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Add partial_tukey and punchout_tukey apodization functions

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Adds two new apodization functions that seem to perform better than
the apodization functions currently in the codebase and fixes three
existing windows as well.

Its important to note that this patch only affects the encoder stage
that evaluates various possible predictors. Audio encoded with these
new windows will still decode with existing legacy decoders.

= Theory =
These functions are used to window the  audio data at the predictor
stage. These news functions enable the use of only part of the signal
to generate a predictor. This helps  because short transients can
introduce noise into the predictor. The  predictor becomes very good
at prediciting one part of the  signal, instead of mediocre for the
whole block.

Signed-off-by: Erik de Castro Lopo <erikd@mega-nerd.com>
This commit is contained in:
Martijn van Beurden
2014-08-10 10:59:29 +02:00
committed by Erik de Castro Lopo
parent ffa55423e0
commit 29a28338c3
8 changed files with 152 additions and 12 deletions
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5
doc/html/documentation_tools_flac.html
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@@ -852,10 +852,11 @@
<span class="argument">-A "function"</span>, <span class="argument">--apodization="function"</span>
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<td>
Window audio data with given the apodization function. The functions are: bartlett, bartlett_hann, blackman, blackman_harris_4term_92db, connes, flattop, gauss(STDDEV), hamming, hann, kaiser_bessel, nuttall, rectangle, triangle, tukey(P), welch.<br />
Window audio data with given the apodization function. The functions are: bartlett, bartlett_hann, blackman, blackman_harris_4term_92db, connes, flattop, gauss(STDDEV), hamming, hann, kaiser_bessel, nuttall, rectangle, triangle, tukey(P), partial_tukey(n[/ov[/P]]), punchout_tukey(n[/ov[/P]]), welch.<br />
For gauss(STDDEV), STDDEV is the standard deviation (0&lt;STDDEV&lt;=0.5).<br />
For tukey(P), P specifies the fraction of the window that is tapered (0&lt;=P&lt;=1; P=0 corresponds to "rectangle" and P=1 corresponds to "hann").<br />
Please note that for both P as well as STDDEV, the use of a point or comma as decimal separator is locale-dependent.<br />
For partial_tukey(n) and punchout_tukey(n), n apodization functions are added that span different parts of each block. Values of 2 to 6 seem to yield sane results. If necessary, an overlap can be specified, as can be the taper parameter, for example partial_tukey(2/0.2) or partial_tukey(2/0.2/0.5). ov should be smaller than 1 and can be negative.<br />
Please note that P, STDDEV and ov are locale specific, so a comma as decimal separator might be required instead of a dot.<br />
More than one -A option (up to 32) may be used. Any function that is specified erroneously is silently dropped. The encoder chooses suitable defaults in the absence of any -A options; any -A option specified replaces the default(s).<br />
When more than one function is specified, then for every subframe the encoder will try each of them separately and choose the window that results in the smallest compressed subframe. Multiple functions can greatly increase the encoding time.<br />
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