122 lines
5.2 KiB
HTML
122 lines
5.2 KiB
HTML
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<html>
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<head>
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<title>Vorbisfile - Sample Crosslapping</title>
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<link rel=stylesheet href="style.css" type="text/css">
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</head>
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<body bgcolor=white text=black link="#5555ff" alink="#5555ff" vlink="#5555ff">
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<table border=0 width=100%>
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<tr>
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<td><p class=tiny>Vorbisfile documentation</p></td>
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<td align=right><p class=tiny>vorbisfile version 1.3.2 - 20101101</p></td>
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</tr>
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</table>
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<h1>What is Crosslapping?</h1>
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<p>Crosslapping blends two samples together using a window function,
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such that any sudden discontinuities between the samples that may
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cause clicks or thumps are eliminated or blended away. The technique
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is nearly identical to how Vorbis internally splices together frames
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of audio data during normal decode. API functions are provided to <a
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href="ov_crosslap.html">crosslap transitions between seperate
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streams</a>, or to crosslap when <a href="seeking.html">seeking within
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a single stream</a>.
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<h1>Why Crosslap?</h1>
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<h2>The source of boundary clicks</h2>
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<p>Vorbis is a lossy compression format such that any compressed
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signal is at best a close approximation of the original. The
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approximation may be very good (ie, indistingushable to the human
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ear), but it is an approximation nonetheless. Even if a sample or set
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of samples is contructed carefully such that transitions from one to
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another match perfectly in the original, the compression process
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introduces minute amplitude and phase errors. It's an unavoidable
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result of such high compression rates.
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<p>If an application transitions instantly from one sample to another,
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any tiny discrepancy introduced in the lossy compression process
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becomes audible as a stairstep discontinuity. Even if the discrepancy
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in a normal lapped frame is only .1dB (usually far below the
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threshhold of perception), that's a sudden cliff of 380 steps in a 16
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bit sample (when there's a boundary with no lapping).
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<h2>I thought Vorbis was gapless</h2>
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<p>It is. Vorbis introduces no extra samples at the beginning or end
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of a stream, nor does it remove any samples. Gapless encoding
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eliminates 99% of the click, pop or outright blown speaker that would
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occur if boundaries had gaps or made no effort to align
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transitions. However, gapless encoding is not enough to entirely
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eliminate stairstep discontinuities all the time for exactly the
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reasons described above.
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<p>Frame lapping, like Vorbis performs internally during continuous
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playback, is necessary to eliminate that last epsilon of trouble.
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<h1>Easiest Crosslap</h1>
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The easiest way to perform crosslapping in Vorbis is to use the
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lapping functions with no other extra effort. These functions behave
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identically to when lapping isn't used except to provide
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at-least-very-good lapping results. Crosslapping will not introduce
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any samples into or remove any samples from the decoded audio; the
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only difference is that the transition is lapped. Lapping occurs from
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the current PCM position (either in the old stream, or at the position
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prior to calling a lapping seek) forward into the next
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half-short-block of audio data to be read from the new stream or
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position.
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<p>Ideally, vorbisfile internally reads an extra frame of audio from
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the old stream/position to perform lapping into the new
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stream/position. However, automagic crosslapping works properly even
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if the old stream/position is at EOF. In this case, the synthetic
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post-extrapolation generated by the encoder to pad out the last block
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with appropriate data (and avoid encoding a stairstep, which is
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inefficient) is used for crosslapping purposes. Although this is
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synthetic data, the result is still usually completely unnoticable
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even in careful listening (and always preferable to a click or pop).
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<p>Vorbisfile will lap between streams of differing numbers of
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channels. Any extra channels from the old stream are ignored; playback
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of these channels simply ends. Extra channels in the new stream are
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lapped from silence. Vorbisfile will also lap between streams links
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of differing sample rates. In this case, the sample rates are ignored
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(no implicit resampling is done to match playback). It is up to the
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application developer to decide if this behavior makes any sense in a
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given context; in practical use, these default behaviors perform
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sensibly.
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<h1>Best Crosslap</h1>
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<p>To acheive the best possible crosslapping results, avoid the case
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where synthetic extrapolation data is used for crosslapping. That is,
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design loops and samples such that a little bit of data is left over
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in sample A when seeking to sample B. Normally, the end of sample A
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and the beginning of B would overlap exactly; this allows
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crosslapping to perform exactly as it would within vorbis when
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stitching audio frames together into continuous decoded audio.
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<p>The optimal amount of overlap is half a short-block, and this
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varies by compression mode. Each encoder will vary in exact block
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size selection; for vorbis 1.0, for -q0 through -q10 and 44kHz or
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greater, a half-short block is 64 samples.
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<br><br>
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<hr noshade>
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<table border=0 width=100%>
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<tr valign=top>
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<td><p class=tiny>copyright © 2000-2010 Xiph.Org</p></td>
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<td align=right><p class=tiny><a href="http://www.xiph.org/ogg/vorbis/">Ogg Vorbis</a></p></td>
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</tr><tr>
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<td><p class=tiny>Vorbisfile documentation</p></td>
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<td align=right><p class=tiny>vorbisfile version 1.3.2 - 20101101</p></td>
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</tr>
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</table>
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</body>
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</html>
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