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      FFmpeg Resampler Documentation
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      <h1>
      FFmpeg Resampler Documentation
      </h1>


<div class="top-level-extent" id="SEC_Top">

<div class="element-contents" id="SEC_Contents">
<h2 class="contents-heading">Table of Contents</h2>

<div class="contents">

<ul class="toc-numbered-mark">
  <li><a id="toc-Description" href="#Description">1 Description</a></li>
  <li><a id="toc-Resampler-Options" href="#Resampler-Options">2 Resampler Options</a></li>
  <li><a id="toc-See-Also" href="#See-Also">3 See Also</a></li>
  <li><a id="toc-Authors" href="#Authors">4 Authors</a></li>
</ul>
</div>
</div>

<ul class="mini-toc">
<li><a href="#Description" accesskey="1">Description</a></li>
<li><a href="#Resampler-Options" accesskey="2">Resampler Options</a></li>
<li><a href="#See-Also" accesskey="3">See Also</a></li>
<li><a href="#Authors" accesskey="4">Authors</a></li>
</ul>
<div class="chapter-level-extent" id="Description">
<h2 class="chapter"><span>1 Description<a class="copiable-link" href="#Description"> &para;</a></span></h2>

<p>The FFmpeg resampler provides a high-level interface to the
libswresample library audio resampling utilities. In particular it
allows one to perform audio resampling, audio channel layout rematrixing,
and convert audio format and packing layout.
</p>

</div>
<div class="chapter-level-extent" id="Resampler-Options">
<h2 class="chapter"><span>2 Resampler Options<a class="copiable-link" href="#Resampler-Options"> &para;</a></span></h2>

<p>The audio resampler supports the following named options.
</p>
<p>Options may be set by specifying -<var class="var">option</var> <var class="var">value</var> in the
FFmpeg tools, <var class="var">option</var>=<var class="var">value</var> for the aresample filter,
by setting the value explicitly in the
<code class="code">SwrContext</code> options or using the <samp class="file">libavutil/opt.h</samp> API for
programmatic use.
</p>
<dl class="table">
<dt><samp class="option">uchl, used_chlayout</samp></dt>
<dd><p>Set used input channel layout. Default is unset. This option is
only used for special remapping.
</p>
</dd>
<dt><samp class="option">isr, in_sample_rate</samp></dt>
<dd><p>Set the input sample rate. Default value is 0.
</p>
</dd>
<dt><samp class="option">osr, out_sample_rate</samp></dt>
<dd><p>Set the output sample rate. Default value is 0.
</p>
</dd>
<dt><samp class="option">isf, in_sample_fmt</samp></dt>
<dd><p>Specify the input sample format. It is set by default to <code class="code">none</code>.
</p>
</dd>
<dt><samp class="option">osf, out_sample_fmt</samp></dt>
<dd><p>Specify the output sample format. It is set by default to <code class="code">none</code>.
</p>
</dd>
<dt><samp class="option">tsf, internal_sample_fmt</samp></dt>
<dd><p>Set the internal sample format. Default value is <code class="code">none</code>.
This will automatically be chosen when it is not explicitly set.
</p>
</dd>
<dt><samp class="option">ichl, in_chlayout</samp></dt>
<dt><samp class="option">ochl, out_chlayout</samp></dt>
<dd><p>Set the input/output channel layout.
</p>
<p>See <a data-manual="ffmpeg-utils" href="ffmpeg-utils.html#channel-layout-syntax">the Channel Layout section in the ffmpeg-utils(1) manual</a>
for the required syntax.
</p>
</dd>
<dt><samp class="option">clev, center_mix_level</samp></dt>
<dd><p>Set the center mix level. It is a value expressed in deciBel, and must be
in the interval [-32,32].
</p>
</dd>
<dt><samp class="option">slev, surround_mix_level</samp></dt>
<dd><p>Set the surround mix level. It is a value expressed in deciBel, and must
be in the interval [-32,32].
</p>
</dd>
<dt><samp class="option">lfe_mix_level</samp></dt>
<dd><p>Set LFE mix into non LFE level. It is used when there is a LFE input but no
LFE output. It is a value expressed in deciBel, and must
be in the interval [-32,32].
</p>
</dd>
<dt><samp class="option">rmvol, rematrix_volume</samp></dt>
<dd><p>Set rematrix volume. Default value is 1.0.
</p>
</dd>
<dt><samp class="option">rematrix_maxval</samp></dt>
<dd><p>Set maximum output value for rematrixing.
This can be used to prevent clipping vs. preventing volume reduction.
A value of 1.0 prevents clipping.
</p>
</dd>
<dt><samp class="option">flags, swr_flags</samp></dt>
<dd><p>Set flags used by the converter. Default value is 0.
</p>
<p>It supports the following individual flags:
</p><dl class="table">
<dt><samp class="option">res</samp></dt>
<dd><p>force resampling, this flag forces resampling to be used even when the
input and output sample rates match.
</p></dd>
</dl>

