L.M.T. Laboratories
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Seven-Frequency Radio-printer

By L. Devaux and F. Smets,
Les Laboratoires, Le Matériel Téléphonique, Paris, France
Edited by Murray Greenman, 1998

Condensed from an article in "Electrical Communication", 1937. Original technical jargon has been retained where practical, so remember that this was written in 1937! Some diagrams have been partially redrawn where the originals were of insufficient quality for scanning. This paper describes a clear fore-runner to modern frequency domain Concurrent Multi-Tone Hellschreiber.

Modern-day comments on technical detail are made in brown.

INTRODUCTION

In printing telegraph systems for wire communication, it is universal practice to translate the character to be transmitted into a telegraph code signal consisting of a combination of (usually five) elements of equal duration, and at the receiving end to re-translate the code signal into the original character. (Such as ITA2 RTTY) Such systems are not well adapted to radio communication, since the effect of superimposed atmospherics or other interference is to modify the received code signal, which is necessarily translated by the receiving mechanism into some character other than that originally transmitted. Experience has shown that, up to the present time, it has not been possible to overcome this difficulty except by using elaborate equipment. (Now practical with many different modes of course).

An alternative method of sending printed messages consists of "scanning" or analysing each printed character into a number of elementary lines, and transmitting these elementary lines in the form of dashes and spaces of varying length; at the receiving end the reproduced lines build up the original character. Such a system of transmission is not unlike the facsimile method(Feld-Hell is similar). It is well suited to radio circuits, since interference cannot change a letter into another one which is totally dissimilar, the only effect being to print small extra elements, or to suppress small elements of the transmitted letters. As is well known, a considerable amount of such "bad printing" does not materially depreciate the intelligibility of the text. The characters will be more or less accurately reproduced, depending on the strength of the interfering static, but no "printing error" is possible. The operator is constantly aware of the the quality of transmission, independently of whether the messages are in code, plain language or cypher(a feature much enjoyed with modern MT-Hell!).

Radio-Printer text example (enlarged)

In the radio-printer system developed in the L.M.T. laboratories, the characters are scanned or analysed in seven horizontal lines, each horizontal line being represented in the electrical transmission by one definite audio frequency. The lines are transmitted simultaneously, rather than in sequence, and the radio carrier is therefore modulated simultaneously by all the audio frequencies corresponding to the lines.(This is an exact description of Concurrent MT-Hell.)

This arrangement enables a high speed of letter printing to be obtained, combined with slow modulation of the audio frequencies which carry the scanning, and hence long operating periods for the printing relays, and a resultant high degree of reliability in printing.

The equipment operates on the "start-stop" principle, i.e. the receiver mechanism is automatically set in motion at the beginning of each character transmission, and automatically arrested when the character has been completed.(Start-stop is not used in MT-Hell as it tends to be noise prone. It was done here to save paper. Start-stop ensures that paper is not wasted and inadvertent spaces between characters at hand-sent speeds do not occur.) There is no need to maintain synchronism between transmitting and receiving mechanisms, and the receiver may in fact be left unattended.

In the signal modulation of the individual audio frequencies, the "marking" or printing condition may correspond to either the existence or suppression of these frequencies. The latter condition has been adopted. (The former was adopted for MT-Hell, since we now have better detection techniques, and it saves transmitter power.) The general effect of static, in this case is thus not to print unwanted components of letters, but to suppress points in the characters, which in the majority of cases remain perfectly legible.

It is convenient to divide the description into two parts -

Mechanical and electro-mechanical devices, including the scanning, the translation of both keyboard and perforated tape operations into electrical impulses, the storage of characters, the modulation of the frequencies, the receiving relays and the mechanical printer.
Electrical circuits, supply of frequencies, filtering of received signals and operation of the receiver.

MECHANICAL

1.a Scanning

Although all conventional characters can be scanned in five lines if necessary, scanning in seven lines gives much better analysis (Fig. 1). The number next to each line of the character examples corresponds to the elementary line number used.

Fig. 1 Character Matrix

(It looks as though the characters are 7 rows x 10 columns, but this is a trick to give 10 column resolution with only 5 columns, by staggering the lines while keeping bandwidth low.)

All characters, including letters, figures, and punctuation marks, have been drawn so that they may be built up from a number of elementary lines, assembled according to requirements. The total number of characters provided amounts to 46, built from the 39 elementary lines (Fig. 2).

Fig. 2 The Elementary Lines

Some elementary lines may be obtained by the combination of two other lines; for example line 16 is made by addition of lines 2 amd 7, etc. The total number of elementary lines can thus be reduced to 23. (It would not matter nowadays with everything in software, but at the time saving cams and relays was important. A compromise was reached between the numbers of cams and complexity of relay wiring.)

