Hi,
Sam Kaufman wrote:
Hi.
In my app, I am trying to call AmAudioFile::rewind(5000) to rewind a streaming
MP3 by a few seconds, but it seems to have no effect. Is this meant to be
supported?
AmAudioFile::rewind is implemented by fseek(3) on the underlying fp.
This works well on normal (wav) audio files, because for every
AmAudioFile::read(), fread(3) is called.
For mp3, it's a little different: the codec sets the buffer parameters
(see mp3.c:367ff), which will cause AmBufferedAudio to read a chunk of
mp3 audio from the file and decode it. The reason is that with MP3
stream, you need to decode a chunk of mp3, until you get sufficient
audio to play.
File v fp readpos
+------------------------------------------+
| |
+------------------------------------------+
Buffer v AmBufferedAudio::r (play pos)
+---------------+
| |
+---------------+
So, if you want to rewind this, you need to
- calculate whether rewind length is in buffer,
if yes you can simply do r-=PCM16_S2B(millisecs*8) and
you're done
- rewind fp by rest_of_buffer + calcBytesToRead(millisecs)
- fix mp3_samples2bytes (see mp3.c), should be possible with CBR
- reset buffer (AmBufferedAudio::r=AmBufferedAudio::w=0)
hth
Stefan
Probably irrelevant: I am doing this via Python and ivr. I implemented an
optional integer parameter for IvrAudioFile.rewind that binds to
AmAudioFile::rewind(int ms). I believe it works because, with a debug level of
3, I see
[#b61fcb70/14969] [rewind, AmAudioFile.cpp:266] DEBUG: Rewinding 5000
milliseconds (150 bytes)
Any help would be appreciated, including, if it's not supported, pointers for
how I might go about implementing.
Best,
Sam
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Stefan Sayer
VoIP Services Consulting and Development
Warschauer Str. 24
10243 Berlin
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