We describe digital encoding and automatic silenceediting techniques for PCM and ADPCM voice storage in the MC68000 microprocessor. We give program details for encoding, silence elimination, storage of active speech bursts and reconstitution of the signal upon read-back. Coding rates in the range 64 down to 16 kbits/s are implemented. We find that silent intervals corresponding, typically, to 20-40 percent of the duration of continuous input speech can be usefully eliminated. Available storage is thereby used more efficiently. The eliminated intervals are automatically reinserted upon read-back. We further demonstrate automatic scaling of the silent intervals in the reconstituted output. We confirm that a variation of ± 50 percent in the reinserted silent intervals is typically acceptable. This latitude can significantly alleviate buffer requirements in a packet transmission system for voice.

In this paper we discuss the implementation of sub-band coding (SBC) on a programmable digital signal processing (DSP) integrated circuit developed at Bell Laboratories. A two-band SBC has been developed for use in 7 kHz bandwidth transmission (AM broadcast quality) at a bit-rate of 56 kb/s. A four-band SBC has been developed for use in speech coding (3.2 kHz bandwidth) at bit rates ranging from 16 kb/s to 24 kb/s. We first review the capabilities of the DSP integrated circuit. We then discuss the hardware and software development environment used for programming real-time SBC on the DSP. Next we discuss block diagrams of the SBC algorithms that were implemented. Finally, we discuss aspects of how these algorithms were programmed in real-time on the DSP and features of the DSP that are utilized.