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TST-7698
Digital voice encryptor

TST-7698 was a digital voice encryption and decryption unit, developed in the mid to late 1980s by Tele Security Timmann (TST) in Tutzing (Germany). It was intended for use in combination with SW/HF, VHF and UHF radios and features either LPC-10 or RELP voice encoding. The device is suitable for virtually any existing (military) FM radio set, including the American AN/PRC-77.

The device uses a digital vocoder (digitizer) that offers excellent speech quality, even at a bitrate of 2400 baud. It is suitable for voice encryption via noisy shortwave (SW) radio channels, and is commonly used with the SSB mode of the radio. When connected to a VHF/UHF FM radio, higher baudrates of up to 9600 baud are possible [A].

The device has 9 compartments for the storage of cryptographic KEYs, that are loaded into the device with a TST-0706 reader and a TST-0502 key card. The key loader should be connected to the same socket as the radio (the leftmost NF-7).
  
 TST-7698 voice encryptor

The TST-7698 was introduced in the mid or late 1980s, as the successor to the TST-7595 SW/HF voice scrambler. Although both devices look very similar — they are housed in the same rugged enclosure and have identical controls and connections — the TST-7698 is much more secure as it offers true digital encryption. The TST-7698 was also used as part of the TST-4045 HF modem.

 TST-7698 voice encryptor
Control panel
Bottom panel with ID label
PRC-77 radio and TST-7698 voice encryptor
TST-7698 voice encryptor connected to PRC-77 radio
PRC-77 with handset and TST-7698 voice encryptor
TST mounting bracket seen from the rear
Cable between PRC-77 and TST-7698
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 TST-7698 voice encryptor
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Control panel
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Bottom panel with ID label
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PRC-77 radio and TST-7698 voice encryptor
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TST-7698 voice encryptor connected to PRC-77 radio
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PRC-77 with handset and TST-7698 voice encryptor
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TST mounting bracket seen from the rear
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Cable between PRC-77 and TST-7698

Features
The TST-7698 is housed in a strong die-cast aluminium enclosure that is very similar to that of the TST-7595 voice scrambler and other TST devices. It houses two eurocard-size (16 x 10 cm) PCBs and is small enough to be fitted to the side of, say, a standard AN/PRC-77 military radio. The device is powered by an internal DC/DC converter – fitted internally at the bottom end – that gets is power from the radio to which it is connected. The radio is connected to the NF10 socket.

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All controls and connections are located at the front of the device, on a removable control panel. A standard handset should be connected to the NF7 socket at the right. The device is enabled by pressing the ON/OFF button. A suitable encryption key can be selected with the rotary dial (0-9). Note that the dial is also used for entering keys manually and for altering system parameters.

LED blinking
The LED on the control panel (fitted just above the ON/OFF switch) shows the current mode of operation by changing its colour as the blinking speed. In general: GREEN means that the device is in CLEAR mode, whilst RED indicates CIPHER mode. The following situations are known:



Keys
The TST-7698 requires the following key types:

  • Family key
    programmed at the factory at manufacturing time
  • Master key
    entered manually or by means of a fill gun
  • Auxiliary keys
    7 when entered manually, 9 when entered with a fill gun
  • Message key
    randomly generated internally when synchronising
Zeroize
  • Set key selector to '0'
  • Press and hold PTT switch on handset
  • Turn device OFF and ON (whilst keeping PTT depressed)
  • Release PTT (a 30 Hz warning tone should now be heard)
Parts
VHF radio set, e.g. PRC-77
Mounting bracket (cradle) for fitting to the PRC-77
Cable for connection to the radio
PRC-77 radio
The AN/PRC-77 radio was in fact a PRC-25 that had been given a solid state RF power amplifier, and was modified for the connection of digital voice encryption equipment, by adding a so-called X-MODE to the circuitry.

When in X-Mode, the filters in the audio path of transmitter and receiver are bypassed so that the full bandwidth of the set can be used.

 More about the PRC-77

  
PRC-77 radio and TST-7698 voice encryptor

Mounting bracket
Customised mounting brackets were available for various types of radios. The one shown in the image on the right was used for mounting the TST-7698 to the side of an PRC-77 radio set.

