synchronous ARQ system without any mark/space convention. It may thus be oper-

ated in USB as well as LSB position of the transceiver. The initial link setup

is still performed using the FSK (PACTOR-I) protocol, in order to achieve com-

patibility to the previous level. If both stations are capable of PACTOR-II,

an automatic switching to the higher level is performed. The basic PACTOR-II

frame structure is similar to PACTOR-I. It consists of a header, a variable

data field, the status byte and the CRC. The standard cycle duration does not

differ from FSK PACTOR and is still 1.25 seconds, which is one of the require-

ments to obtain easy compatibility to Level I. Longer Control Signals (CS) had

to be applied to achieve a higher robustness for the acknowledgment signal,

required due to the greater robustness of the data channel. The entire length

of the standard packet had to be shortened to 0.8 seconds in order not to

shorten the maximum possible propagation delay, which is thus still 170 milli-

seconds. The requirements to operate PACTOR-II regarding the transmit delay

and the receiver recovery time of the used equipment therefore remain unchanged

in comparison with Level I.

Due to the signal propagation delay, and equipment switching delays, PACTOR-II

as well as PACTOR-I has in the standard mode a maximum range for ARQ contacts

of around 20,000 km. As with PACTOR-I, a long path option is available also for

PACTOR-II, enabling contacts up to 40,000 km. The sending station calls the

partner station in 'Long Path Mode'. Initial contact is established using the

PACTOR-I FSK protocol as previously mentioned, but with a cycle time of 1.4

seconds instead of 1.25. This longer cycle time allows for the much greater

propagation delays found on 'Long Path' contacts. The link then automatically

switches to PACTOR-II, with the same cycle duration. In the new data mode (see

below), timing is also automatically adjusted to obtain longer receiving gaps.

Unlike the previous level, PACTOR-II additionally switches to longer packets

if the data blocks are not filled up with idles, (i.e. if the transmitter buf-

fer indicates that more information has to be transferred than fitting into

the standard packets). If the information sending station (ISS) prefers to use

long packets, it sets the long cycle flag in the status word. The information

receiving station (IRS) then finally can accept the proposed change of the cy-

cle duration by sending a CS6. This situation, for example, occurs when reading

longer files out of mailboxes. The long packets are basically made up like the

short ones, but consist of a larger data field, which may contain up to 2208

bits of usable information. The length of these data packets is 3.28 seconds,

which leads to an entire cycle duration of 3.75 seconds in this so-called data

mode.

When entering text manually in QSO traffic from operator to operator, the maxi-

mum throughput of the standard mode is normally not used up, thus the required

higher flexibility of the system is still available due to the short packets.

The efficient use of longer data packets on short wave is generally only pos-

sible, if powerful error control coding, with full frame interleaving is ap-

plied to cancel out error bursts or short fading periods. As already mentioned,

PACTOR-II uses a convolutional code with a constraint length of 9, a real Vi-

terbi decoder and soft decision, in addition to analog Memory-ARQ. Due to the

high coding gain and the resulting capacity of error correction without re-

questing a repetition of the entire packet, a significant increase in the ef-

fective throughput could be obtained. Proceeding from average bit error rates

on short wave channels, simple block codes are usually unable to provide enough

coding gain. This often leads to a decrease in speed when using longer data

strings, as repetitions often cannot be avoided. For more details on these tech-

nical foundations, see the first part of this series.

PACTOR-II uses six different CS, each consisting of 40 bits, all having exactly

the maximum possible mutual hamming distance of 24 bits to each other. They

thus reach exactly the Plotkin boundary and also represent a perfect code. This

allows the advantageous use of the Cross Correlation method for decoding, which

is also a kind of soft decision, leading to the correct detection of even in-

audible CS. This checking is not only confined to a simple binary principle. A

complex analog test procedure is applied, using the fine detail data from the

DSP, to evaluate the single CS received, as well as the information summed up

in the Memory-ARQ buffer. Similar to Level I, CS1 and CS2 are used to acknow-

ledge/request packets and CS3 forces a break-in. CS4 and CS5 handle the speed

changes, and CS6 is a toggle for the packet length. All CS are always sent in

DBPSK in order to obtain a maximum of robustness.

