There is a big difference in fidelity between
There is a big difference in fidelity between conventional voice telephony and an acceptable system that can be used for the enjoyment of music, which has led to the development of a number of sophisticated compression schemes for general audio that can utilize such narrow-band channels. They are based upon the concepts of psychoacoustics, that is, the study of the mechanisms used by the human ear for the perception of sound. The problems for the developers of codecs stem from the very different waveforms generated by, for example, the piccolo or the castanets. The quest to squeeze maximum fidelity from a channel started in the analog era before digital processing. Analog compression The compression schemes of analog audio originally were developed to counter the limited dynamic range of magnetic tape recorders. A typical recorder could have a range between peak level and the noise floor of 70 dB. High-quality recording demanded at least 90 dB. Dolby A Dolby A was the first commercially successful noise reduction product used by professional recording studios, dating back to 1965. It established the principal of splitting audio into sub-bands for processing, a system still used by MPEG codecs and by audio preprocessors. Multichannel recording added to the demands from recording studios for noise reduction. In a final mix-down, 40 or so tape tracks could be combined into a stereo signal. The noise from each channel adds, and without Dolby, the result would have been unusable. To get around the noise problem, the dynamic range of the input audio was compressed before recording on tape, then expanded back on replay, a process commonly referred to as companding. To make the process inaudible was not trivial, especially using analog computing elements. Earlier proposals for noise reduction systems had suffered from noise modulation and poor transient response. The Dolby system made use of psychoacoustics to mask the effects of compression. With the low-cost digital processing available now, much more complex algorithms can be used. Dolby B and C This was a low-cost noise reduction system aimed at consumer products like the compact cassette. At the low tape speeds used, the frequency-dependent noise (tape hiss) was a big problem. It could be reduced to acceptable levels Audio compression 103
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