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Narrow-band voice coder - this page is a stub

The HY-2 was a fully transistorised voice coder or vocoder, developed in the USA in 1961. It was intended for use in combination with an external key generator, such as the KG-13, to send and receive encrypted voice traffic via telephone or narrowband radio channels. Due to the application of the so-called FLYBALL integrated circuit modules, its weight was reduced to 'just' over 45 kg.

The HY-2 is the successor to the KY-9, which was also fully transistorized, but did not yet feature the service-friendly FLYBALL technology.

The device is based on a 16-channel vocoder and can send its data at 2400 baud, which is slightly more than the KY-9 that could only handle 1650 baud. This probably resulted in a (slight) improvement of the audio voice quality.

During the 1960s, the HY-2 was widely used in the US Government's AUTOSEVOCOM I secure voice network. It was also deployed in Vietnam for use on narrowband VHF and UHF FM radio.

Due to the limited voice quality, that resulted in a 'Donald Duck' style voice, former US President Johnson refused to use the HY-2. And he wasn't the first one: President John F. Kennedy before him disliked its predecessor, the KY-9, for the same reason and had it replaced by the KY-3 on subscriber lines in the area. Nevertheless, the datasheet describes the device as a high quality CIPHONY system, with a unit price of US$ 22,100, which exludes the KY-13 and the MODEM.

Note that the HY-2 is 'just' a vocoder or speech digitizer. In order to use it as a stand-alone voice encryption/decryption device, an external key generator/mixer is needed, such as the large KG-13 that is shown in the image on the right.

The KG-13, codenamed PONTUS, measures 89 x 59 x 48 cm and weights well over 110 kg. It consists of three identical punch-card driven key generators, two synchronizers and ancillaries.

The key generators produce a pseudo random data stream that is mixed with the clear data produced by the HY-2, by means of the binary XOR-operation. This operation is also known as the Vernam Cipher.

 More about the KG-13


The diagram below shows how the HY-2 was used. At the left is the audio input/output, which could either be a handset or an external telephone or audio line. The HY-2 converts the analogue voice into digital information which is then passed on to the KG-13 for encryption. The encrypted data is then passed to an external 2400 baud modem for transmission via radio or telephone.

At the receiving end, the data received by the MODEM is first decrypted in the KG-13 and then passed onto the HY-2, which then reconstructs or synthesizes the human speech, based on the data in the 16 vocoder channels. This results in a typical 'Donald Duck' style voice.

  1. TSEC/HY-2 Datasheet
    CSP 6620A. Department of the Navy, September 1962. Obtained via [2].
  1. Jerry Proc and contributors, HY-2
    Retrieved May 2016.

  2. Nick England, HY-2 datasheet
    Retrieved May 2016. Reproduced here by kind permission.
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Crypto Museum. Created: Thursday 12 May 2016. Last changed: Sunday, 25 February 2018 - 14:54 CET.
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