Radio Communication: Capturing telex messages with the PUNCHROPHYL (1960)

 

Capturing telex messages with the PUNCHROPHYL (1960)

 
At the end of the fifties of the twentieth century, the Royal Netherlands Airforce’s 45^th telecommunication service wanted to record intercepted encrypted and unencrypted military telex messages from behind the Iron Curtain. In 1960, a custom-built papertape punch machine called PUNCHROPHYL (Punching UNinterupted CHaracter Registration Otomaton PHYsics Laboratory) was developed by TNO that could capture the received telex messages and the time of receipt. In addition, a twin plex converter was built, which could separate both telex channels that were put on one carrier. At that time recording took place with papertape (see “Background: papertape” at the bottom of this page). The papertape punch device was able to create 7-hole (4 and 3 tracks around the s [sprocket row) paper tape with the format below.

Receiving telex information was sent to the PUNCHROPHYL using a radio receiver and recorded in papertape after processing. A separate control box with an indicator was used to control the combined functioning of the PUNCHROPHYL. As peripheral equipment was used:

• The radio receivers:
  • RACAL (RA 17L) with an output resistance of 3 Ohm and 10 mW output power,
  • PHILIPS (BX 925 A/09) with an output resistance of 600 Ohm.
• A CREED papertape punch for registration of received messages.
• A normal telex machine (leaf or band writer) which was controlled with 40 mA over 225 ohms as a quiescent current. Received normal telex signals such as the Western Baudot code could be registered on the telex machine.
• A modified paper tape recorder by HELL (according to the modification HELL recorder RC 18 I) was used to check the normal (Western) telex registration. See further (external): HELL morse recorder rc18.html and HELL morse systems

 

The incoming radio signal knew two mutually different states. These two states (in Fig. 2 “High” and “Low”) were received as two adjacent, alternating frequencies. This so-called Frequency Shift Keying (FSK) means that the frequency is switched, the “SHIFT”, between the two modulations. The receiver was able to convert these modulations into audio frequencies (tones). The received signal consists of a series of characters. Each sign consists of a certain number of elements (bits) with properties as a start or stop element or as an information bit.

Fig. 2 shows the signals that the PUNCHROPHYL could process:

• 12 bits with 10 information bits, a nominal character duration of 300 ms,
• 7 bits with five information bits, nominal character duration of 155 ms,
• 7.5 bits with five information bits, nominal character duration of 150 or 163 ms.

The signals in FIG. 2a and FIG. 2b consist of a series of contiguous characters. When no content information was sent, the sending of start and stop elements generally went through the so-called “Idle Time” signal. The elements of both start and stop had equal duration.

The signal is shown in FIG. 2c is the normal telex code (Western Baudot code, about 50 Baud). The indication “7.5 bits” refers to the character length. The STOP element is 1.5 times the length of an information bit. Signals with 7.5 bits of characters sometimes consisted of a series of non-contiguous characters, the so-called “separate characters”.

The Punchrophyl was composed of the following elements:

