| 摘要 |
Accumulation of distortion possibly resulting from multiple digital code conversions is prevented by including in an ADPCM-to-PCM decoder (100, FIG. 1 or 600, FIG. 6) an arrangement (119, FIG. 1 or 601, FIG. 6) for making a trial transmission of the resulting PCM sample ( PSI (k)) to obtain (via 209, FIG. 2) a composite noise value ( alpha (k)) for the PCM sample ( PSI (k)). The composite noise value includes noise components resulting from both quantizing the code word and truncating digital signals used in the decoder and subsequent coder. The composite noise value is evaluated (via 211) to determine whether adjustment of the PCM sample value ( PSI (k)) is necessary. In one embodiment (FIG. 1, 100), a reconstructed linear version (Yr(k)) of the PCM sample is adjusted (via 216, 218, 220 or 221) when the composite noise value ( alpha (k)) is not within the ADPCM quantizer interval for the ADPCM sample (In(k)) being converted to PCM. The amount of adjustment is dependent on the PCM quantizer interval (214) that the reconstructed signal (Yr(k)) falls in and whether the ADPCM sample is in saturation, i.e., the most positive (215) or negative (217) ADPCM quantizer interval, or not in saturation (219). In another embodiment (FIG. 6, 600), the decoder (600) PCM output sample ( PSI (k)) is adjusted, if necessary, up or down one PCM quantizer interval (via 802 or 803) when the composite noise value ( alpha (k)) is not within the quantizer interval decision levels of the ADPCM sample received by the decoder 600. |
