This paper notes the failure of spectral masking of coding errors by wanted signals when the error is highly cross-correlated with the signal. This can reduce effective masking thresholds by 30 dB. The existence of such cross-correlation is proved for all Shannon efficient audio data compression systems. At low bit rates, these can exhibit up to 4 dB gain modulation. Stereo directional masking effects can also reduce masking thresholds. An improved cross-spectral class of models for masking, taking account of these effects, is discussed.
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