This paper describes an experimental set up of a time division multiplexing (TDM) scheme developed for a 16-channel vocoder under development in the Audio and Acoustics Division of Central Electronics Engineering Research Institute, Pilani. A channel vocoder is a bandwidth compression system used for transmission of voice signals. In a channel vocoder the speech signal is continuously analysed at the transmitting end and is synthesized at the receiving end. The analyser consists of 16-spectrum channels, voicing-un-voicing indicator and a pitch extracting circuit. The spectrum outputs of the spectrum channels and pitch circuit are slowly varying d.c. signals. In order to send all the information on a single line, the signals from the 17 channels are time division multiplexed. After multiplexing the 16-spectrum channels are encoded by a 3-bit PCM encoder while the 17th channel (i.e. pitch channel) is encoded by a 6-bit PCM encoder. The encoded signals are then transmitted. On the receiving side a synchronization scheme brings the transmitter and the receiver operations in synchronism. Then a 3-bit decoder decodes the 16-spectrum channel signals while a 6-bit decoder decodes the pitch signal. These decoded signals which are in the form of pulse amplitude modulated form are demultiplexed and then passed through individual channel filters to retrieve the analogue signals. The synthesizer at the receiver terminal of the vocoder utilizes the information from the various channels to reconstruct the original voice signal.
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