Perceptually weighted distortion measures and the tandem connection of speech codecs

Tandem connections of voice codecs can occur today in mobile- to-mobile calls and for certain VoIP connections. While post-filtering in tandem encodings is well-understood, the effects of the perceptual weighting filter in CELP codecs in tandem encodings has not been investigated. We study the impact of perceptual weighting filters on tandem coding using the rate distortion theory of discrete-time autoregressive (AR) sources with a frequency weighted error criterion and by examining tandem connections involving the AMR-NB codec. We show that for the usual method of calculating the perceptual weighting based upon the codec input, the perceptual weighting has a cumulative effect that is more pronounced at lower bit rates.