1 <html> 2 3 <head> 4 <title>Vorbisfile - Sample Crosslapping</title> 5 <link rel=stylesheet href="style.css" type="text/css"> 6 </head> 7 8 <body bgcolor=white text=black link="#5555ff" alink="#5555ff" vlink="#5555ff"> 9 <table border=0 width=100%> 10 <tr> 11 <td><p class=tiny>Vorbisfile documentation</p></td> 12 <td align=right><p class=tiny>vorbisfile version 1.2.0 - 20070723</p></td> 13 </tr> 14 </table> 15 16 <h1>What is Crosslapping?</h1> 17 18 <p>Crosslapping blends two samples together using a window function, 19 such that any sudden discontinuities between the samples that may 20 cause clicks or thumps are eliminated or blended away. The technique 21 is nearly identical to how Vorbis internally splices together frames 22 of audio data during normal decode. API functions are provided to <a 23 href="ov_crosslap.html">crosslap transitions between seperate 24 streams</a>, or to crosslap when <a href="seeking.html">seeking within 25 a single stream</a>. 26 27 <h1>Why Crosslap?</h1> 28 <h2>The source of boundary clicks</h2> 29 30 <p>Vorbis is a lossy compression format such that any compressed 31 signal is at best a close approximation of the original. The 32 approximation may be very good (ie, indistingushable to the human 33 ear), but it is an approximation nonetheless. Even if a sample or set 34 of samples is contructed carefully such that transitions from one to 35 another match perfectly in the original, the compression process 36 introduces minute amplitude and phase errors. It's an unavoidable 37 result of such high compression rates. 38 39 <p>If an application transitions instantly from one sample to another, 40 any tiny discrepancy introduced in the lossy compression process 41 becomes audible as a stairstep discontinuity. Even if the discrepancy 42 in a normal lapped frame is only .1dB (usually far below the 43 threshhold of perception), that's a sudden cliff of 380 steps in a 16 44 bit sample (when there's a boundary with no lapping). 45 46 <h2>I thought Vorbis was gapless</h2> 47 48 <p>It is. Vorbis introduces no extra samples at the beginning or end 49 of a stream, nor does it remove any samples. Gapless encoding 50 eliminates 99% of the click, pop or outright blown speaker that would 51 occur if boundaries had gaps or made no effort to align 52 transitions. However, gapless encoding is not enough to entirely 53 eliminate stairstep discontinuities all the time for exactly the 54 reasons described above. 55 56 <p>Frame lapping, like Vorbis performs internally during continuous 57 playback, is necessary to eliminate that last epsilon of trouble. 58 59 <h1>Easiest Crosslap</h1> 60 61 The easiest way to perform crosslapping in Vorbis is to use the 62 lapping functions with no other extra effort. These functions behave 63 identically to when lapping isn't used except to provide 64 at-least-very-good lapping results. Crosslapping will not introduce 65 any samples into or remove any samples from the decoded audio; the 66 only difference is that the transition is lapped. Lapping occurs from 67 the current PCM position (either in the old stream, or at the position 68 prior to calling a lapping seek) forward into the next 69 half-short-block of audio data to be read from the new stream or 70 position. 71 72 <p>Ideally, vorbisfile internally reads an extra frame of audio from 73 the old stream/position to perform lapping into the new 74 stream/position. However, automagic crosslapping works properly even 75 if the old stream/position is at EOF. In this case, the synthetic 76 post-extrapolation generated by the encoder to pad out the last block 77 with appropriate data (and avoid encoding a stairstep, which is 78 inefficient) is used for crosslapping purposes. Although this is 79 synthetic data, the result is still usually completely unnoticable 80 even in careful listening (and always preferable to a click or pop). 81 82 <p>Vorbisfile will lap between streams of differing numbers of 83 channels. Any extra channels from the old stream are ignored; playback 84 of these channels simply ends. Extra channels in the new stream are 85 lapped from silence. Vorbisfile will also lap between streams links 86 of differing sample rates. In this case, the sample rates are ignored 87 (no implicit resampling is done to match playback). It is up to the 88 application developer to decide if this behavior makes any sense in a 89 given context; in practical use, these default behaviors perform 90 sensibly. 91 92 <h1>Best Crosslap</h1> 93 94 <p>To acheive the best possible crosslapping results, avoid the case 95 where synthetic extrapolation data is used for crosslapping. That is, 96 design loops and samples such that a little bit of data is left over 97 in sample A when seeking to sample B. Normally, the end of sample A 98 and the beginning of B would overlap exactly; this allows 99 crosslapping to perform exactly as it would within vorbis when 100 stitching audio frames together into continuous decoded audio. 101 102 <p>The optimal amount of overlap is half a short-block, and this 103 varies by compression mode. Each encoder will vary in exact block 104 size selection; for vorbis 1.0, for -q0 through -q10 and 44kHz or 105 greater, a half-short block is 64 samples. 106 107 <br><br> 108 <hr noshade> 109 <table border=0 width=100%> 110 <tr valign=top> 111 <td><p class=tiny>copyright © 2007 Xiph.org</p></td> 112 <td align=right><p class=tiny><a href="http://www.xiph.org/ogg/vorbis/">Ogg Vorbis</a></p></td> 113 </tr><tr> 114 <td><p class=tiny>Vorbisfile documentation</p></td> 115 <td align=right><p class=tiny>vorbisfile version 1.2.0 - 20070723</p></td> 116 </tr> 117 </table> 118 119 </body> 120 121 </html> 122
