View source for TimeStretch

==== Syntax and Parameters ====
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{{Func2Def
|TimeStretch(clip ''clip'' [, float ''tempo'', float ''rate'', float ''pitch'', <br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; int ''sequence'', int ''seekwindow'', int ''overlap'', bool ''quickseek'', int ''aa'' ])
|TimeStretch(clip ''clip'' [, int ''tempo_n'', int ''tempo_d'', int ''rate_n'', int ''rate_d'', int ''pitch_n'', int ''pitch_d'', <br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; int ''sequence'', int ''seekwindow'', int ''overlap'', bool ''quickseek'', int ''aa'' ])
}}

:{{Par2|clip|clip|}}
::Source clip. Audio is always converted to [[Float]].
::{{AvsPluscon}} no conversion is performed. Accepts [[Float]] audio only.
::Starting from v2.61 multichannel audio is supported.
::Prior to v2.61: If {{FuncArg|clip}}.[[Clip_properties|AudioChannels]]=2, special processing is used to preserve [[Wikipedia:Stereo_imaging|stereo imaging]]. Otherwise, channels are processed independently. Independent processing works well for unrelated audio tracks, but not very well for [[Wikipedia:5.1_surround_sound|surround sound]]; pending the release of AviSynth v2.61, try [[TimeStretchPlugin]] instead. <sup>[http://forum.doom9.org/showthread.php?t=71632] [http://forum.doom9.org/showthread.php?t=172054#post1718302]</sup>

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===== ''Tempo, Rate and Pitch'' =====
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:{{Par2|tempo|float|100.0}}
::Changes speed while maintaining the original [[Wikipedia:Pitch_(music)|pitch]]. 
::If {{FuncArg|tempo}}=200, the audio will play twice (200%) as fast; if {{FuncArg|tempo}}=50, the audio will play half (50%) as fast.
::The effect is also known as [[Wikipedia:Audio_time-scale/pitch_modification|time-stretching]].

:{{Par2|rate|float|100.0}}
::Changes speed while allowing [[Wikipedia:Pitch_(music)|pitch]] to rise or fall, like the traditional analog [[Wikipedia:Pitch_control|vari-speed]] effect. 
::If {{FuncArg|rate}}=200, the audio will play twice (200%) as fast; if {{FuncArg|rate}}=50, the audio will play half (50%) as fast.

::Rate control is implemented purely by sample rate transposing.<sup>[http://www.surina.net/soundtouch/README.html]</sup> 
::If {{FuncArg|rate}} is adjusted by itself, no [[Wikipedia:Audio_time-scale/pitch_modification|time-stretching or pitch-shifting]] is performed, and the [[#Advanced_Parameters|'''''Advanced Parameters''''']] will have no effect.

:{{Par2|pitch|float|100.0}}
::Changes [[Wikipedia:Pitch_(music)|pitch]] while maintaining the original speed (within a small tolerance&ndash;see [[#Notes|Notes]] below).
::If {{FuncArg|pitch}}=200, the audio will sound an [[Wikipedia:Octave|octave]] higher; if {{FuncArg|pitch}}=50, the audio will sound an octave lower.
::The effect is also known as [[Wikipedia:Audio_time-scale/pitch_modification|pitch-shifting]]. 

:{{FuncArg|tempo}}, {{FuncArg|rate}} and {{FuncArg|pitch}} can all be adjusted independently, in which case their effects are added together.

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===== ''Tempo_n, Tempo_d, Rate_n, Rate_d, Pitch_n and Pitch_d'' =====
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:When needing more accuracy you can use the rational pair parameters {{FuncArg|tempo_n}}, {{FuncArg|tempo_d}}, {{FuncArg|rate_n}}, {{FuncArg|rate_d}}, {{FuncArg|pitch_n}} and {{FuncArg|pitch_d}} instead. All parameters are integers and have default value 1. Internally {{FuncArg|tempo}} is calculated as double({{FuncArg|tempo_n}}/{{FuncArg|tempo_d}}) ({{FuncArg|rate}} and {{FuncArg|pitch}} likewise), before further processing. Seeking should be sample exact.

