[Ffmpeg-devel] Newbie question about pcm_s16le

Benjamin Larsson banan
Fri Aug 4 11:11:29 CEST 2006

michele tavella wrote:
> Dear all, 
> I subscribed to ffmpeg-devel two days ago. I'm working on my master
> thesis and (as the first task) I have to synchronize two streams:
> - a standard DV stream
> - a PCM audio stream
> I understand that my question could be off-topic, so I'm sorry if I
> bother you, but it is the first time I approach the audio/video field.
Yes it's off-topic.
> I'm currently using the FFMPEG API to handle decoding of a DV Raw
> stream acquired via "dvgrab" (dvvideo + stereo pcm_s16le).
> Following the examples I did find googling around, I have successfully
> decoded the video frames.
> I've also decoded the PCM stream and I can handle the stream as a
> vector of shorts.
> On a couple of guides said that the PCM stream should alternate Left
> channel samples with right channel samples, something like this:
> Channel	L R L R L R ...
> Index	0 1 2 3 4 5 ...
This seems to be correct.
> In image_001.png you can find a plot of the even and the odd samples of
> an audio frame.
> The Odd Samples signal looks to me like a synchronization signal! 
> I'm now wondering how the stream is organized.
You plot the values in an unsigned way when they actually are signed. 
(Unsigned range 0->2^16, signed -2^15->2^15-1).
> Any help would be great, also in terms of documentation/books.
> Thank you for your time, 
> mt
> --
>      home  tavella.michele at gmail.com	
>  LIRA-Lab  michele at liralab.it 
>   OpenPGP  98E66CEC
>       web  http://mt.homelinux.org 

Benjamin Larsson

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