Secure voice

The implementation of voice encryption dates back to World War II when secure communication was paramount to the US armed forces.

In the time of SIGSALY, the transistor had not been developed and the digital sampling was done by circuits using the model 2051 Thyratron vacuum tube.

[2] In this system permutation matrices were used to scramble coded representations (such as pulse-code modulation and variants) of the speech data.

It is difficult in practice to send the encrypted signal over the same voiceband communication circuits used to transmit unencrypted voice, e.g. analog telephone lines or mobile radios, due to bandwidth expansion.

The STE system, by contrast, requires wide bandwidth ISDN lines for its normal mode of operation.

Due to its lower complexity[citation needed] than Waveform Interpolative (WI) coder, the MELP vocoder won the DoD competition and was selected for MIL-STD-3005.

This fairly significant development was aimed to create a new coder at half the rate and have it interoperable with the old MELP standard.

MELPe provides much better quality than all older military standards, especially in noisy environments such as battlefield and vehicles and aircraft.

The NATO competition concluded that MELPe substantially improved performance (in terms of speech quality, intelligibility, and noise immunity), while reducing throughput requirements.

The low-cost FLEXI-232 Data Terminal Equipment (DTE) made by Compandent, which are based on the MELCODER golden reference, are very popular and widely used for evaluating and testing MELPe in real-time, various channels & networks, and field conditions.

The NATO competition concluded that MELPe substantially improved performance (in terms of speech quality, intelligibility, and noise immunity), while reducing throughput requirements.