1.b Transmission

The transmitter is equipped with seven low frequency oscillators, working continuously and modulating the radio transmitter. The mechanical problem of transmission consists in controlling these oscillators in the proper time and sequence, to suppress momentarily the modulation as required.

Advantage has been taken of the great facilities afforded by standard telephone relays, to reduce to a minimum the number of mechanical parts. A system based on the use of such relays has been designed for making nearly all the requisite contacts, and providing for either keyboard or tape transmission, and for the storing of signals, a feature which improves greatly the efficiency of manual operation.

The primary elementary lines are generated by a set of 23 cams, carried by a continuously rotated shaft, and fitted with contact springs. Combinations of primary lines can be obtained by means of contacts in parallel. A few of these elementary line cams are shown to the left of Fig. 3.

Fig. 3 Transmitter Block Diagram

To each character requiring seven lines corresponds a relay bearing eight contacts. Seven of these contacts are used to connect the seven oscillators to the proper cams, while the eighth closes a circuit locking relay. A few characters, such as dash, full stop, comma etc., which require less than seven lines, are provided with relays with fewer contacts. (this is not a proportional spacing technique - simply a way to save relay wiring)

There are five chain (decoding) relays with multiple contact sets, which in turn operate the 46 character relays (A five bit demultiplexer). The character relays connect the cams contacts to the seven oscillators which modulate the carrier. An oscillator is made inoperative when the cam contact to which it is connected is closed.

The common return for all character relays is made through a common start sending relay which is intended to send first the start signal when any character relay is operated. The start signal consists of momentary suppression of lines 1, 3, 5, and 7, and is obtained by means of two starting cams which operate just before the primary line cams begin to work. Thus the start signal will only be sent if the starting relay is energised, i.e. if a character relay is operated. (It does not matter if the start sequence also occurs during a character.)

Character transmission consists, therefore, in the operation of a character relay at the proper moment, just before the starting cam closes its contacts, and keeps the relay energised until the lines are fully transmitted. This latter condition is easily satisfied by means of the locking contacts on the character relays, the common circuit of which is controlled by a locking cam which operates at the appropriate time during the revolution of the shaft.

1.c Perforated Tape Operation

The transmitter has been designed to use a standard perforated tape, according to the five-unit Baudot (Murray) code, giving simultaneous contacts for each row of perforations. Six decode relays are used (an extra one to discriminate figures and letters). Standard telephone relays with eight contacts have been used. The contacts of the six relays are chained to decode the 46 character combinations. A cam on the same shaft as the primary line cams operates a rachet drive solenoid to advance the perforated tape to keep the tape transmitter in proper synchronism.

Each combination of perforations in the tape operates the corresponding combination of chain (decoding) relays, and results in the operation of the corresponding character relay.

When the code combination for changing over from letters to figures occurs, the contacts of the decoding relays cause the sixth (shift) relay to be energised, and a second set of contacts on the other five decoding relays are now operated, and figures or punctuation are sent. To come back to letters, the combination operates another relay which releases the shift relay to return to the "letter" position. (An R-S flip-flop.)

1.d Keyboard Operation

A switch is used to change the connections of the six chain (decoding) relays for tape or keyboard operation. Keyboard operation has been slightly complicated, but largely improved, by adding a storing register between the keyboard and the decoding relays (One character keyboard buffer!). This completely does away with the necessity of operating the keyboard at the constant speed of the shaft. The operator can thus depress the keys at any moment, in any sequence, and the speed of actual transmission is nevertheless constant and equal to the mean of the speed of manual operation.

A six unit code is used from the keyboard, since the shift relay is operated directly by one of the contacts of each key. This does away with special keys for shifting from letters to figures, and vice versa, and simplifies the operation. The keys give the code combination by closing a variable number of code contacts with the contact bars.

1.e Storage Register

The principle of this device is as follows: a number of groups of six relays (corresponding to the six contact bars) is provided. Each group, when connected to the contact bars, is able to register a combination of six units given by the depression of a key. The same group, if connected later to the chain (decoding) relays, will transmit to these relays the registered combination. These connections to contact bars and to the chain relays are made by two rotating switches, the incoming switch connecting the bars to the register relays, and the outgoing switch connecting the register relays to the chain relays. As many groups of six relays may be provided as are found advisable. In practice five groups give satisfactory operation.

The incoming and outgoing switches always run one after the other, but the second is always one position behind the first. The incoming switch is advanced one step any time any key is depressed, but the outgoing switch is advanced by one step at every revolution of the cam shaft. Since five groups of register relays are provided, there must not be more than four steps difference between the two switches. A differential device closes the circuit of a magnet which locks the keys when keys have been depressed in rapid sequence, so that they cannot be operated again until a steady advance of the outgoing switch has made free at least one group of register relays (A FIFO buffer).

