The AC-1 digital audio system exploits psychoacoustic phenomena and employs Adaptive-Delta-Modulation (ADM) to achieve compression ratios of between 2.5 and 3.5:1 over 16-bit PCM. Until recently, the only practical decoder for this system consisted of a bipolar analog IC and numerous external high-precision components. This paper describes a semi-custom ASIC design for an AC-1 decoder, which combines a highly efficient 36 MIPS DSP with an oversampling D/A converter. The specifications far exceed the analog decoder, and the 8,000 gate, l mu CMOS ASIC is fabricated in a standard digital process, allowing inclusion of critical circuitry needed in a digital audio receiver as well. It exhibits a dynamic range (peak signal level to CCIR/ARM weighted idling noise) of 96 dB, yielding subjective 16-bit PCM performance at very low cost.
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