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Racal MA-4470
Voice Encryption Unit

The MA-4470 was one of the last high-grade voice encryption units to be developed an manufactured around 1996 by Racal COMSEC Ltd in the UK, before the company was sold to Thomson CSF (now Thales) in 2000. A civil telephone variant is known as the MA-4471.

The unit is smaller than any of its predecessors and measures just 16 x 6 x 21 cm. It is inserted between the handset and the transceiver and allows voice signals to be encrypted using a combination of Time Element scrambling and Frequency Processing, in half-duplex [1].

The MA-4470 employs a novel continuous synchronisation system that allows late-entry and that is virtually transparent to the user when switching the transmission direction of the half-duplex link. The unit can be powered by the host radio set, or by an external power supply unit.
MA-4470 front panel

All controls and connection are at the front panel. At the left are the sockets for the handset (or headset) and the transceiver. To the right of the sockets is a 4-character LED display and some LED indicators that are used for user feedback. The right half of the front panel is taken by the 25-button rubber keypad, which is used for switching between clear and secure, entering the base key, using the fill-gun, selection of the current key and ereasure of the key(s).

MA-4470 voice crypto unit MA-4470 front panel MA-4470 front panel Another view of the print panel
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MA-4470 voice crypto unit
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MA-4470 front panel
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MA-4470 front panel
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Another view of the print panel

Entering keys
The cryptographic keys are 80-bits long and can be entered by typing 20 hexadecimal digits on the keypad. Alternatively, the keys can be entered using a suitable key-fill device (fill-gun), such as the MA-4255 Master Programmer or the MA-4459 Slave keygun [1].

The image on the right shows the Racal MA-4083 fill-gun that looks similar to the MA-4459 slave keygun that is used with the MA-4470. The MA-4255 Master Programmer is housed in the same case as the MA-4225 portable encryptor.
MA-4083 fill gun, which is similar to the MA-4459 slave keygun.

The MA-4470 unit can be opened from the rear. The device consists of 4 PCBs plus a font panel. After removing the rear panel, the four PCBs are revealed. From top to bottom there are the interface board, the key processor board, the speech store board, and finally the audio board.

The front panel of the unit acts as a backplane for the four PCBs. Each board uses a 64-way DIN connector for connection to the backplane. A small metal bracked, that is usually stored in between the PCBs, can be used to pull-out the individual boards.

The image on the right shows the processor board, which is the 2nd board from the top, half-way out of the case. The metal bracket is shown here in front of the PCB. Detailed pictures of each board can be found below.
Key Processor Board being removed

Interior of the MA-4470 Key Processor Board being removed Key Processor board Speech store board Interface board Audio board Close-up of the audio board Another detail of the audio board
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Interior of the MA-4470
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Key Processor Board being removed
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Key Processor board
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Speech store board
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Interface board
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Audio board
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Close-up of the audio board
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Another detail of the audio board

  • Technique: Time Element scrambling and Frequency processing
  • Keystream: non-linear keystream generator
  • Total number of keys: 8·1031
  • Internal key settings: 26 bits
  • Base key: 80 bits per key (from fill-gun) or 20 hex digits (via keyboard)
  • Stored keys: 16 base keys + 1 guard key (selectable via keyboard)
  • Key period: 1024 bits
  • Message key length: 24 bits
  • Transmission: FSK in frequency processed channel at 48 baud
  • Message key cycle time: > 90 days
  • Key storage battery life: 5 years
  • Audio bandwidth: 400Hz - 2400Hz
  • Required channel bandwidth: 400Hz - 2700Hz
  • Audio delay: 495 ms
  • Input level: TX: 0dB, 1mV - 50mV, RX: 70mV - 3.5V
  • Output level: adjustable to match input level (max. 1.5W into 8 Ω)
  • Power supply: 12V - 32V DC, 120 - 300mA
  • Temperature range: -20°C to +65°C (operation), -40°C to +70°C (storage)
  • MA-4255 Master Programmer
  • MA-4459 Slave Keygun
  • MA-4220 NiCd Battery for Master Programmer
  • MA-4234 Battery Charger (230V AC) for Master Programmer
  • MA-4454 Battery Charger (115V AC) for Master Programmer
  1. Racal-Comsec Limited, MA-4470 Sales Leaflet
    February 1991. 1
  1. Leaflet kindly supplied by Paul Ratcliffe, January 2013.

Further information
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Crypto Museum. Created: Thursday 26 August 2010. Last changed: Saturday, 24 February 2018 - 16:39 CET.
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