III. Speed Levels and Error Control Coding

As mentioned in the introduction, PACTOR-II uses a two-tone DPSK modulation

system. Due to the raised cosine pulse shaping, the maximum required bandwidth

is only around 450 Hz at minus 50 dB. ASK, which was also tested in the early

stage, provided poorer results in weak conditions compared with a higher DPSK

modulation, as different amplitude levels are more difficult to distinguish in

noisy channels than more phase levels. Additionally, ASK increases the Crest

Factor of the signal. For these reasons, it is not used in the final PACTOR-II

protocol. Basic information on these items can also be found in the first part

of this series.

PACTOR-II uses instead, different DPSK modulation schemes and various code

rates. The Crest Factor of the PACTOR-II signal is therefore only 1.45. The

basic code used is an optimum rate 1/2 convolutional code with a constraint

length of 9. Codes with higher rates, e.g. rate 2/3 and rate 7/8, can be de-

rived from that code by so-called puncturing. Prior to the transmission, cer-

tain of the symbols of the rate 1/2 encoded stream are 'punctured' or deleted,

and not transmitted. At the receiving end, the punctured encoded bits are re-

placed with 'null' symbols prior to decoding with the rate 1/2 decoder. The

decoder treats these null symbols neither as a received '1' nor as '0', but as

an exactly intermediate value. No information is thus conveyed by that symbol

that may influence the decoding process. The coding performance of 'punctured'

code operation nearly matches the coding performance of the best known classic

rate 2/3 or 7/8 codes with a comparable constraint length, provided that the

puncture pattern is chosen carefully. The major advantage of this approach is

that a single code rate decoder (in our case a rate 1/2 decoder) can implement

a wide range of codes.

In the PTC-II, the Viterbi algorithm is implemented for decoding of the con-

volutional code. Nevertheless, as already indicated in the first part, there

are several different methods to decode the PACTOR-II signal, which require

less processing power, but in return for this, also provide less coding gain.

However, these methods at least allow compatibility to the PACTOR-II standard

and may therefore be applied in cheaper hardware.

The most robust PACTOR-II speed level employs DBPSK with rate 1/2 coding, which

per cycle allows an absolute throughput of 5 bytes in the standard mode and 36

bytes in the data mode respectively. In the next step, DQPSK with rate 1/2 co-

ding is applied, which leads to an absolute throughput of 14 bytes in the

standard mode and 76 bytes in the data mode. This is followed by 8-DPSK with

a rate 2/3 coding, providing a throughput of 32 and 156 bytes per packet, re-

spectively. Finally, in best propagation conditions, PACTOR-II applies 16-DPSK

with a rate 7/8 coding, which allows the maximum throughput of 59 bytes in a

short packet and 276 bytes in the data mode. The mentioned transfer rates are

all net rates referring to 8-bit ASCII, which are calculated after the error

control coding and all other protocol overhead. As data compression is usually

active, these throughput rates must be multiplied by the compression factor.

The effective speed is therefore considerably higher in practice. The speed

levels are automatically chosen by the PTC-II, considering the link statistics

and the actual channel quality, thus no user intervention is required.

IV. On-line Data Compression

Like in the previous FSK PACTOR system, automatic on-line Huffman data compres-

sion is applied. Additionally, PACTOR-II uses run-length encoding and, as a

further novelty, Pseudo-Markov Compression (PMC, see below). Compared to 8-bit

ASCII (plain text) PMC yields a compression factor of around 1.9, which leads

to an effective speed of about 600 bits per second in average propagation con-

ditions in data mode. PACTOR-II is already around 3 times faster than PACTOR-I

and 15 times faster than AMTOR on average channels. However, the maximum effec-

tive speed in good conditions can exceed 1200 bits per second. As the PTC-II

firmware automatically checks, whether PMC, Huffman encoding or the original

ASCII code is the best choice, which depends on the probability of occurrence

of the characters, there is no risk of losing throughput capacity. PACTOR-II

is of course still able to transfer any given binary information, e.g. programs

or picture- and voice files. In these cases the on-line data compression is

automatically switched off.

