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← TST Voice TST-4045 →
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).
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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.
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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.
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.
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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:
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The TST-7698 requires the following key types:
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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
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- 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)
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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
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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
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The option number is added as a suffix to the model number,
e.g. TST-7698-42.
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-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)
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With this option, the case is no longer waterproof.
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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
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- 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)
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- Power input (+) 10 to 32V DC
- Ground
- Ground
- Audio out
- TX key out (+) (via opto-coupler)
- Clear (low), cipher (high)
- Line (1) 600 Ω, 0dBm
- TX key in
- Line (2) 600 Ω, 0dBm
- Ground
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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.
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- Speaker (1)
- Speaker (2)
- Ground
- PTT
- Clear/cipher 1
- Battery (+) 1
- Microphone
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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)
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TST-proprietary implementation of both vocoder types.
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Kindly supplied by Jim Meyer [1].
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© Crypto Museum. Created: Tuesday 27 February 2018. Last changed: Monday, 30 September 2019 - 08:29 CET.
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