The bracket is fitted over one of the carrying grips of the radio's front panel, and is fixed in place with a removable spring.
  
TST mounting bracket seen from the rear

Interconnection cable
For connection to the radio, the user had the choice between a universal cable (with no connector at the end) or the dedicated on for the PRC-77 radio, shown in the image on the right.

The large connector should be fitted to the POWER socket on the front panel of the PRC-77. At the other side is an NF10 plug that should be fitted to the NF10 connector on the TST-7698.
  
Cable between PRC-77 and TST-7698

PRC-77 military radio
PRC-77 radio and TST-7698 voice encryptor
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PRC-77 military radio
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PRC-77 radio and TST-7698 voice encryptor




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Interior
The TST-7698 is housed in a strong die-cast aluminium enclosure that measures 220 × 105 × 45 mm and weights 1300 grams. It is nearly identical to other TST encryptors, such as the TST-7595 voice scrambler, and has a control panel that is very similar to that of the TST-7595.

The device is contructed from a rectangular die-cast frame – that can hold several eurocard (160 x 100 mm) PCBs – with removable die-cast end pieces, one of which is the control panel, whilst the other end holds the 10-33V DC converter.

The interior can be accessed by loosening the headless hex bolts at the corners of the case, after which the control panel can be removed as shown in the image on the right. The PCBs may now be extracted from the enclosure by pulling them towards the front. To make things easier, the control panel might be disconnected first.
  
Control panel removed

Note that one PCB has a small 3-pin connector at one of its corners, that mates with a socket on the DC/DC converter that is located at the bottom end of the enclosure — fully cast in epoxy. Ensure that the PCBs are installed in the correct orientation when reassembling the device later.

Inside the device are three PCBs: two large ones and one smaller daughter card, connected by rainbow-style ribbon cables. At the bottom of the device is the standard TST-7698 processor board, which is built around an Hitachi HD63701 micro­controller with built-in EPROM. This board provides the user interface and the encryption.

A small daughter card is fitted on top of the processor board and is held in place with four screws. It holds the adaptive PSK modems — made by Rockwell — and is connected to the processor board via a wide rainbow ribbon cable.
  
Hitachi microcontroller

Note that the text has been removed from the MODEM chips, which was probably done to hide them from the potential competitors. Judging from the design of the PCB and the choice of components, it is likely that the MODEM board was developed elsewhere, possibly by Rockwell.

The other large board is the TST-9001 vocoder. It has the same size as the processor board and is fitted at the top, connected via a narrow rain­bow ribbon cable. The vocoder is built around a (then) state-of-the-art Texas Instruments TMS­320 Digital Signal Processor (DSP), with the firm­ware held in two 27C128 EPROMs – identified as 'L' and 'H' – as shown in the image on the right.

Depending on the application (HF or VHF/UHF), a different vocoder – and hence a different set of EPROMs – would be installed on the board. When used on HF/SSB, an LPC-10 vocoder was used.
  
Texas Instruments TMS320 DSP with software in EEPROMs

LPC – or Linear Predictive Coding – was used as LPC-10 in early cryptographic telephones, such as the STU-II and the Spendex-40. It allows highly compressed speech to be transmitted over a standard narrowband voice channel, as a 'synthethic' voice with little or no speaker recognition.

When used on wideband VHF or UHF FM radio channels, or via analogue telephone lines (POTS), the TST-9001 board was usually fitted with a RELP vocoder. RELP, or Residual-Exited Linear Predictive encoding, can be seen as an improved version of LPC-10 and offers far better quality speech with the ability to recognise the speaker at the other end, at the cost of a higher bitrate.

The development of a DSP-based vocoder in the mid-1980s is quite remarkable, considering that the technology had only just become available and that TST was a relatively small company.
  
Flex PCB inside control panel

It shows that TST was on par with large players like Philips, Siemens and American crypto manu­facturers. This is also demonstrated by the use of a flex-PCB for connecting the controls and connections to the processor board, as shown in the image above. Flex-PCBs reduce the overall weight of the design, make it less prone to cable faillures, but are generally more expensive to develop and manufacture. In this case, the flex-PCB also holds some of the RFI/EMC filtering.

