From 74f4b1bc3b627ba4c7e03498234d88cacdfbe97b Mon Sep 17 00:00:00 2001 From: Aki Date: Wed, 29 Sep 2021 22:52:49 +0200 Subject: Squashed 'vorbis/' content from commit d22c3ab5f git-subtree-dir: vorbis git-subtree-split: d22c3ab5f633460abc2532feee60ca0892134cbf --- doc/vorbisfile/crosslap.html | 121 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 121 insertions(+) create mode 100644 doc/vorbisfile/crosslap.html (limited to 'doc/vorbisfile/crosslap.html') diff --git a/doc/vorbisfile/crosslap.html b/doc/vorbisfile/crosslap.html new file mode 100644 index 0000000..9d28b0b --- /dev/null +++ b/doc/vorbisfile/crosslap.html @@ -0,0 +1,121 @@ + + + +Vorbisfile - Sample Crosslapping + + + + + + + + + +

Vorbisfile documentation

vorbisfile version 1.3.2 - 20101101

+ +

What is Crosslapping?

+ +

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 crosslap transitions between seperate +streams, or to crosslap when seeking within +a single stream. + +

Why Crosslap?

The source of boundary clicks

+ +

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. + +

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). + +

I thought Vorbis was gapless

+ +

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. + +

Frame lapping, like Vorbis performs internally during continuous +playback, is necessary to eliminate that last epsilon of trouble. + +

Easiest Crosslap

+ +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. + +

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). + +

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. + +

Best Crosslap

+ +

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. + +

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. + +

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copyright © 2000-2010 Xiph.Org	Ogg Vorbis
Vorbisfile documentation	vorbisfile version 1.3.2 - 20101101

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