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As part of a Defense Advanced Research Projects Agency program investigating voice transmission over a packet-switched computer network, a real-time variable frame rate LPC vocoder has been implemented at USC-ISI. The vocoder is implemented on a Floating Point Systems AP-120B array processor. A likelihood ratio threshold is used to control when frames of data are transmitted. The average data rate and the speech quality of the vocoder depend on the threshold, but acceptable quality is possible with data rates less than 2,000 bits per second for continuous speech.