Tandem Transcoding Without Distortion Accumulation

A simple and universal PCM/ADPCM transcoding algorithm, which does not accumulate distortion in multiple code conversions between PCM and ADPCM, has been derived by thoroughly investigating the PCM-ADPCM-PCM code conversion process. Conditions to avoid distortion in code conversions are clarified by newly introduced mathematical expressions for ADPCM and PCM quantization processes. These expressions can be utilized to simplify a code conversion process without losing generality. Distortion reduction on a code conversion can be realized by adding some functions to the ADPCM decoder output portion. This distortion reduction technique is shown to assure no distortion accumulation in tandem code conversions. In contrast to other reported techniques for tandem conversions, the proposed method is free from applied ADPCM structures, as well as being the simplest among them. As the mathematical treatment is so general, this technique can be applied to code conversions other than PCM/ADPCM conversions.