1. Converter. The converter switched the alternating tones from the radio receiver into alternating direct voltages. The START polarity is 0, the STOP polarity is negative. The input filters of the converter each passed one of the two tones. This occurs only if the beat oscillator of the radio receiver is tuned correctly. If it was assumed that the “SHIFT” = 1,000Hz was between the two frequencies, then the beat oscillator had to be set so that one tone had a frequency of 2750 Hz and the other 1750 Hz.
2. Signal synchronisation unit. The received radio signal consisted of a series of contiguous characters. The regular succession of characters can be regarded as a cyclical phenomenon. To process this signal, the PUNCHROPHYL used an internal system cycle, which could synchronise itself with the signal cycle. The PUNCHROPHYL was able to detect those locations where the START and STOP elements are located in the incoming signal. As a result, each information bit was then sorted according to the order of entry. The signal synchronisation unit was constructed with:
   • A statistical memory that generated the internal system cycle. It could be considered as an electronic wheel whose speed was determined by the partial setting of the system clock. The absolute synchronisation with the cycle of the input signal (phase) was controlled with the scan retarder. The information in the statistical memory could, for example, fade the synchronisation between the system cycle and the input signal cycle for a short period if the input signal was lost. However, when processing 7.5 bits of signals, the statistical memory stopped storing as soon as no character was available. This made it possible to process the “loose characters” in a similar way as for normal telex devices.
   • A scan retarder that always responded to the transitions from STOP to START bit. With this, the scan retarder generated a synchronisation pulse for the statistical memory. For example, small asynchronous deviations were corrected.
   • A shift unit. Some radio signals suddenly showed polarity changes of the START and STOP bits. This was referred to as “SLIDING” at that time. Why this polarity change was carried out was then still unknown. If there was no response to this change, synchronisation would be completely disrupted. The automatic control unit edited the input signal (with or without “SLIDE”) in such a way that normal polarity was assigned to the STOP and START bit.
3. Signal measurement. The signal measurement made it possible to measure the duration of the characters during “IDLE-time” broadcasts. The duration of a character was strongly dependent on the channel used. This time measurement also provided information for the part of the system clock to be set.
4. The system clock.  The system clock controlled the pulses of the PUNCHROPHYL with the output of pulses. These impulses in μs were adjustable (fraction-based). By setting the system clock a reasonable synchronisation between the input signal cycle and system cycle could be obtained.
5. Buffer memories and amplifiers. The information elements of the characters were successively read and stored in a shift register. This buffer was read out in parallel and presented to the code magnets of the CREED papertape puncher. At each punching stroke of the CREED papertape puncher, the five information elements (bits) were punched into the papertape. To be able to punch both seven and five-channel tracks, a modification was made to the CREED puncher. In Fig.3. the layout for these tracks is indicated.
   This device was able to create 7-hole (4 and 3 tracks around the s [sprocket row) paper tape with the format shown below.

   

A modified CREED high-speed reperforator Model 25-Mk IV was available for reading the collected information. A small modification was made in the papertape guide of the CREED reperforator which made it possible to punch seven and five-track paper tapes either simultaneously or separately. Fig.3. shows a trace layout Tracks 3 to 7 also appeared in the 5-track band. These contain the information bits. The five information bits of the 7-bit or 7.5-bit Telex signals were punched at the same time. The ten information bits for 12-bit telex signals could not be punched at the same time. There was only a maximum of seven tracks available. The ten information bits signals were therefore divided into two groups of five bits each, which were punched subsequently.

Track 1 was reserved for number marking. At any time this marking could be applied using a push-button action on the control panel. The number marking consisted of four bits, which were punched alongside four consecutive characters. In the 12-bit telex signal, each of these four marking bits was punched next to the second group of information bits of the four consecutive characters. First of all, a hole was punched to indicate the start of the marking (start bit). Then followed the remaining three bits, which together formed a binary number. The binary number was incremented after each marking command.

Binary number
000 0
001 0 0
010 0 0
011 0 0 0
100 0 0
101 0 0 0
110 0 0 0
111 0 0 0 0
papertape direction

If something special occurred during the processing of the information received, this could be indicated using the marking on the paper tape. Possibly with an accompanying note for the indicated marker number.
Track 2 was reserved to indicate at twelve bits of telex signals whether “SLIDING” had occurred. If this occurred, a hole was punched in track 2 next to the first group of five information bits (ten information bits total). The information that could be added in tracks 1 and 2 in this way also made it possible to distinguish the first group of five information bits from the second group when receiving twelve bits of signals. After all, it happened that only one of the two groups contained bits of information.

With the HELL recorder (HELL recorder RC 18 I, “Gerätsbeschreibung” and modification sheet PL 337 RVO-TNO: “Marking on HELL tape”), an ink line is written on a rolling thin paper belt with the help of a stylus. If a small AC voltage with a frequency of between 500 to 2,000 Hz is applied as the input for the HELL recorder, this input signal is processed as a so-called “HIGH” signal.

The HELL papertape was marked as well:

1. This is a corresponding marking as used with the CREED puncher. Four bits were displayed per marking. A binary number of three bits preceded by a start bit. They were displayed on the HELL paper belt as double dashes (spikes), which fell within the second info bit of four consecutive characters.

   

2. Zero-set mark. When the PUNCHROPHYL was correctly set synchronously, a single peak was found in the starting element of each written character. If the PUNCHROPHYL ran asynchronously with the incoming signal, the peak was displayed at an arbitrary position within the written characters. In Fig. 5a. the synchronisation recovery is shown.

   

With a “LOW” signal, the markings are “LOW” and vice versa.
Received 7.5-bit telex messages could be read using a telex machine or tape writer.

