[Gnash-commit] [bug #45722] Gnash emitted loud screech after playing PCM audio object

[Nutchanon Wetchasit]

Follow-up Comment #27, bug #45722 (project gnash):

I've tested around Gnash's FFmpeg/libAV-based codec backend, and found problem
on many levels:

* Gnash's FFmpeg-based sample format converter uses a hard coded assumption
that input sample format is _16-bit signed PCM_
<http://git.savannah.gnu.org/cgit/gnash.git/tree/libmedia/ffmpeg/AudioResamplerFfmpeg.cpp?id=516a6cae2301c582b54d7f00188c8e22c5d76c1b#n64>
when `swresample.h` and `avresample.h` are both unavailable.
* Gnash's FFmpeg-based sample format converter wouldn't attempt to convert
8-bit PCM input sound
<http://git.savannah.gnu.org/cgit/gnash.git/tree/libmedia/ffmpeg/AudioResamplerFfmpeg.cpp?id=516a6cae2301c582b54d7f00188c8e22c5d76c1b#n52>
when `swresample.h` and `avresample.h` are both unavailable.
* Gnash's FFmpeg-based codec handler uses a wrong codec ID for 8-bit embeded
PCM sound (Gnash mistakenly uses _signed_ PCM
<http://git.savannah.gnu.org/cgit/gnash.git/tree/libmedia/ffmpeg/AudioDecoderFfmpeg.cpp?id=516a6cae2301c582b54d7f00188c8e22c5d76c1b#n186>,
but it really is _unsigned_ PCM).
* Gnash's FFmpeg-based codec handler divided sample rate with 2
<http://git.savannah.gnu.org/cgit/gnash.git/tree/libmedia/ffmpeg/AudioDecoderFfmpeg.cpp?id=516a6cae2301c582b54d7f00188c8e22c5d76c1b#n279>
if the source audio is 8-bit PCM for an unobvious reason. As the SWF parser
seems to give a correct sample rate (e.g. 44100 Hz in `pcmplay-8bit.swf`
test), I think the divisor was put there to work around other problems listed
here.
* Gnash's FFmpeg-based audio frame decoder uses an incorrect calculation for
resampling buffer size
<http://git.savannah.gnu.org/cgit/gnash.git/tree/libmedia/ffmpeg/AudioDecoderFfmpeg.cpp?id=516a6cae2301c582b54d7f00188c8e22c5d76c1b#n540>.
(`resampleFactor` should account only sample _rate_ change, as channels number
change is multiplied in later
<http://git.savannah.gnu.org/cgit/gnash.git/tree/libmedia/ffmpeg/AudioDecoderFfmpeg.cpp?id=516a6cae2301c582b54d7f00188c8e22c5d76c1b#n547>)
* Gnash's FFmpeg-based audio frame decoder uses a hard-coded assumption that
the decoded audio uses 2 bytes/samples
<http://git.savannah.gnu.org/cgit/gnash.git/tree/libmedia/ffmpeg/AudioDecoderFfmpeg.cpp?id=516a6cae2301c582b54d7f00188c8e22c5d76c1b#n542>
when calculating total samples to convert. (Which produces an incorrect number
when the decoded audio is 8-bit PCM, as it uses 1 byte/sample)

These problems seems to be very closely related that all of them would have
to be fixed at the same instant to get the correct behavior.

Gnash: 0.8.11dev (git 516a6ca 15-Sep-2015)
SDL: libsdl1.2debian 1.2.15-5 (debian)
libAV: libavcodec53 6:0.8.17-1 (debian), libavformat53 6:0.8.17-1 (debian),
libavutil51 6:0.8.17-1 (debian)
CSound: libcsound64-5.2 1:5.17.11~dfsg-3 (debian)
System: Debian GNU/Linux 7.0 Wheezy i386


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