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An LSI CODEC, an LSI multiplexed digital filter, and an LSI compandor are developed for telephone PCM AD/DA conversion. The total chip count is 1.56 chips per channel. The single-chip interpolative CODEC operates at a 32 kHz sampling rate with a 12-bit linear code input/output, and is implemented in a phosphorus buried emitter IIL process without the need for component value trimming. The digital filter is four-channel multiplexed for both transmitting and receiving directions. It is compactly fabricated with a combinatorial configuration in a 5 /spl mu/m NMOS process. A new word length extension technique was devised to realize an 87 dB dynamic range with the digital filter. The compandor, also fabricated in 5 /spl mu/m NMOS, is 16-channel multiplexed for both directions. All measured characteristics meet CCITT recommendations with sufficient margins. They demonstrate the inherent stability and noise immunity that can be achieved with the interpolation technique of the CODEC and the digital signal processing of the digital filter.