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A single-channel PCM codec is described which is being manufactured for use in the new British Post Office (BPO) range of digital PABX's. The codec works on the principle of converting to and from PCM using an intermediate delta-sigma modulation code format. This technique allows relatively simple analog circuitry to be used in conjunction with a digital LSI chip to perform the conversion between the intermediate code and PCM. The codec meets all the relevant CCITT Recommendations with significant operating margins.