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AMTOR - putting one of those old modems back into use

Background

AMTOR is an acronym for AMateur Teleprinting Over Radio and is mostly used on HF. The mode is based on the marine SITOR code that was introduced to amateur radio by Peter, G3PLX. The mode is an enhancement of RTTY with active error correction that copes reasonably well with interference and fading. AMTOR traditionally uses upper case characters and letters/figures shift in the same way that RTTY does, later modems could also operate with lower case characters. The mode has been largely discarded in favour of low cost sound card based data modes, such as PSK31, Olivia and MFSK. Unfortunately, AMTOR generally needs a dedicated external modem as the timing is critical and modern operating systems, such as Windows, can be interrupted by other tasks. Older PCs running DOS and the earlier "single task" computers were capable of operating AMTOR with simple home made modems.

Broadcast calls, such as CQ calls, use a method of forward error correction where each character is repeated (mode B). Normally a QSO is conducted in mode A (ARQ automatic repeat request). Unlike the original marine SITOR code, there was a need to be able to monitor a QSO operating in mode A, this mode is known as mode L.

The data throughput of AMTOR is fairly slow under most band conditions, but is generally error free. For normal "ragchew" contacts the data flow will match the typing speed of a reasonable to good typist. The error correction is fairly crude but adequate for most purposes.

Suitable modems can sometimes be acquired for little cost from fellow hams who may have them lying gathering dust in garages and on shelves. eBay is a source for old modems, but beware that prices seem a bit steep for items that are often several years old and not easily repaired.

Available modems

Probably the most popular AMTOR modem was the PK232, in addition there were modems from Kantronics (KAM), ICS (AMT), HAL , SCS and an MFJ offering. Older Pactor modems can also operate on AMTOR. Most of these modems are quite long in the tooth and were often only supplied with DOS software. Currently, as at June 2014, the following new modems can also operate AMTOR:

PK232 DSP from Timewave
PTC-IIIusb (but not the P4 Dragon series) from SCS
KAM-XL from Kantronics

Software

Many of the older modems used DOS software which can be a problem for modern PCs which often don't even have conventional RS232 serial ports. Some of the modems can be operated from a simple terminal program such as Windows "HyperTerminal", however most users will find a dedicated program easier to use. Freeware terminal programs are hard to find.

XPWare used to be a paid for product which has now been offered for free with versions for both DOS and Windows up to XP. XPWare works with AEA (PK232), Kantronics (KAM), SCS (PTC) and HAL modems, however it won't directly install on later versions of Windows. There is an easy "workaround" to use the free program Oracle VirtualBox. With this program you can run one operating system inside another in a virtual box (as per the program name). The program works with a variety of operating systems, I have an XP session running on a Windows 7 64 bit PC with XPWare controlling a KAM Plus via a serial port... Note you need the VirtualBox Extension Pack to use serial ports. You also need a licensed version of the additional operating system, however once registered the internet connection can be broken in VirtualBox, so you don't need any anti-virus programs, nor do you need to keep updating your isolated XP system. For a screen view of how to set up a serial port in Oracle VirtualBox, click here . This creates COM3 for your XPWare when using the host PC serial port 6 - note the colon after "COM6". Check under Control Panel on the XP machine that you have a serial port available.

Alpha (which only works with SCS modems) is free from SCS.

In addition, there is one Linux ( KPTC ) offering but unfortunately it only works with SCS modems - and needs compiling for the particular version of Linux used.

There were Easyterm for Windows, Radio Operations Center for Windows and KaWin for Windows, but these seem to have vanished off the web.

Suitable radios

The requirements are fast transmit/receive change over and a reasonable level of frequency stability. A dedicated modem socket (or line socket) is desirable to avoid needing to change microphone and speaker leads. Most modern HF transceivers are suitable for use on AMTOR, however it is worth checking the specification and ARRL reviews for "suitable for AMTOR" or similar wording. I currently use an Elecraft K3 and Kenwood TS-480SAT, both of which work without modification on AMTOR. Some earlier transceivers may need modifying in order to change over quickly enough. There is a slight advantage in using transceivers with diode antenna change over to avoid the endless "click click" of the antenna relay when operating AMTOR, it is doubtful that relay wear will be significant unless you want to operate AMTOR 24 hours a day. A narrow "CW" filter that can be configured to work on higher tone frequencies centred on 1500 Hz, or higher, is useful but not essential, some radios such as the TS-480 automatically offset the standard 500 Hz CW filter for use with AMTOR. There doesn't appear to be any difference in performance between the older analogue IF radios and the more recent DSP IF radios.

