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00004 <title>Vorbisfile - Sample Crosslapping</title>
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00008 <body bgcolor=white text=black link="#5555ff" alink="#5555ff" vlink="#5555ff">
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00011 <td><p class=tiny>Vorbisfile documentation</p></td>
00012 <td align=right><p class=tiny>vorbisfile version 1.68 - 20030307</p></td>
00013 </tr>
00014 </table>
00015
00016 <h1>What is Crosslapping?</h1>
00017
00018 <p>Crosslapping blends two samples together using a window function,
00019 such that any sudden discontinuities between the samples that may
00020 cause clicks or thumps are eliminated or blended away. The technique
00021 is nearly identical to how Vorbis internally splices together frames
00022 of audio data during normal decode. API functions are provided to <a
00023 href="ov_crosslap.html">crosslap transitions between seperate
00024 streams</a>, or to crosslap when <a href="seeking.html">seeking within
00025 a single stream</a>.
00026
00027 <h1>Why Crosslap?</h1>
00028 <h2>The source of boundary clicks</h2>
00029
00030 <p>Vorbis is a lossy compression format such that any compressed
00031 signal is at best a close approximation of the original. The
00032 approximation may be very good (ie, indistingushable to the human
00033 ear), but it is an approximation nonetheless. Even if a sample or set
00034 of samples is contructed carefully such that transitions from one to
00035 another match perfectly in the original, the compression process
00036 introduces minute amplitude and phase errors. It's an unavoidable
00037 result of such high compression rates.
00038
00039 <p>If an application transitions instantly from one sample to another,
00040 any tiny discrepancy introduced in the lossy compression process
00041 becomes audible as a stairstep discontinuity. Even if the discrepancy
00042 in a normal lapped frame is only .1dB (usually far below the
00043 threshhold of perception), that's a sudden cliff of 380 steps in a 16
00044 bit sample (when there's a boundary with no lapping).
00045
00046 <h2>I thought Vorbis was gapless</h2>
00047
00048 <p>It is. Vorbis introduces no extra samples at the beginning or end
00049 of a stream, nor does it remove any samples. Gapless encoding
00050 eliminates 99% of the click, pop or outright blown speaker that would
00051 occur if boundaries had gaps or made no effort to align
00052 transitions. However, gapless encoding is not enough to entirely
00053 eliminate stairstep discontinuities all the time for exactly the
00054 reasons described above.
00055
00056 <p>Frame lapping, like Vorbis performs internally during continuous
00057 playback, is necessary to eliminate that last epislon of trouble.
00058
00059 <h1>Easiest Crosslap</h1>
00060
00061 The easiest way to perform crosslapping in Vorbis is to use the
00062 lapping functions with no other extra effort. These functions behave
00063 identically to when lapping isn't used except to provide
00064 at-least-very-good lapping results. Crosslapping will not introduce
00065 any samples into or remove any samples from the decoded audio; the
00066 only difference is that the transition is lapped. Lapping occurs from
00067 the current PCM position (either in the old stream, or at the position
00068 prior to calling a lapping seek) forward into the next
00069 half-short-block of audio data to be read from the new stream or
00070 position.
00071
00072 <p>Ideally, vorbisfile internally reads an extra frame of audio from
00073 the old stream/position to perform lapping into the new
00074 stream/position. However, automagic crosslapping works properly even
00075 if the old stream/position is at EOF. In this case, the synthetic
00076 post-extrapolation generated by the encoder to pad out the last block
00077 with appropriate data (and avoid encoding a stairstep, which is
00078 inefficient) is used for crosslapping purposes. Although this is
00079 synthetic data, the result is still usually completely unnoticable
00080 even in careful listening (and always preferable to a click or pop).
00081
00082 <p>Vorbisfile will lap between streams of differing numbers of
00083 channels. Any extra channels from the old stream are ignored; playback
00084 of these channels simply ends. Extra channels in the new stream are
00085 lapped from silence. Vorbisfile will also lap between streams links
00086 of differing sample rates. In this case, the sample rates are ignored
00087 (no implicit resampling is done to match playback). It is up to the
00088 application developer to decide if this behavior makes any sense in a
00089 given context; in practical use, these default behaviors perform
00090 sensibly.
00091
00092 <h1>Best Crosslap</h1>
00093
00094 <p>To acheive the best possible crosslapping results, avoid the case
00095 where synthetic extrapolation data is used for crosslapping. That is,
00096 design loops and samples such that a little bit of data is left over
00097 in sample A when seeking to sample B. Normally, the end of sample A
00098 and the beginning of B would overlap exactly; this allows
00099 crosslapping to perform exactly as it would within vorbis when
00100 stitching audio frames together into continuous decoded audio.
00101
00102 <p>The optimal amount of overlap is half a short-block, and this
00103 varies by compression mode. Each encoder will vary in exact block
00104 size selection; for vorbis 1.0, for -q0 through -q10 and 44kHz or
00105 greater, a half-short block is 64 samples.
00106
00107 <br><br>
00108 <hr noshade>
00109 <table border=0 width=100%>
00110 <tr valign=top>
00111 <td><p class=tiny>copyright © 2003 Xiph.org</p></td>
00112 <td align=right><p class=tiny><a href="http://www.xiph.org/ogg/vorbis/">Ogg Vorbis</a></p></td>
00113 </tr><tr>
00114 <td><p class=tiny>Vorbisfile documentation</p></td>
00115 <td align=right><p class=tiny>vorbisfile version 1.68 - 20030307</p></td>
00116 </tr>
00117 </table>
00118
00119 </body>
00120
00121 </html>
