What is Vocoder ?

A vocoder (short for voice encoder) is an analysis/synthesis system, used to reproduce human speech. The vocoder examines speech by measuring how its spectral characteristics change over time. This results in a series of signals representing these modified frequencies at any particular time as the user speaks. In simple terms, the signal is split into a number of frequency bands (the larger this number, the more accurate the analysis) and the level of signal present at each frequency band gives the instantaneous representation of the spectral energy content. Thus, the vocoder dramatically reduces the amount of information needed to store speech, from a complete recording to a series of numbers. To recreate speech, the vocoder simply reverses the process, processing a broadband noise source by passing it through a stage that filters the frequency content based on the originally recorded series of numbers.

The vocoder was originally developed as a speech coder for telecommunications applications in the 1930s, the idea being to code speech for transmission. Transmitting the parameters of a speech model instead of a digitized representation of the speech waveform saves bandwidth in the communication channel; the parameters of the model change relatively slowly, compared to the changes in the speech waveform that they describe. Its primary use in this fashion is for secure radio communication, where voice has to be encrypted and then transmitted. The advantage of this method of "encryption" is that no 'signal' is sent, but rather envelopes of the bandpass filters. The receiving unit needs to be set up in the same channel configuration to re-synthesize a version of the original signal spectrum. The vocoder as both hardware and software has also been used extensively as an electronic musical instrument.

Vocoder applications:

·  Terminal equipment for Digital Mobile Radio (DMR) based systems.

·  Digital Trunking

·  DMR TDMA

·  Digital Voice Scrambling and Encryption

·  Digital WLL

·  Voice Storage and Playback Systems

·  Messaging Systems

·  VoIP Systems

·  Voice Pagers

·  Regenerative Digital Voice Repeaters