1.f Receiving Mechanism

At the receiving end, the voice frequency currents, after selection by filters, are fed to the seven printing magnets. Each magnet is used to press one of seven levers, which each carry a small printing stylus lying on the running paper tape, a carbon paper tape being inserted between paper and stylii. Dots and dashes are thus printed on the paper tape, according to the currents received by the magnets, and their assembly builds up the transmitter characters (Fig. 4).

Fig. 4 Receiving Mechanism

The paper and carbon tapes are moved by two pairs of rollers, which are driven by the main cam shaft. A constant speed motor drives the shaft via a clutch of the type used in Creed teleprinters, to give the cam shaft intermittent motion through the action of the clutch magnet, which is controlled by a latched start relay. During shaft motion the relay stays energised through the action of cam driven contacts, so the cam shaft stops only when one revolution has been completed.

The start relay has two opposed windings, the first being connected in the common return of printing magnets 1,3,5 and 7, while the second is similarly connected to printing magnets 2,4,and 6. The reason for this is the following: strong static, which would be liable to operate the start relay, has a wide and continuous frequency spectrum, and its actions in the seven channels is nearly uniform; since the currents delivered to the printing magnets acts differentially on the start relay, this does not operate, and false starts are avoided. (AND gate: START = [1 + 3 + 5 + 7 + NOT[2 + 4 + 6]]).

The main cam shaft makes one revolution for each clutching operation, and the inter- character lifting stud operated by a cam on the shaft prevents the printing stylii from pressing the paper at the instant of start, i.e. during the start signal.

The speed of the motor shaft does not need to be accurately governed, the only result of variations in the speed being very slight variations in the width of the printed characters (an effect familiar to MT-Hell users).

ELECTRICAL

2.a Modulation of the Voice Frequencies

It has already been mentioned that the "mark" signal may consist either in the existence or in the suppression of the voice frequency current. As the printer is intended to work on radio links which are subject to selective fading, it is necessary to provide individual automatic volume control, at the receiving end, for each voice frequency. This is the main reason why the signals have been "marked" by the suppression of the appropriate voice frequency current. This method, moreover, has the advantage that static will generally result in the suppression of a few points in the characters, instead of printing extra dots.

2.b Voice Frequency and Filters

The choice of the voice frequencies is determined (a) by the speed of transmission and the quality of the receiving filter; and (b) by the frequency band which is generally passed by telephone links. The printer has been designed for a speed of 5 characters per second (50 WPM), and consequently, owing to the analysis of characters, the shortest signals last 20 ms. This corresponds to a fundamental frequency of 25 Hz. Good transmission requires that the third harmonic of this fundamental frequency should be passed, and the pass-band of the the receiving filters should therefore be 150 Hz. For economic reasons, it is desirable to use filters comprising only one coil and capacitor, and the Q of the circuit must be correct. Experience has shown that a spacing of 240 Hz gives a good compromise, together with a Q of 10 for the tuned circuits. The filter diagram shows the arrangement of the tuned circuits (Fig.5).

Fig. 5 Receiving Filter Diagram

The frequencies adopted are odd multiples of 120 Hz, from 600 Hz to 2040 Hz. The only harmonic relationship between these numbers is that the sixth is the third harmonic of the the first. This fact is important, as it avoids the printing of wrong dots owing to harmonic distortion in the radio-telephone link. The total frequency range required extends roughly from 500 Hz to 2200 Hz. The next diagram gives the transmission characteristics of a filter (4th channel, 1320 Hz, Fig. 6).

Fig. 6 Receiving Filter Characteristics

Around the resonant point of one of the circuits, C, L, r, (see Fig. 5) the shunting effects of the other circuits in parallel is negligible, and the voltage V developed across the circuit is equal to

As the Q is made constant for all the circuits, their efficiency is the same. For the other frequencies, the circuit C, L, r under consideration, is shunted by the resistance r. This resistance is made as small as possible, little more than coil resistance. The total resistance R + r is chosen great enough to give the desired passband: a convenient value was found to be 560 Ohms, corresponding to a Q of 10. The dotted curve in the filter graph represents the ratio V/E for a single tuned circuit of Q = 10. The dotted curve represents the same ratio as obtained with the complete array of tuned circuits as shown in the filter circuit. Although tuning is not sharp, the interference from neighbouring channels is quite small.

2.c Oscillators

There are seven independent valve oscillators which feed a line amplifier tube and a level metering circuit. The oscillators are a loosely coupled tuned-anode LC design. The output network consists of a resistive attenuator loosely coupled to the anode choke, designed so that changes of load do not affect the oscillators. The oscillators also generate square signals of constant amplitude to further assist keeping constant modulation level. The attenuator circuits have individual level adjustments, and the resistive circuits connect to a single transformer coupled line amplifier.

