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How to set timestamp of CMSampleBuffer for AVWriter writing

Asked 2013-02-09T09:18:05.443
11

I'm working with AVFoundation for capturing and recording audio. There are some issues I don't quite understand.

Basically I want to capture audio from AVCaptureSession and write it using AVWriter, however I need some shifting in the timestamp of the CMSampleBuffer I get from AVCaptureSession. I read documentation of CMSampleBuffer I see two different term of timestamp: 'presentation timestamp' and 'output presentation timestamp'. What the different of the two ?

Let say I get a CMSampleBuffer (for audio) instance from AVCaptureSession, and I want to write it to a file using AVWriter, what function should I use to 'inject' a CMTime to the buffer in order to set the presentation timestamp of it in the resulting file ?

Thanks.

ios avfoundation audio-recording cmsamplebufferref core-media
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1 Answer

7

Chunk of sample code from here: http://www.gdcl.co.uk/2013/02/20/iPhone-Pause.html CMSampleBufferRef sample - is your sampleBuffer, CMSampleBufferRef sout your output. NewTimeStamp is your time stamp. 

CMItemCount count;
CMTime newTimeStamp = CMTimeMake(YOURTIME_GOES_HERE);
CMSampleBufferGetSampleTimingInfoArray(sample, 0, nil, &count);
CMSampleTimingInfo* pInfo = malloc(sizeof(CMSampleTimingInfo) * count);
CMSampleBufferGetSampleTimingInfoArray(sample, count, pInfo, &count);
for (CMItemCount i = 0; i < count; i++)
{
    pInfo[i].decodeTimeStamp = newTimeStamp; // kCMTimeInvalid if in sequence
    pInfo[i].presentationTimeStamp = newTimeStamp;

}
CMSampleBufferRef sout;
CMSampleBufferCreateCopyWithNewTiming(kCFAllocatorDefault, sample, count, pInfo, &sout);
free(pInfo);
answered 2013-03-02T06:06:17.803

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