A more sophisticated MP3 encoder can produce variable bitrate audio. MPEG audio may use bitrate switching on a per-frame basis, but only layer III decoders must support it. VBR is used when the goal is to achieve a fixed level of quality. The final file size of a VBR encoding is less predictable than with constant bitrate. Average bitrate is VBR implemented as a compromise between the two: the bitrate is allowed to vary for more consistent quality, but is controlled to remain near an average value chosen by the user, for predictable file sizes. Although an MP3 decoder must support VBR to be standards compliant, historically some decoders have bugs with VBR decoding, particularly before VBR encoders became widespread. COUNT 5
QSN1 What kind of audio can a sophisticated MP3 encoder produce? ANS1 variable bitrate audio QSN2 Which type of decoder must support bitrate switching on a per-frame basis? ANS2 layer III QSN3 What do encoders use when the goal is to achieve a fixed level of quality? ANS3 VBR QSN4 Using VBR instead of a constant bit rate encoding makes which part of encoding less predictable? ANS4 final file size QSN5 An average bit rate is used when who choses an average value for the encoder to use? ANS5 the user