</dd>
<dt><samp class="option">dither_scale</samp></dt>
<dd><p>Set the dither scale. Default value is 1.
</p>
</dd>
<dt><samp class="option">dither_method</samp></dt>
<dd><p>Set dither method. Default value is 0.
</p>
<p>Supported values:
</p><dl class="table">
<dt>&lsquo;<samp class="samp">rectangular</samp>&rsquo;</dt>
<dd><p>select rectangular dither
</p></dd>
<dt>&lsquo;<samp class="samp">triangular</samp>&rsquo;</dt>
<dd><p>select triangular dither
</p></dd>
<dt>&lsquo;<samp class="samp">triangular_hp</samp>&rsquo;</dt>
<dd><p>select triangular dither with high pass
</p></dd>
<dt>&lsquo;<samp class="samp">lipshitz</samp>&rsquo;</dt>
<dd><p>select Lipshitz noise shaping dither.
</p></dd>
<dt>&lsquo;<samp class="samp">shibata</samp>&rsquo;</dt>
<dd><p>select Shibata noise shaping dither.
</p></dd>
<dt>&lsquo;<samp class="samp">low_shibata</samp>&rsquo;</dt>
<dd><p>select low Shibata noise shaping dither.
</p></dd>
<dt>&lsquo;<samp class="samp">high_shibata</samp>&rsquo;</dt>
<dd><p>select high Shibata noise shaping dither.
</p></dd>
<dt>&lsquo;<samp class="samp">f_weighted</samp>&rsquo;</dt>
<dd><p>select f-weighted noise shaping dither
</p></dd>
<dt>&lsquo;<samp class="samp">modified_e_weighted</samp>&rsquo;</dt>
<dd><p>select modified-e-weighted noise shaping dither
</p></dd>
<dt>&lsquo;<samp class="samp">improved_e_weighted</samp>&rsquo;</dt>
<dd><p>select improved-e-weighted noise shaping dither
</p>
</dd>
</dl>

</dd>
<dt><samp class="option">resampler</samp></dt>
<dd><p>Set resampling engine. Default value is swr.
</p>
<p>Supported values:
</p><dl class="table">
<dt>&lsquo;<samp class="samp">swr</samp>&rsquo;</dt>
<dd><p>select the native SW Resampler; filter options precision and cheby are not
applicable in this case.
</p></dd>
<dt>&lsquo;<samp class="samp">soxr</samp>&rsquo;</dt>
<dd><p>select the SoX Resampler (where available); compensation, and filter options
filter_size, phase_shift, exact_rational, filter_type &amp; kaiser_beta, are not
applicable in this case.
</p></dd>
</dl>

