A microprocessor operated low bit-rate Digi-Tized speech transmission over VHF land mobile radio

Abstract EN

The ever increasing demand tor:digital transmission channels in radio frequency band presents a potentially serious problem of spectral congestion due it), the required increased bandwidth.

The philosophy.piit forward in this investigation is c-oncerned with ;;the:voice store-and forward methodologys which:is trading delay with a redaction in bandwidth for a specific land mobile radio application.

The system developed employs Adaptive Differential Pulse Code Mod-; ulation (ADPCM).

The modulated speech is stored at the transmitter, and then broadcast at a lower bit rate..

At the receiver, the bit stream is again temporarily stored:Until the.

message is complete ed.

whereupon it is then decoded at the original: bit rate.

The transmission rate can fee chosen to Suit the available bandwidth arid the acceptable transmission delay.

A compactly packaged hardware based on high densitymemoriesand:CMOS Microprocessor is:developed and a structured software is introduced for data packaging and processing.

Finally, system performance has been tested for various bit rates on a simulated radio channel using VHF land mobile radio equipment/ Test results demonstrated that by adopting efficient digitizing techniques and speech coding -algorithms, relatively: low.

transmission rates can be achieved using waveform; codings while portraying a robust and intelligible speech within a narrow bandwidth channel, less than halt the value required by an equivalent analog bandwidth channel;