A low-delay audio codec using the ADPCM structure (ADPCM = adaptive differential pulse code modulation) in subbands is presented. With the use of 8 subbands a coarse spectral shaping of the coding noise is obtained and the signal delay is approximately 3 ms. The targeted bit rate is in the range from 128 to 176 kbit/s per channel for near transparent audio quality. The codec uses a cosine-modulated filterbank and backward adaptive calculation of the prediction coefficients and quantization scaling factors. The computations are optimized for a real-time implementation on a fixed-point DSP with an almost constant workload over time. A comparison with the Philips Subband Coder (SBC) and the Fraunhofer Ultra Low Delay Codec (ULD) is performed.
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