Mostly SSB is used with audio tones on AMTOR, although FSK can be used with some modems as AMTOR is based on RTTY. Note that modems which include Pactor will only use audio tones as FSK (direct frequency shift keying) is not suitable for use on Pactor.

Hooking it up and getting going

Some of the older modems had 25 pin serial ports and others have oddly spaced DIN sockets. Most of these connectors can be sourced without too much difficulty, but it is generally easier to find a modem complete with at least the plugs and sockets, preferably with complete leads. Wiring modems to radios and computers is covered in the user manual, all that I have either wired or seen, have been comprehensively covered in the manual. In some cases running a ground wire from the radio to the modem or passing the connecting cable through a large ferrite ring is useful to avoid RF hanging up the modem, it depends on how much RF you have in the shack as to how much of a problem it might be, again refer to the user manual for any grounding or filtering information. Depending on the tone pairs used, you will have to pick USB or LSB, working out the actual carrier frequency for the mark and space tones may require a little mental arithmatic in order to meet up with someone on a certain frequency.

AMTOR uses 170 Hz shift, the same as for RTTY. Some modems will only accommodate a 200 Hz shift, which is near enough for all practical purposes. Some modems include a feature to specify the mark and space frequencies, traditionally for RTTY these were 2125 and 2295 Hz. The more modern modems, which include Pactor, often default to 1400 and 1600 Hz tones, which are the initial tones for a Pactor call. In any case, your transceiver needs to be able to work with whatever tone pairs your modem is set for. With an Elecraft K3 these tones are best set around a centre frequency of 1500 Hz as this is fixed at that frequency when operating in synchronous data modes such as AMTOR and Pactor.

The basic parameters necessary on both the radio and modem are as follows, these are based on using an SCS modem with an Elecraft K3, other modems and radios will have similar parameters - although not all radios have a specific data mode:

Radio settings

Mode - DATA A (not the alternative side band where "REV" is shown in the display)
Bandwidth - 300 Hz
AGC - fast
Synchronous data on (+ symbol displayed)
PTT RLS 12mS (some radios will have this parameter fixed, it allows time for the audio to exit the TX DSP filtering before returning to receive)
Microphone input - Line
By using the synchronous data mode and line input, the transmit/receive turnaround time is minimised and the transmit audio equaliser is bypassed.

Modem settings

ARX 1 (enables AMTOR, ARX 0 would prevent AMTOR operation)
BC 1 (AMTOR FEC enabled)
MARk 1585 (mark tone frequency in Hz)
SPAce 1415 (space tone frequency in Hz)
TOnes 2 (allows for free choice of mark and space tones)
MYSelc (enter your four character selective call)
TR 0 (TX and RX shift normal)
TXDelay 4 (20mS delay before audio is sent on transmit, allows time for the transceiver to change to transmit).

There are dozens of modems commands, these are just the most basic ones. In addition, the tone amplitudes may need adjusting depending on the sensitivity of the line input to your radio. With the tone frequency pairs above, the radio operates in USB even though it's in a "data" mode.

Finding other stations to work
The site "Hamspots" used to have an ARQ page where both Pactor and Amtor stations could post details of stations hear/worked and their own CQ calls. Recently, probably due to lack of activity, the ARQ page is no longer there. Posting on the digital forum at eHam is worth a try

Format for contacts

Normally a station would call CQ in AMTOR mode B (FEC) in a similar manner to this:

CQ CQ CQ DE G4AON G4AON (GAON)
CQ CQ CQ DE G4AON G4AON (GAON)

PLEASE CALL IN ARQ K

The four letters in brackets are my selcall, it is helpful if stations list their selcall during a CQ as a few do not follow the normal arrangement for creating a selcall and confuse those of us who try and guess what they used! Normally for two letter suffix calls you would repeat the first letter, W1AB would use WWAB. Equally WA1BCD would use WBCD to make a 4 character selcall. You can use what you like provided that if you deviate from the expected selcall you make it clear in your CQ.

The caller would selectively call the station calling CQ and give their callsigns as the AMTOR exchange (unlike Pactor) does not include a callsign. It is useful if the modem is configured to auto reply, when called, with your callsign and name. The change over command is +? and can be manually typed or done via one of the pre-configured buttons in most terminal software.

CW ID may be necessary depending on your licence conditions, again most modems are configurable to auto send CW IDs periodically and at the end of a QSO.

All the above is usually covered in the user manual for the modem.

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