Digital Sound & Music: Concepts, Applications, & Science, Chapter 5, last updated 6/25/2013
Figure 5.47 Bit rate choices in Adobe Audition’s MP3/mp3PRO encoder
The MPEG standard does not prescribe how compression is to be implemented. Instead,
it prescribes the format of the encoded audio signal, which is a bit stream made up of frames
with certain headers and data fields. The MPEG standard suggests two psychoacoustical models
that could accomplish the encoding, the second model, used for MP3, being more complex and
generally resulting in greater compression than the first. In reality, there can be great variability
in the time efficiency and quality of MP3 encoders. The advantage to not dictating details of the
compression algorithm is that implementers can compete with novel implementations and
refinements. If the format of the bit stream is always the same, including what each part of the
bit stream represents, then decoders can be implemented without having to know the details of
the encoder’s implementation.
An MP3 bit stream is divided into frames each of which represents the compression of
1152 samples. The size of the frame containing the compressed data is determined by the bit
rate, a user option that is set prior to compression. As before, let’s assume that a bit rate of 128
kb/s is chosen to compress an audio signal with a sampling rate of 44.1 kHz. This implies that
the number of frames of compressed data that must be produced per second is 44100/1152
38.28. The chosen bit rate of 128 kb/s is equal to 16000 bytes per second. Thus, the number of
bytes allowed for each frame is 16000/38.28 418. You can see that if you vary the sampling
rate or the bit rate, there will be a different number of bytes in each frame. This computation
considers only the data part of each compressed frame. Actually, each frame consists of a 32-
byte header followed by the appropriate number of data bytes, for a total of approximately 450
bytes in this example.
The format of the header is shown in Figure 5.48 and explained in Table 5.4.
Figure 5.48 MP3 frame header
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