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pcm/PcmChannels: silence surround channels when converting from stereo

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Previously, there was no special code to convert stereo to
multi-channel.  The generic solution for this was to convert to mono,
and then copy the result to all channels.  That's a pretty bad
solution, but at least something which always renders audio.  MPD does
something, instead of failing.

Now that MPD has proper support for multi-channel (by defining the
channel order), we can do better than that.  It is a (somewhat) common
case to play back stereo music on a DAC which can only do
multi-channel.  The best approach here is to copy the stereo channels
to front-left and front-right, and apply the "silence" pattern to all
other channels.
This commit is contained in:
Max Kellermann
2017-01-19 10:53:41 +01:00
parent 97ae594375
commit 33716732a1
3 changed files with 71 additions and 0 deletions
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40
src/pcm/PcmChannels.cxx
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@@ -20,9 +20,14 @@
#include "config.h"
#include "PcmChannels.hxx"
#include "PcmBuffer.hxx"
#include "Silence.hxx"
#include "Traits.hxx"
#include "AudioFormat.hxx"
#include "util/ConstBuffer.hxx"
#include "util/WritableBuffer.hxx"
#include <array>
#include <algorithm>
#include <assert.h>
@@ -90,6 +95,38 @@ NToStereo(typename Traits::pointer_type dest,
return dest;
}
/**
* Convert stereo to N channels (where N > 2). Left and right map to
* the first two channels (front left and front right), and the
* remaining (surround) channels are filled with silence.
*/
template<SampleFormat F, class Traits=SampleTraits<F>>
static typename Traits::pointer_type
StereoToN(typename Traits::pointer_type dest,
unsigned dest_channels,
typename Traits::const_pointer_type src,
typename Traits::const_pointer_type end)
{
assert(dest_channels > 2);
assert((end - src) % 2 == 0);
std::array<typename Traits::value_type, MAX_CHANNELS - 2> silence;
PcmSilence({&silence.front(), sizeof(silence)}, F);
while (src != end) {
/* copy left/right to front-left/front-right, which is
the first two channels in all multi-channel
configurations **/
*dest++ = *src++;
*dest++ = *src++;
/* all other channels are silent */
dest = std::copy_n(silence.begin(), dest_channels - 2, dest);
}
return dest;
}
template<SampleFormat F, class Traits=SampleTraits<F>>
static typename Traits::pointer_type
NToM(typename Traits::pointer_type dest,
@@ -133,6 +170,9 @@ ConvertChannels(PcmBuffer &buffer,
StereoToMono<F>(dest, src.begin(), src.end());
else if (dest_channels == 2)
NToStereo<F>(dest, src_channels, src.begin(), src.end());
else if (src_channels == 2 && dest_channels > 2)
StereoToN<F, Traits>(dest, dest_channels,
src.begin(), src.end());
else
NToM<F>(dest, dest_channels,
src_channels, src.begin(), src.end());