Control panel removed
Three PCBs and control panel
Flex PCB inside control panel
TST-7698 processor board and modem board (left)
TST-9001 vocoder board
TST-7698 processor board
Moden board
Hitachi microcontroller
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Control panel removed
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Three PCBs and control panel
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Flex PCB inside control panel
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TST-7698 processor board and modem board (left)
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TST-9001 vocoder board
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TST-7698 processor board
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Moden board
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Hitachi microcontroller

Versions
  • TST 7698-2
    Suitable for VHF/UHF, 2400—9600 baud
  • TST 7698-3
    Suitable for VHF/UHF and HF, 2400 baud
  • TST 7698-6
    Same as TST 7698-2, but wit 2dB higher output level
Options
The option number is added as a suffix to the model number, e.g. TST-7698-42.

  • -31
    Built-in battery compartment 1
  • -33
    Adapter for acoustic coupler
  • -42
    Full sealing to MIL-spec 810Cm 512.1
  • -50
    600 Ω radio interface cable set
  • -51
    Telephone coupler and interface cable set
  • -53
    Radio interface cable for PRC-77
  • -55
    Mounting tray for PRC-77
  • -57
    Mounting tray for vehicle installation
  • -87
    Family key programmer with adapter socket
  • -89
    Key fill device TST-0700
  • -92
    VOX (voice operated transmission)
  1. With this option, the case is no longer waterproof.

Accessories
  • TST 0606
    Random generator for creating keys
  • TST 0810
    Key management system
  • TST 0706
    Key reader/loader for key cards TST 0502
  • TST 0708
    Reader/writer for key cards TST 0502
  • TST 0502
    Key cards for transferring cryptographic keys
Applications
  • VHF radio networks (9600 baud, RELP)
  • UHF radio networks (9600 baud, RELP)
  • HF radio networks (2400 baud, LPC-10)
  • Half-duplex, full-duplex (requires two units)
  • Dial-up (analogue) telephone line (2-wire)
  • Leased line operation (4-wire)
Connections
Radio socket   NF10
  1. Power input (+) 10 to 32V DC
  2. Ground
  3. Ground
  4. Audio out
  5. TX key out (+) (via opto-coupler)
  6. Clear (low), cipher (high)
  7. Line (1) 600 Ω, 0dBm
  8. TX key in
  9. Line (2) 600 Ω, 0dBm
  10. Ground
Handset   NF7
The NF7 socket on the control panel of the TST-7698 accepts a standard H-1088/GY military handset of the German Bundeswehr, such as the ones manufactured by Telemit. Note that the wiring for pins (E) and (F) have been added by TST.

  1. Speaker (1)
  2. Speaker (2)
  3. Ground
  4. PTT
  5. Clear/cipher 1
  6. Battery (+) 1
  7. Microphone
  1. TST-proprietary.

Specifications
  • Encryption
    Digital
  • Keys
    1080
  • Period
    1071 years
  • Vocoder
    RELP, LPC-10 1
  • Baudrate
    2400 (HF, VHF, UHF), or 9600 (VHF, UHF, telephone)
  • Audio
    300 - 3000 Hz, S/N 20dB (RELP), or 500 - 2600 Hz, S/N 8dB (LPC)
  • Dynamic range
    40dB
  • Power
    10 to 32V DC (7W)
  1. TST-proprietary implementation of both vocoder types.

Documentation
  1. TST 7698 sales brochure 1
    Date unknown. 6 Pages (German).

  2. TST 7698 handbook 1
    8.20 F. TST, 1988. 38 Pages (English).
  1. Kindly supplied by Jim Meyer [1].

References
  1. Helmut 'Jim' Meyer, HS0ZHK, My way to Ham - Radio and beyond
    Website QRZ.COM. Personal correspondence. Retrieved February 2018.
Further information
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Crypto Museum. Created: Tuesday 27 February 2018. Last changed: Monday, 30 September 2019 - 09:29 CET.
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