MPEG layer II audio coding (MUSICAM)
MPEG layer II audio coding (MUSICAM) The MPEG layer II algorithm is the preferred algorithm for European DTV, and includes a number of simple enhancements of layer I. Layer II was originally adopted as the transmission coding standard for the European digital radio project (digital audio broadcasting or DAB), where it was termed MUSICAM. The full range of bit-rates for each layer is supported, as are all three sampling frequencies, 32, 44.1 and 48 kHz. Note that MPEG decoders are always backward-compatible; for example, a layer II decoder can decode layer I or layer II bitstreams, but cannot decode a layer III encoded stream.
MPEG layer II coding improves compression performance by coding data in larger groups. The Layer II encoder forms frames of 3 12 32 ¼ 1152 samples per audio channel. Layer I codes data in single groups of 12 samples for each sub-band, whereas layer II codes data in three groups of 12 samples for each sub-band. The encoder encodes with a unique scale factor for each group of 12 samples only if necessary to avoid audible distortion. The encoder shares scale factor values between two or all three groups when the values of the scalefactors are sufficiently close, or when the encoder anticipates that temporal noise masking will hide the consequent distortion. The layer II algorithm also improves performance over layer I by representing the bit allocation, the scale factor values and the quantized samples with a more efficient code. Furthermore, layer II coding added 5.1 multi-channel capability. This was done in a scaleable way, so as to be compatible with layer I audio. An improvement to the standard with respect to Pro Logic compatibility led to a second edition of the MPEG layer II standard, accepted in 1997.
MPEG layers I and II contain a number of engineering compromises. The most severe concerns the 32 constant-width sub-bands, which do not accurately reflect the equivalent filters in the human hearing system (the critical bands). Specifically, the bandwidth is too wide for the lower frequencies, so the number of quantizer bits cannot be specifically tuned for the noise sensitivity within each critical band. Furthermore, the filters have insufficient Q, so that signal at a single frequency can affect two adjacent filter bank outputs. Another limitation concerns the time– frequency–time domain transformations achieved with the wave filter. These are not transparent so, even without quantization, the inverse transformation would not perfectly recover the original input signal.
MPEG layer III The layer III algorithm is a much more refined approach. Layer III is finding its application on the Internet, where the ability to compress audio files by a large factor is important in download times. It is not used in digital television applications.
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