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Bit rate
The bit rate is variable for MP3 files. The general rule is that more
information is included from the original sound file when a higher bit
rate is used, and thus the higher the quality during playback. In the
early days of MP3 encoding, a fixed bit rate was used for the entire
file.
Bit rates available in MPEG-1 Layer 3 are 32, 40, 48, 56, 64, 80, 96,
112, 128, 160, 192, 224, 256 and 320 kbit/s, and the available sample
frequencies are 32, 44.1 and 48 kHz. 44.1 kHz is almost always used
(coincides with the sampling rate of compact discs), and 128 kbit/s
has become the de facto "good enough" standard, although 192 kbit/s is
becoming increasingly popular over peer-to-peer file sharing networks.
MPEG-2 and [the non-official] MPEG-2.5 includes some additional bit
rates: 8, 16, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160 kbit/s.
Variable bit rates (VBR) are also possible. Audio in MP3 files is
divided into frames (which have their own bit rate), so it is possible
to change the bit rate dynamically as the file is encoded (although
not originally implemented, VBR is in extensive use today). This
technique makes it possible to use more bits for parts of the sound
with higher dynamics (more sound movement) and fewer bits for parts
with lower dynamics, further increasing quality and decreasing storage
space. This method compares to a sound activated tape recorder that
reduces tape consumption by not recording silence. Some encoders
utilize this technique to a great extent.
Non-standard bitrates up to 640 kbit/s can be achieved with the LAME
encoder and the --freeformat option, however few MP3 players can play
those files.
Design limitations of MP3
There are several limitations inherent to the MP3 format that cannot
be overcome by using a better encoder.
Newer audio compression formats such as Vorbis and AAC no longer have
these limitations.
In technical terms, MP3 is limited in the following ways:
Bitrate is limited to a maximum of 320 kbit/s
Time resolution can be too low for highly transient signals
No scale factor band for frequencies above 15.5/15.8 kHz
Joint stereo is done on a frame-to-frame basis
Encoder/decoder overall delay is not defined, which means lack of
official provision for gapless playback. However, some encoders such
as LAME can attach additional metadata that will allow players that
are aware of it to deliver gapless playback.
Nevertheless, a well-tuned MP3 encoder can perform competitively even
with these restrictions.
Encoding of MP3 audio
The MPEG-1 standard does not include a precise specification for an
MP3 encoder. The decoding algorithm and file format, as a contrast,
are well defined. Implementers of the standard were supposed to devise
their own algorithms suitable for removing parts of the information in
the raw audio (or rather its MDCT representation in the frequency
domain). During encoding 576 time domain samples are taken and are
transformed to 576 frequency domain samples. If there is a transient
192 samples are taken instead of 576. This is done to limit the
temporal spread of quantization noise accompanying the transient.
This is the domain of psychoacoustics: the study of subjective human
perception of sounds.
As a result, there are many different MP3 encoders available, each
producing files of differing quality. Comparisons are widely
available, so it is easy for a prospective user of an encoder to
research the best choice. It must be kept in mind that an encoder that
is proficient at encoding at higher bitrates (such as LAME, which is
in widespread use for encoding at higher bitrates) is not necessarily
as good at other, lower bitrates.
Decoding of MP3 audio
Decoding, on the other hand, is carefully defined in the standard.
Most decoders are "bitstream compliant", meaning that the uncompressed
output they produce from a given MP3 file will be the same (within a
specified degree of rounding tolerance) as the output specified
mathematically in the ISO/IEC standard document. The MP3 file has a
standard format which is a frame consisting of 384, 576, or 1152
samples (depends on MPEG version and layer) and all the frames have
associated header information (32 bits) and side information (9, 17,
or 32 bytes, depending on MPEG version and stereo/mono). The header
and side information help the decoder to decode the associated huffman
encoded data correctly.
