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A lossy audio compression algorithm is transparent if the original and decoded signal are indistinguishable to the human ear. The bit rate depends on the codec and the audio material. Some codecs (e.g. AAC, MP3) are optimized for transparent coding at a low data rate, others (e.g. VQF, MP3Pro, AAC+SBR, WMA) for distinguisable but pleasant reproduction at lower bit rates.
Typical data rates in kbps for different codecs are:
MP1 288-320 MP2 224-256 MP3 160-224 MPEG-2 AAC 128-160 MPEG-4 AAC 112-144 MPEGplus 160-200