Preface
| Reference | DVB-H CRTPSourceFilter source code |
| | Direct Show |
!!NOTES:
when checking a/v sync, there are three cases to be tested.
a) From no signal to having signal (e.g. Mute->unMute)
b) fast-backward/re-wind, fast-forward (DISCONTINUITY)
c) Device lost (ex: Ctrl-Alt-Del)
4) buffering mechansim
1) a/v PTS
Real a/v PTS should come from a source filter,
ex: FileSourceFilter, DVBHRTPSourceFilter
a/v PTS should be used in MediaSample::SetTime(..) and
MediaSample::SetSyncPoint(..) with 100ns-based unit.
Pseudo code:
HRESULT CRTPSourcePin::FillBuffer(IMediaSample *pSample) {
// Notes:
// (1) pSample may be audio or video samples.
// (2) m_rtTimeBase and pPacket->pts are micro-second based.
1 micro-sec = 1000 nano-sec.
rtStart = (m_pSourceFilter->m_rtTimeBase + pPacket->pts) * 10 ;
rtEnd = rtStart + 1 ;
pSample->SetTime( &rtStart, &rtEnd );
pSample->SetSyncPoint( TRUE ) ;
if ( pPacket->pts > m_pFilter->m_rtCurrMaxPTS) {
m_pFilter->m_rtCurrMaxPTS = pPacket->pts;
}
}
!!NOTES:
MediaSample PTS value has to always move upward.
Otherwise, there might be frame drops.
2) DirectShow System Clock
When running a DirectShow Graph, System clock will also move forward
together with MediaSample PTS.
There will be following different cases
a) When PTS of MediaSample is equal to or bigger than System Clock
Audio MedisaSample PTS
|----------------|-------------|----------------------
100 200 300 or 500 (meant to be assigned)
Video MediaSample PTS
|----------------|-------------|-----------------------
100 200 300 or 500 (meant to be assigned)
System clock
|----------------|-------------|-----------------------
1000 1100 1200
Result: a/v will be in sync as long as PTS for MediaSample is assigned synchronously for a/v.
b) The speed of MediaSample is slower than System Clock
Audio MedisaSample PTS
|----------------|-------------|----------------------
100 200 210 (meant to be assigned)
Video MediaSample PTS
|----------------|-------------|-----------------------
100 200 210 (meant to be assigned)
System clock
|----------------|-------------|-----------------------
1000 1100 1200
Result:
might have video frame drops even though a/v PTS is in sync
because System Clock runs faster
3) Discontinuity
When discontinuity happens, what should be done
a) Use the last MAX PTS as the next PTS base
(ps: this is to make sure PTS value is moving upward)
b) Calculate how many seconds are passed between discontinuity event and
the last packet PTS
(ps: this is to make sure the following MedisaSample PTS value still
follows System Clock speed)
Pseudo code:
s = GetMediaPacket(...);
if (s == REAL_PACKET) {
lastPacket_tickCount = GetTickCount();
} else if (s == DISCONTINUITY) {
m_pSourceFilter->m_rtTimeBase += pRTPSourceFilter->m_rtCurrMaxPTS;
// extra seconds to add
m_pSourceFilter->m_rtTimeBase += (GetTickCount() - lastPacket_tickCount ) *1000;
m_pSourceFilter->m_rtCurrMaxPTS = 0;
}
4) Buffering mechanism
When it comes to DVB-T, buffering may not be needed.
But when it comes to DVB-H burst streaming, deciding how many packets should be buffered
in advance is very important because it will affect the playback quality.