</dd>
<dt><samp class="option">filter_size</samp></dt>
<dd><p>For swr only, set resampling filter size, default value is 32.
</p>
</dd>
<dt><samp class="option">phase_shift</samp></dt>
<dd><p>For swr only, set resampling phase shift, default value is 10, and must be in
the interval [0,30].
</p>
</dd>
<dt><samp class="option">linear_interp</samp></dt>
<dd><p>Use linear interpolation when enabled (the default). Disable it if you want
to preserve speed instead of quality when exact_rational fails.
</p>
</dd>
<dt><samp class="option">exact_rational</samp></dt>
<dd><p>For swr only, when enabled, try to use exact phase_count based on input and
output sample rate. However, if it is larger than <code class="code">1 &lt;&lt; phase_shift</code>,
the phase_count will be <code class="code">1 &lt;&lt; phase_shift</code> as fallback. Default is enabled.
</p>
</dd>
<dt><samp class="option">cutoff</samp></dt>
<dd><p>Set cutoff frequency (swr: 6dB point; soxr: 0dB point) ratio; must be a float
value between 0 and 1.  Default value is 0.97 with swr, and 0.91 with soxr
(which, with a sample-rate of 44100, preserves the entire audio band to 20kHz).
</p>
</dd>
<dt><samp class="option">precision</samp></dt>
<dd><p>For soxr only, the precision in bits to which the resampled signal will be
calculated.  The default value of 20 (which, with suitable dithering, is
appropriate for a destination bit-depth of 16) gives SoX&rsquo;s &rsquo;High Quality&rsquo;; a
value of 28 gives SoX&rsquo;s &rsquo;Very High Quality&rsquo;.
</p>
</dd>
<dt><samp class="option">cheby</samp></dt>
<dd><p>For soxr only, selects passband rolloff none (Chebyshev) &amp; higher-precision
approximation for &rsquo;irrational&rsquo; ratios. Default value is 0.
</p>
</dd>
<dt><samp class="option">async</samp></dt>
<dd><p>For swr only, simple 1 parameter audio sync to timestamps using stretching,
squeezing, filling and trimming. Setting this to 1 will enable filling and
trimming, larger values represent the maximum amount in samples that the data
may be stretched or squeezed for each second.
Default value is 0, thus no compensation is applied to make the samples match
the audio timestamps.
</p>
</dd>
<dt><samp class="option">first_pts</samp></dt>
<dd><p>For swr only, assume the first pts should be this value. The time unit is 1 / sample rate.
This allows for padding/trimming at the start of stream. By default, no
assumption is made about the first frame&rsquo;s expected pts, so no padding or
trimming is done. For example, this could be set to 0 to pad the beginning with
silence if an audio stream starts after the video stream or to trim any samples
with a negative pts due to encoder delay.
</p>
</dd>
<dt><samp class="option">min_comp</samp></dt>
<dd><p>For swr only, set the minimum difference between timestamps and audio data (in
seconds) to trigger stretching/squeezing/filling or trimming of the
data to make it match the timestamps. The default is that
stretching/squeezing/filling and trimming is disabled
(<samp class="option">min_comp</samp> = <code class="code">FLT_MAX</code>).
</p>
</dd>
<dt><samp class="option">min_hard_comp</samp></dt>
<dd><p>For swr only, set the minimum difference between timestamps and audio data (in
seconds) to trigger adding/dropping samples to make it match the
timestamps.  This option effectively is a threshold to select between
hard (trim/fill) and soft (squeeze/stretch) compensation. Note that
all compensation is by default disabled through <samp class="option">min_comp</samp>.
The default is 0.1.
</p>
</dd>
<dt><samp class="option">comp_duration</samp></dt>
<dd><p>For swr only, set duration (in seconds) over which data is stretched/squeezed
to make it match the timestamps. Must be a non-negative double float value,
default value is 1.0.
</p>
</dd>
<dt><samp class="option">max_soft_comp</samp></dt>
<dd><p>For swr only, set maximum factor by which data is stretched/squeezed to make it
match the timestamps. Must be a non-negative double float value, default value
is 0.
</p>
</dd>
<dt><samp class="option">matrix_encoding</samp></dt>
<dd><p>Select matrixed stereo encoding.
</p>
<p>It accepts the following values:
</p><dl class="table">
<dt>&lsquo;<samp class="samp">none</samp>&rsquo;</dt>
<dd><p>select none
</p></dd>
<dt>&lsquo;<samp class="samp">dolby</samp>&rsquo;</dt>
<dd><p>select Dolby
</p></dd>
<dt>&lsquo;<samp class="samp">dplii</samp>&rsquo;</dt>
<dd><p>select Dolby Pro Logic II
</p></dd>
</dl>

<p>Default value is <code class="code">none</code>.
</p>
</dd>
<dt><samp class="option">filter_type</samp></dt>
<dd><p>For swr only, select resampling filter type. This only affects resampling
operations.
</p>
<p>It accepts the following values:
</p><dl class="table">
<dt>&lsquo;<samp class="samp">cubic</samp>&rsquo;</dt>
<dd><p>select cubic
</p></dd>
<dt>&lsquo;<samp class="samp">blackman_nuttall</samp>&rsquo;</dt>
<dd><p>select Blackman Nuttall windowed sinc
</p></dd>
<dt>&lsquo;<samp class="samp">kaiser</samp>&rsquo;</dt>
<dd><p>select Kaiser windowed sinc
</p></dd>
</dl>

</dd>
<dt><samp class="option">kaiser_beta</samp></dt>
<dd><p>For swr only, set Kaiser window beta value. Must be a double float value in the
interval [2,16], default value is 9.
</p>
</dd>
<dt><samp class="option">output_sample_bits</samp></dt>
<dd><p>For swr only, set number of used output sample bits for dithering. Must be an integer in the
interval [0,64], default value is 0, which means it&rsquo;s not used.
</p>
</dd>
</dl>


</div>
<div class="chapter-level-extent" id="See-Also">
<h2 class="chapter"><span>3 See Also<a class="copiable-link" href="#See-Also"> &para;</a></span></h2>

<p><a class="url" href="ffmpeg.html">ffmpeg</a>, <a class="url" href="ffplay.html">ffplay</a>, <a class="url" href="ffprobe.html">ffprobe</a>,
<a class="url" href="libswresample.html">libswresample</a>
</p>

</div>
<div class="chapter-level-extent" id="Authors">
<h2 class="chapter"><span>4 Authors<a class="copiable-link" href="#Authors"> &para;</a></span></h2>

<p>The FFmpeg developers.
</p>
<p>For details about the authorship, see the Git history of the project
(https://git.ffmpeg.org/ffmpeg), e.g. by typing the command
<code class="command">git log</code> in the FFmpeg source directory, or browsing the
online repository at <a class="url" href="https://git.ffmpeg.org/ffmpeg">https://git.ffmpeg.org/ffmpeg</a>.
</p>
<p>Maintainers for the specific components are listed in the file
<samp class="file">MAINTAINERS</samp> in the source code tree.
</p>

</div>
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