The technical operation of the PUNCHROPHYL

The signal level for the PUNCHROPHYL had to be above a minimum. This was indicated with an electronic indicator and a rotating reel gauge. The electronic indicator lights up when the level is too low. For a favourable signal level, the rotating coil gauge had to be set to 80% of travel. A too-high level encouraged distortion. The input level was adjustable with the RF volume control of the radio receiver. It was therefore recommended that the AF volume control of the PHILIPS BX925 radio receiver be set to maximum. The use of automatic volume adjustment (AVC) was not allowed.
Signals for the PUNCHROPHYL have a SHIFT of 500, 1,000, 1,500 or 2,000 Hz. The radio FSK signal was set to the listening frequency (AUDIO) using the beat oscillator of the radio receiver. The “SHIFT” was set as follows:

• Set one of the two signal tones with the BEAT oscillator so that it becomes inaudible.
• Activate the push-button “TONE” on the control panel. An internally generated tone was audible instead of the signal tone.
• Use the “FREQ.SHIFT” selector switch on the control panel to select an internally generated tone, which most of all happened to the other audible signal tone. Of the two input filters of the CONVERTER (see Fig. 4), one had a fixed passage for 2750 Hz. The other filter could be set for transmission of frequencies that are 500, 1,000, 1,500 or 2,000 Hz lower than 2,750 Hz.

  

• On the control panel, two electronic indicators completely lit up when processing the two correctly set filter signals. To present the signals in the correct polarity to the PUNCHROPHYL, an “INVERTER” switch was installed. With this “INVERTER” switch, the polarity for “IDLE TIME” signals of 7 or 12 bits on the HELL recorder was also correctly displayed. The signal could also be checked for the STOP and START polarity. If the input signal had the STOP polarity, the left indicator would light up completely and the right was almost extinguished. The reverse was true for the START polarity.

With an “IDLE TIME” signal, a duration measurement in ms of the received characters, a so-called signal measurement, could be performed. Using several switches on the control panel, the time required for a current of 10 characters was measured for several seconds. For the following signals, a duration of about 300 ms (12 bits of character length), 155 ms (at 7 bits) and 150 or 163 ms (at 7.5 bits per character). With these measurement results, the type of input signal could be determined with the PUNCHROPHYL. By setting this type of signal, e.g. a 7-bit signal, with a selector switch in the “MEASURE 7” position and the “MEASURE” action, a particle was determined, which appeared in the four numeral tubes on the control panel with an accuracy of one decimal. With the help of pushbuttons “PARTIAL SETTING”, this dividend could be set for the PUNCHROPHYL. With a series of indication tubes marked “SYNC” on the control panel, the synchronisation process for the Telex signal to be received was now optimally set and monitored.

With three indicators and two push buttons on the control panel, the number marking for the paper tape and the tape of the HELL writer was activated. After the action with the push-button “0-set”, a binary one (001) on the three indicator tubes was visible with the push-button “BEVEL”. With this, the marking in the punched tape and/or on the tape of the HELL writer was simultaneously applied. Then the action with the “BEVEL” push button always increased the binary number until the maximum value (0-set) was reached. Note: The volume control of the HELL writer must be set to maximum and the writing flow regulator to minimum.
 
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Background: papertape

The perforated tape consisted of a paper band (strip) in which the information was encoded in round holes that were punched and arranged in five or eight rows. From the early 1950s to the end of the 1970s, it was a widely used computer medium for both data and software storage. The paper tape strip was about 2 cm or 2.54 cm (inch) wide. The mechanical specifications, which both the unpunched and punched paper strips had to comply with, were laid down in the so-called ECMA standard 10 (ECMA-10) (ECMA-10). This standard was maintained until the mid-70s of the twentieth century.

In the five-hole papertape, the five information bits were written perpendicular to the tape, three on one side and two on the other side of the smaller transport holes (so-called sprocket holes). Because five bits (holes) were too little for data storage, the storage on the paper tape was later enlarged to 8 holes in a row (five and three). In the eight-hole papertape, so-called ASCII characters were punched either 6, 7 or 8 bits of data. Many coding methods were in use. To deal with errors in punching and reading, there was sometimes a parity bit added. This special bit, recorded in one row, caused the total sum per column to be odd. The paper tape reading device stopped when an even number of holes were read. It was then up to the operator to ascertain the error and record it using a note on the print report.
 
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