Ordinary Huffman compression exploits the 'one-dimensional' probability distri-

bution of the characters in plain texts. The more frequently a character oc-

curs, the shorter has to be the Huffman symbol that is assigned to the actual

character. On the other hand, Markov compression can be considered as a 'dou-

ble' Huffman compression, since it not only makes use of the simple probabili-

ty distribution, but of the 'two-dimensional' probability. For each preceding

character, a probability distribution of the very next character can be calcu-

lated. For example, if the actual character is 'e', it is very likely that 'i'

or 's' occurs next, but extremely unlikely that an 'X' follows. The resulting

probability distributions are much sharper than the simple one-dimensional dis-

tribution and thus lead to a considerably better compression. Unfortunately,

there are two drawbacks: Since for each ASCII character a separate coding table

is required, the entire Markov coding table becomes impractically large. Addi-

tionally, the two-dimensional distribution and thus also the achievable com-

pression factor depends much more on the kind of text than the simple character

distribution.

We have therefore chosen a slightly modified approach which we called Pseudo-

Markov Compression, because it can be considered as a hybrid between Markov-

and Huffman encoding. In this variant, the Markov encoding is limited to the

16 most frequent 'preceding' characters. All other characters trigger normal

Huffman compression of the very next character. This reduces the Markov coding

table to a reasonable size and also makes the character probabilities less cri-

tical, since especially the less frequent characters tend to have unstable pro-

bability distributions. Nevertheless, for optimum compression, two different

tables for English and German texts are defined in the PACTOR-II protocol and

automatically chosen by the PTC-II.

V. Some Practical Aspects

Similar to Level I, the tones of the PACTOR-II signal are spaced at 200 Hz.

Their frequency may be defined freely in steps of 1 Hz by software command, as

long as the shift remains 200 Hz. Thus you can easily switch between high- and

low-tones, and also adjust any additionally required tone pair. This allows

the utilizing of narrow CW filters in all transceivers that provide the option

of activating the corresponding filters in the SSB mode.

In the PACTOR-II system, the transferred information is swapped from one chan-

nel (tone) to the other in every cycle. Unlike FSK systems, the link is thus

not blocked when strong narrow band QRM completely overpowers one channel (e.g.

CW or carriers), but only its maximum speed is reduced. Usual FSK systems with

a mark/space convention and without Memory-ARQ have to fail in such cases, be-

cause even if a so-called 'space-only' mode is applied, the strongest signal is

automatically chosen. This always leads to a break-down of the link, as the QRM

is stronger than the useful signal in the proposed case.

PACTOR-II provides a comprehensive Listen-Mode, which is much more robust than

known from PACTOR-I, because just the short header has to be received correct-

ly, then the powerful error control coding can be fully utilized. Burst errors

may be corrected also by monitoring stations and thus virtually do not affect

the performance. The Unproto-Mode in PACTOR-II allows to choose between all the

above mentioned speed and encoding levels. On the receiving side, the correct

mode is detected automatically and therefore needs no user-adjustment. For ex-

ample, a fast and very robust QTC mode can thus be achieved, when a message is

transmitted in the Unproto-Mode using DBPSK with rate 1/2 coding.

Table 1: PACTOR-II speed and encoding levels with the resulting througput

rates (all throughput rates are net rates referring to 8-bit ASCII).

Cycle Modulation Code rate Data bytes/packet Data bytes/sec

type (no compression) (with PMC, f=1.9)