The oscillators run continuously, and the output interrupted by the transmitting signal at the attenuator network by shorting to ground. There are keys to provide individual oscillator current measurement, and both individual and combined amplitude measurements for adjustment purposes. The oscillator amplitudes can be adjusted independently.

2.d Printer-Receiver

The combined audio-frequency signals supplied by the radio receiver are applied to the grid of an amplifier tube (Variable slope AGC controlled). The anode has a resistive load, and is coupled to the previously described seven channel filter arrangement. Each output of the filter is applied to one of seven identical circuits, comprising an amplifier tube, a rectifier and an output tube.

The output of the amplifier tube is transformer coupled to two rectifiers with identical load resistors, but different filter time-constants. The first is 500ms, the second 5ms. When an unmodulated tone is applied to the transformer, the voltage drops across the two load resistors is practically equal. If the tone is suppressed for a short time, corresponding to a signal, the voltage across the load resistor with the 5ms time constant quickly becomes zero, causing current to flow in the output tube and the printing magnet. The voltage across the long time constant load resistor does not change appreciably. (Functionally similar to an RTTY automatic threshold correction circuit and slicer).

Fig. 7 Detector Diagram

If interference due to neighbouring channel signals or noise is present, and added to the signal, the voltage drops in both load resistors are increased, but remain equal. If the level of interference is equal to the signal, and if again the signal is suppressed for a short time, the voltage across the short time constant load resistor will become zero, just as if no interference was present. In other words, the longer time constant capacitor acts as a reservoir which will absorb interference up to the level of the signal, since the resistors are equal(similar to an audio noise limiter).

The suppression of the signal causes the bias on the output tube to become zero, even in the presence of noise, and sets up a current in the tube and the printing magnet. Without the diode between the centre of the load resistors and the transformer centre-tap, a sequence of signals would give weaker and weaker pulses. The diode allows the longer time constant to charge rapidly, but discharge slowly, preventing the memory effect (DC restorer circuit).

The screen voltage of the output tube is adjusted by means of a potentiometer, for a no signal current of 10 mA. The voltage to the screen grid is supplied via a resistor, shunted by a capacitor to give the effect of a kick-circuit on the printing magnet.

For good printing the operating current in the printer magnets must exceed 7 mA, and the non-operating current must be less than 3mA. The ideal A.V.C. (Automatic Volume Control) would keep the minimum current of the output tube constant, whatever the signal input level. This case is approached by controlling the slope of the amplifying tube by the voltage across the output tube cathode resistance. In order to make the voltage practically independent of the current pulses during printing, the cathode resistance is shunted by a selenium rectifier which is polarised by a voltage of 12 V. The voltage across the cathode resistor can thus never exceed 12 V. Its mean value is practically proportional to the "spacing" current in the printing magnets, and can be used for biassing the grid of the amplifier tube, via a smoothing circuit with a time constant of 0.5 seconds.

Diagram showing the operation of the rectifier circuit

PRACTICAL RESULTS

The printer transmitter and receiver units are separate rack mounted units, although much smaller bays could have been used for housing the electrical equipment.

Radio-Printer transmitting and receiving equipment

The apparatus was first tested in a radio-telephone link devoid of fading, but into which intentional noise could be injected at will. The text example below (Fig. 8) shows the tape as received during these tests, the mean noise level being the same as the signal level (0dB S/N). Extra dots are numerous, due to saturation of the receiver during short periods of very high, instantaneous noise level, but the text is easily legible and devoid of wrong letters. .

Fig. 8 0dB Signal-to-Noise

Another series of tests was carried out in July 1937, using the radio-telephone link between Algiers and Paris, a distance of 1300 km, on a frequency of 12.2 MHz. The transmitter was in Algiers and the receiver in Paris. The link has a good signal-to-noise ratio (more than 30 dB), but is greatly subject to fading. Selective fading is severe and frequently reaches 20dB and more. The next example shows the effect of selective fading, which results in a series of dashes which slowly cross the width of the characters, i.e. the band of modulation.

Fig. 9 Effect of Fading

The printer was in daily use for nearly a month, and the results were very satisfactory, both with perforated tape and keyboard operation. In some instances of extremely bad transmission conditions, when a code printer would have been definitely useless, the radio-printer continued to deliver an intelligible test. It will be seen from these comprehensive tests that the built-up character radio- printer described above constitutes a most reliable and convenient means of printed communication. (All this was achieved in 1937! What a shame the War got in the way, and the system was never adopted.)