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Racal MA-4204
Time-division voice scrambler

The MA-4204 is a low-grade military voice encryption unit, designed and built in the early 1970s by Racal Datacom Ltd in the UK. The device uses a time-division scrambling technique which shuffles small time segments in a pseudo-random sequence that changes every half second. It is suitable for narrow-band voice transmission on HF and VHF bands, in tactical (military) situations.
 
The scrambling order of the time segments changes each half second and is controlled by a pseudo-random number generator (PRNG). The PRNG itself can be controlled by three rotary switches at the top half of the front panel.

Each switch has 8 positions, creating 512 possible sequences (8 x 8 x 8). Although 512 combinations provides hardly any protection, it was considered sufficient for tactical use at the time. With modern techniques it should be relatively easy to break the code, simply by observing the discontinuities in the signal.
  
MA-4204 front panel

The MA-4204 can be used for voice signals in any mode (AM, FM, SSB) but also for keyed signals (morse, CW). The MA-4204 can be powered directly from the transceiver (TCVR) or from an optional battery that can be attached to the bottom of the device. The MA-4204 belongs to the same family of devices as the slightly more advanced MA-4014B, that has 5 code-selectors.
 
MA-4204 voice crypto unit MA-4204 front panel Rear panel MA-4204 front panel Rear panel of the MA-4204 (bottom) Racal MA-4204 Close-up of the AUDIO sockets Close-up of the TCVR socket

 
Controls
The MA-4204 is extremely easy to operate and has very few controls and indicators. It is connected to a radio set and suitable accessories through the 3 connectors at the front panel. Once connected, the mode of operation can be selected with the 3-position rotary switch at the bottom center (MODE). The required scrambling code is selected with the 3 switches at the top.


Two red indicators are present to the right of the TCVR connector, marked AUX and PTE. The AUX indicator lights up when the unit gets power from the connected transceiver (see below). The PTE indicator lights up in PRIVATE or ENCRYPTED mode. It flashes when the voltage drops too low.
 
MODE of operation
  • OFF
    This mode bypasses the encryption and decryption circuits of the MA-4204. It forces communication in CLEAR.

  • ON
    In this mode, automatic encryption is enabled. It can be used for the reception of CLEAR signals, whilst it switches automatically to decryption when a suitable synchronising signal is detected.

  • SEND
    This mode forces continuous transmission in encrypted mode.

Connections
The MA-4204 has three connectors at the front, through which all signals and power lines are connected. The leftmost connector (TCVR) is intended for connection to a transceiver. It is also used to supply power to the unit. The unit can be fed by any DC power source between 12 and 30V and draws just 120mA. Socket AUDIO 1 is used for the connection of a handset, a morse KEY or a battery charger (when the external battery MA-968A is used). AUDIO 2 can be used for connecting an extra handset or ancillary equipment.

TCVR (Transceiver)
 
  1. MIC out (to transceiver)
  2. Power (+12 to 30V)
  3. PTT out (to transceiver)
  4. GND (common rail)
  5. KEY out (when MODE-switch is OFF)
  6. Audio in (from transceiver)
  
AUDIO 1
 
  1. MIC in
  2. Battery supply
  3. PTT in
  4. GND (common rail)
  5. Key
  6. AUDIO out
  
AUDIO 2
 
  1. MIC in
  2. AUX supply output
  3. PTT in
  4. GND (common rail)
  5. PRIVATE indicator (remote)
  6. AUDIO out
  
Operation
Inside the MA-4014B, speech is first digitized using a Delta-Sigma modulator, which produces a data stream with a bit-rate of 32 kbps (kilo-bits per second). The data is temporarily stored in memory, where it is divided into eight segments of 2048 bits each (2 kb). This way, each segment represents 62.5 ms, with 500 ms for all 8 segments. The eight segments are then scrambled.


Scrambling of the eight segment is controlled by a pseudo random generator, assisted by pre-recorded scrambling orders that were stored in ROM. These pre-recorded orders were carefully selected to guarantee minimum intelligibility. After scrambling, the data was coverted back to an analogue signal again, so that it could be transmitted over a standard voice channel.
 
Interior
The interior of the MA-4204 can be accessed easily by removing two large bolts from the bottom and pulling the front panel out of the case. The interior consists of a metal frame with 4 PCBs. The outmost PCBs (top and bottom) can be released and 'folded away' as shown below.
 
The back of the metal frame holds the fuses; two active ones plus a spare. The fuse-assembly is also the terminal for the (optional) battery.

In the image on the right, the digital board is visible on the left. At the edge of this board is a series of digital Dynamic Shift Registers [2]. The PCB on the right is the oscillator board, which contains a white custom-designed chip.

The other boards contain the analogue circuits and some switching electronics. Detailed images of each board can be found below.
  
MA-4204 with its PCBs 'folded out'

In the past few years (2010), the surplus market has been flooded with MA-4204 units. Although they hardly provide any protection these days, they can be demonstrated as part of a hobbyist project. The information provided on this page should be sufficient to connect the unit to a rig.
 
Releasing the bolts at the rear Removing the MA-4204 from its case Inside of the case Uncovered MA-4204 MA-4204 interior Rear side of the frame, holding the fuses. MA-4204 with its PCBs 'folded out' Close-up of the switch board
Close-up of the digital store Close-up of the digital shift registers (1024 bits each) Oscillator board Analogue board MODE selector (interior) TCVR socket (interior) Audio sockets (interior) Close-up of the code selectors

 
Wanted
At present, we have no access to the user manual or the service manual of the MA-4204. Much of the characteristics of this device have been deduced from the manual of the slightly more advanced MA-4014B. If you have access to any of these manuals, or if you have further information about these devices, please contact us.
 
References
  1. Racal Datacom Ltd., Technical Manual, MA. 4014B, Audio Encryption Unit
    Ref. WOH 5062. Issue 1.1.75-150.

  2. Signetics, 2524 and 2525 datasheet
    512 and 1024 bit Recirculating Dynamic Shift Registers.
    Publication date unknown. Retrieved April 2012.

Further information

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