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Telsy TDS-2004
Telephone voice encryptor

The TDS-2004 was a speech encryption device for use on analogue (PSTN) telephone networks, introduced by Telsy in Turin (Italy) around 1981. It was part of the TDS-2000 family and is the table-top version of the TDS-2004M. It is also compatible with the TDS-2003 briefcase unit. The TDS-2004 was also sold as an OEM product by Gretag in Switzerland as the Gretacoder 103.

The TDS-2004 was the office version of the TDS-2004M mobile encryption device. It was based on the same electronics and was housed in a dark green metal case. The image on the right shows a typical TDS-2004, with the D-2000 companion telephone placed on top.

The TDS-2004 has two modes of encryption: time division or 2-dimensional coding (time and frequency division), and the cryptographic key determines which mode is used. A total of 9 keys can be preset and the key selector at the front is used to select the key compartment.
  
Telsy TDS-2004

The TDS-2004 consists of two units: the main TDS-2004 unit and a separate modified D-2000 telephone set. The two are connected via a 1.5 meter long cable that is fixed to the telephone set. The other end has a DB25 male plug that mates with a socket at the rear of the TDS-2004.

The image on the right shows the D-2000 telephone set, which is in fact a modified Krone FeTAp 752. This telephone was one of the commonly used models in Germany during the 1980s. Although the phone has push-buttons for dialling a number, it only features pulse-dialling, as the majority of exhanges in the 1980s did not support tone dialling (DTMF).

Extra buttons are added to this telephone to allow switching between clear (CL), crypto (CR) and private mode (P). The Push-to-Talk switch (PTT, left) is only used in half-duplex mode.
  
Telsy TDS-2004, modified phone

Once the crypto keys are loaded into the main unit, the user can control all features from the telephone set. A call is initiated en clair, which is indicated by a RED LED in the CL-button. Once the call has been established, one of the parties presses the CR-button in order to go secure. If it fails, an alarm will be sounded. Otherwise the CR-LED will be lit. The TDS-2000 range appeared to be very successful and thousands of units were sold to the Army, the Police and even to the corporate sector. As a result, the TDS-2004 remained in production well into the 1990s.

Telsy TDS-2004 Telsy TDS-2004, with the handset off-hook TDS-2004 front panel TDS-2004 rear panel TDS-2004 main unit Telsy TDS-2004, modified phone Telsy TDS-2004, modified phone with the handset off-hook Selecting the desired key compartment

Setting the keys
Setting the key can be done on the front panel. When setting the key, the mode selector (left) has to be set to SET KEY (INS COD). Then set the key selector (centre) to the number of the required key compartment and toggle the right knob to CLEAR (CHIARO) in order to purge the current key.

Next, enter between 1 and 8 digits, by rotating the key selector to the required number and toggling the right knob to CRYPTO in order to enter the digit. When doing this, you need to remember which digits you have already entered. The TDS-2004 does not have the visual aid of a display, like the TDS-2003. Finally, set the mode selector to SPX (simplex) or DPX (duplex).

Connections
All connections between the D-2000 telephone and the TDS-2004 main unit, are via a single DB25 female socket at the rear of the TDS-2004. The D-2000 has a fixed 1.5 meter cable with a DB25 male plug at the end. The pin-out of the DB25 socket is given below:


Some of these lines are optional, such as the remote key-selection (BCD-switch) that is shown at the bottom. It is mounted at the top of the D-2000. The PTT-switch could also be mounted in the handset. The off-hook switch (pin 22) is only needed when automatic line-sensing is disabled.

Interior
The TDS-2004 is housed in a sturdy metal case with a die-cast aluminium frame holding the main circuit boards. The hull of the case can easily be removed by releasing 4 large bolts at the bottom. The frame is attached to the front panel that also holds a small PCB with the controls. The unit is extremely service friendly and can be repaired within minutes by swapping boards.

The TDS-2004 contains two main PCBs mounted in the frame, and a removable power supply unit (PSU). The image on the right shows the main boards (right) and the separated PSU (left). The PSU also holds the DB25 socket for the phone.

The two main boards can be retracted from the frame towards the rear. The upper board holds the analogue circuitry, whilst the lower board contains the digital circuits. These two PCBs are identical to the boards used in the TDS-2004M and the TDS-2003. The only difference between the three models is in fact the interface.
  
PSU separated from the main unit

The digital board contains a Z-80 microprocessor plus a number of custom chips, manufactured American Microsysstems Inc. (AMI) in their production facility in Austria. The analogue board also contains some custom-designed active low-pass filters, which are implemented as thick-film circuits in the black/white chips. All custom chips have numbers starting with 'TY'. At the heart of the analogue board are two FX-309 Delta Modulators by CML (UK) [3]. For a more detailed description of the two boards, please refer to the description of the interior of the TDS-2004M.

TDS-2004 main unit TDS-2004 interior PSU separated from the main unit PSU separated from the main unit PSU Analogue and digital boards Analogue board Digital board

References
  1. Telsy S.p.A., TDS 2004 - THANKS !
    Device featured on this page kindly donated by Telsy. April 2012.

  2. Telsy S.p.A., TDS 2004 Instruction Manual
    June 1981.

  3. Consumer Microcircuits Ltd., FX-309 Datasheet
    Continuous Variable Slope Delta Modulator (CVSD).
    Date unknown. Retrieved February 2012.
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