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<link rel="stylesheet" type="text/css" href="styles/lame.css" />
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<title>LAME MP3 Encoder :: Constant Bit Rate</title>
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<body>
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Index page
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About LAME
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Intro to encoding
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CBR
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ABR
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VBR
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M/S Stereo
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Usage of LAME
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Version history
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Coders of LAME
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Contact LAME
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LAME links
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LAME - Constant Bit Rate
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Suggested usage:
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lame -b 128 input.wav output.mp3
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CBR encoding is the basic encoding mode of MP3: The bitrate is kept constant
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across the entire file, which means the same number of bits is allocated to
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encode each second of audio, and internally, frames of audio data occur at
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regular, predictable intervals, given a predictable file size for a given
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duration. CBR is therefore the "opposite" of VBR.
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That said, in some formats there may be some variability in the number of bits
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that contain actual audio information from frame to frame. This concept manifests
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in the bit reservoir of MP3s. In a CBR MP3, even though the frames are of
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a fixed size, the audio data is not necessarily distributed consistently between
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them; audio for one frame might use fewer bits than the frame has, so that frame
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adds the spare bits to a reservoir that can supplement the bits allocated
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to the next frame. Thus, the effective bitrate is allowed to vary somewhat in a
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CBR MP3, even though there is a fixed number of frames for the duration of audio.
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For example, for a 256kbps file, the bitrate of a single frame can be up to 320
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kbps, but the frame immediately before and/or after that one would have to use
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fewer bits, whereas in VBR, there would be no such restriction. Consequently, the
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amount of variability across the entire MP3 is not as great as that afforded by
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VBR, but it is not insignificant; a CBR encoder that does not efficiently use
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the reservoir will likely produce a lower quality file than one that does.
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ABR is a more flexible way to encode where filesize is
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important, but still giving some flexibility to choose frame sizes.
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Who should use CBR
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CBR is useful for people who are concerned about maintaining maximum
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compatibility, especially with certain streaming applications and some
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hardware-based decoders that don't reliably support VBR.
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CBR is also useful for people who desire the ability to obtain accurate
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estimates of the bitrate or approximate duration of a file's decoded audio
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without scanning and partially decoding the entire file.
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alt="Get LAME (Lame Aint an MP3 Encoder) at SourceForge.net. Fast, secure and Free Open Source software downloads"
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