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===== ''Advanced Parameters'' =====
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:The time-stretch algorithm has a few parameters that can be tuned to optimize sound quality for certain applications. The current default parameters have been chosen by iterative if-then analysis (read: "trial and error") to obtain the best subjective sound quality in pop/rock music processing, but in applications processing different kind of sound the default parameter set may return a sub-optimal result.

:These parameters affect the time-stretch algorithm as follows:

:{{Par2|sequence|int|100 *}}
::This is the length of a single ''processing sequence'' in milliseconds, which determines how the original sound is chopped in the time-stretch algorithm. Larger values mean fewer, and longer, sequences are used. In general, 
::* a larger {{FuncArg|sequence}} value sounds better with a lower {{FuncArg|tempo}} and/or {{FuncArg|pitch}}; 
::* a smaller {{FuncArg|sequence}} value sounds better with a higher {{FuncArg|tempo}} and/or {{FuncArg|pitch}}.

:{{Par2|seekwindow|int|22 *}}
::The length in milliseconds for the algorithm that searches for the best possible overlap location. For larger {{FuncArg|seekwindow}} values, the possibility of finding a better mixing position increases, but an overly large {{FuncArg|seekwindow}} may cause '''''drifting''''' (a disturbing artifact where audio pitch seems unsteady) because neighboring sequences may be chosen at more uneven intervals.

:{{Par2|overlap|int|8 *}}
::The overlap length in milliseconds. When the sound sequences are mixed back together to form a continuous sound stream again, {{FuncArg|overlap}} defines how much of the ends of the consecutive sequences will be overlapped. This shouldn't be a critical parameter. If you reduce the {{FuncArg|sequence}} by a large amount, you might wish to try a smaller {{FuncArg|overlap}}.

:{{Par2|quickseek|bool|false *}}
::The time-stretch routine has a 'quick' mode that substantially speeds up the algorithm but may degrade the sound quality when {{FuncArg|quickseek}} is set to true. 
::*'''Try {{Template:FuncArg|quickseek}}=false''' if you hear artifacts like warbling, clicking etc.

:{{Par2|aa|int|64 *}} 
::Controls the number of ''taps'' the [[Wikipedia:Finite_impulse_response|anti-alias filter]] uses. Set to 0 to disable the filter. Must be a multiple of 4.

:This table summarizes how these parameters can be adjusted for different applications:
:{| class="wikitable"
|-
| '''Parameter'''
| '''Default value'''
| '''If larger...'''
| '''If smaller...'''
| '''Music'''
| '''Speech'''
| '''CPU burden'''
|-
|style="vertical-align:top"| {{FuncArg|sequence}}
|style="vertical-align:top"| Relatively large, chosen for slowing music tempo.
|style="vertical-align:top"| Usually better for slowing tempo. You might need less {{FuncArg|overlap}}.
|style="vertical-align:top"| Accelerates "echoing" artifact when slowing down the tempo.
|style="vertical-align:top"| Default value usually good.
|style="vertical-align:top"| A smaller value might be better.
|style="vertical-align:top"| Smaller value increases CPU burden.
|-
|style="vertical-align:top"| {{FuncArg|seekwindow}}&nbsp;
|style="vertical-align:top"| Relatively large, chosen for slowing music tempo.
|style="vertical-align:top"| Eases finding a good mixing position, but may cause "drifting" artifact.
|style="vertical-align:top"| Makes finding a good mixing position more difficult. 
|style="vertical-align:top"| Default usually good, unless "drifting" is a problem.
|style="vertical-align:top"| Default usually good.
|style="vertical-align:top"| Larger value increases CPU burden.
|-
|style="vertical-align:top"| {{FuncArg|overlap}}
|style="vertical-align:top"| Relatively large, chosen to suit above parameters.
| 
| 
|
| 
|style="vertical-align:top"| Larger value increases CPU burden.
|}
:'''*''' {{FuncArg|sequence}} and {{FuncArg|seekwindow}} have default values 100 and 22. However they are updated if the calculated tempo is different from the default value (100). The calculated tempo depends on the specified tempo or pitch in your script. It will be different from 100 if tempo or pitch in your script is different from 100. The update of the default values happens in TDStretch::calcSeqParameters().
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