diff options
Diffstat (limited to 'contrib/vorbis/doc/vorbisfile/crosslap.html')
| -rw-r--r-- | contrib/vorbis/doc/vorbisfile/crosslap.html | 121 |
1 files changed, 0 insertions, 121 deletions
diff --git a/contrib/vorbis/doc/vorbisfile/crosslap.html b/contrib/vorbis/doc/vorbisfile/crosslap.html deleted file mode 100644 index 9d28b0b..0000000 --- a/contrib/vorbis/doc/vorbisfile/crosslap.html +++ /dev/null @@ -1,121 +0,0 @@ -<html> - -<head> -<title>Vorbisfile - Sample Crosslapping</title> -<link rel=stylesheet href="style.css" type="text/css"> -</head> - -<body bgcolor=white text=black link="#5555ff" alink="#5555ff" vlink="#5555ff"> -<table border=0 width=100%> -<tr> -<td><p class=tiny>Vorbisfile documentation</p></td> -<td align=right><p class=tiny>vorbisfile version 1.3.2 - 20101101</p></td> -</tr> -</table> - -<h1>What is Crosslapping?</h1> - -<p>Crosslapping blends two samples together using a window function, -such that any sudden discontinuities between the samples that may -cause clicks or thumps are eliminated or blended away. The technique -is nearly identical to how Vorbis internally splices together frames -of audio data during normal decode. API functions are provided to <a -href="ov_crosslap.html">crosslap transitions between seperate -streams</a>, or to crosslap when <a href="seeking.html">seeking within -a single stream</a>. - -<h1>Why Crosslap?</h1> -<h2>The source of boundary clicks</h2> - -<p>Vorbis is a lossy compression format such that any compressed -signal is at best a close approximation of the original. The -approximation may be very good (ie, indistingushable to the human -ear), but it is an approximation nonetheless. Even if a sample or set -of samples is contructed carefully such that transitions from one to -another match perfectly in the original, the compression process -introduces minute amplitude and phase errors. It's an unavoidable -result of such high compression rates. - -<p>If an application transitions instantly from one sample to another, -any tiny discrepancy introduced in the lossy compression process -becomes audible as a stairstep discontinuity. Even if the discrepancy -in a normal lapped frame is only .1dB (usually far below the -threshhold of perception), that's a sudden cliff of 380 steps in a 16 -bit sample (when there's a boundary with no lapping). - -<h2>I thought Vorbis was gapless</h2> - -<p>It is. Vorbis introduces no extra samples at the beginning or end -of a stream, nor does it remove any samples. Gapless encoding -eliminates 99% of the click, pop or outright blown speaker that would -occur if boundaries had gaps or made no effort to align -transitions. However, gapless encoding is not enough to entirely -eliminate stairstep discontinuities all the time for exactly the -reasons described above. - -<p>Frame lapping, like Vorbis performs internally during continuous -playback, is necessary to eliminate that last epsilon of trouble. - -<h1>Easiest Crosslap</h1> - -The easiest way to perform crosslapping in Vorbis is to use the -lapping functions with no other extra effort. These functions behave -identically to when lapping isn't used except to provide -at-least-very-good lapping results. Crosslapping will not introduce -any samples into or remove any samples from the decoded audio; the -only difference is that the transition is lapped. Lapping occurs from -the current PCM position (either in the old stream, or at the position -prior to calling a lapping seek) forward into the next -half-short-block of audio data to be read from the new stream or -position. - -<p>Ideally, vorbisfile internally reads an extra frame of audio from -the old stream/position to perform lapping into the new -stream/position. However, automagic crosslapping works properly even -if the old stream/position is at EOF. In this case, the synthetic -post-extrapolation generated by the encoder to pad out the last block -with appropriate data (and avoid encoding a stairstep, which is -inefficient) is used for crosslapping purposes. Although this is -synthetic data, the result is still usually completely unnoticable -even in careful listening (and always preferable to a click or pop). - -<p>Vorbisfile will lap between streams of differing numbers of -channels. Any extra channels from the old stream are ignored; playback -of these channels simply ends. Extra channels in the new stream are -lapped from silence. Vorbisfile will also lap between streams links -of differing sample rates. In this case, the sample rates are ignored -(no implicit resampling is done to match playback). It is up to the -application developer to decide if this behavior makes any sense in a -given context; in practical use, these default behaviors perform -sensibly. - -<h1>Best Crosslap</h1> - -<p>To acheive the best possible crosslapping results, avoid the case -where synthetic extrapolation data is used for crosslapping. That is, -design loops and samples such that a little bit of data is left over -in sample A when seeking to sample B. Normally, the end of sample A -and the beginning of B would overlap exactly; this allows -crosslapping to perform exactly as it would within vorbis when -stitching audio frames together into continuous decoded audio. - -<p>The optimal amount of overlap is half a short-block, and this -varies by compression mode. Each encoder will vary in exact block -size selection; for vorbis 1.0, for -q0 through -q10 and 44kHz or -greater, a half-short block is 64 samples. - -<br><br> -<hr noshade> -<table border=0 width=100%> -<tr valign=top> -<td><p class=tiny>copyright © 2000-2010 Xiph.Org</p></td> -<td align=right><p class=tiny><a href="http://www.xiph.org/ogg/vorbis/">Ogg Vorbis</a></p></td> -</tr><tr> -<td><p class=tiny>Vorbisfile documentation</p></td> -<td align=right><p class=tiny>vorbisfile version 1.3.2 - 20101101</p></td> -</tr> -</table> - -</body> - -</html> |