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<h2 class="hilight">LAME - Constant Bit Rate</h2>
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<p>Suggested usage:</p>
<blockquote class="code">lame -b 128 input.wav output.mp3</blockquote>
<p>
CBR encoding is the basic encoding mode of MP3: The bitrate is kept constant
across the entire file, which means the same number of bits is allocated to
encode each second of audio, and internally, frames of audio data occur at
regular, predictable intervals, given a predictable file size for a given
duration. CBR is therefore the "opposite" of <a href="vbr.html">VBR</a>.
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<p>
That said, in some formats there may be some variability in the number of bits
that contain actual audio information from frame to frame. This concept manifests
in the <i>bit reservoir</i> of MP3s. In a CBR MP3, even though the frames are of
a fixed size, the audio data is not necessarily distributed consistently between
them; audio for one frame might use fewer bits than the frame has, so that frame
adds the spare bits to a <i>reservoir</i> that can supplement the bits allocated
to the next frame. Thus, the effective bitrate is allowed to vary somewhat in a
CBR MP3, even though there is a fixed number of frames for the duration of audio.
For example, for a 256kbps file, the bitrate of a single frame can be up to 320
kbps, but the frame immediately before and/or after that one would have to use
fewer bits, whereas in VBR, there would be no such restriction. Consequently, the
amount of variability across the entire MP3 is not as great as that afforded by
VBR, but it is not insignificant; a CBR encoder that does not efficiently use
the reservoir will likely produce a lower quality file than one that does.
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<p>
<a href="abr.html">ABR</a> is a more flexible way to encode where filesize is
important, but still giving some flexibility to choose frame sizes.
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<h3>Who should use CBR</h3>
<p>
CBR is useful for people who are concerned about maintaining maximum
compatibility, especially with certain streaming applications and some
hardware-based decoders that don't reliably support VBR.
</p>
<p>
CBR is also useful for people who desire the ability to obtain accurate
estimates of the bitrate or approximate duration of a file's decoded audio
without scanning and partially decoding the entire file.
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