[FFmpeg-devel] mpegtsenc: Improve PCR generation and output

Tomas Härdin tomas.hardin
Mon Oct 4 16:48:35 CEST 2010

On Thu, 2010-09-30 at 11:33 -0700, Baptiste Coudurier wrote:
> Hi Tomas,
> On 9/30/10 7:07 AM, Tomas H?rdin wrote:
> > Hi
> >
> > While working some more on remuxing dvbsub in mpegts I noticed that the
> > longer the sample I used was, the higher the muxdelay I needed in order
> > to avoid the "dts<  pcr, TS is invalid" warning.
> >
> > This is due to how the current muxer calculates PCR. It simply
> > accumulates TS_PACKET_SIZE*8*90000LL/ts->mux_rate in cur_pcr. Unless
> > your mux_rate evenly divides 135360000 (TS_PACKET_SIZE*8*90000) this
> > will cause rounding errors which quickly lead to unacceptable PCR drift.
> >
> > A second problem is that it only uses 90 kHz precision, when it should
> > use 27 MHz. 90 kHz is insufficient to stay within the allowed +- 500 ns
> > jitter.
> >
> > The attached patch calculates PCR based on max_delay and the current
> > size of the output, in 27 MHz. This method should eliminate any PCR
> > drift caused by the rounding errors in the previous solution. It also
> > outputs the full PCR.
> >
> > The patch passes the regtests, but that seems to be because there are
> > none for mpegts. Maybe some should be added?
> The test is present, by default the muxer uses VBR, you activate CBR 
> muxing by specifying a muxrate. Did you test specifying a muxrate ? :)
> Patch looks good, thanks for the work, I'll test it against some tools.

Actually, I made a mistake. The six reserved bits are between
program_clock_reference_base and program_clock_reference_extension, not
after. So the patch writes the program_clock_reference_extension bits in
the wrong place. See table 2-7 on page 44 in ISO/IEC 13818-1.

Updated patch attached.

As for testing, I failed to find a file in tests/ that contains "mpegts"
or whose name includes "mpegts", so how would I go about testing this?

-------------- next part --------------
A non-text attachment was scrubbed...
Name: mpegtsenc_pcr2.patch
Type: text/x-patch
Size: 5011 bytes
Desc: not available
URL: <http://lists.mplayerhq.hu/pipermail/ffmpeg-devel/attachments/20101004/0e562ae0/attachment.bin>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 197 bytes
Desc: This is a digitally signed message part
URL: <http://lists.mplayerhq.hu/pipermail/ffmpeg-devel/attachments/20101004/0e562ae0/attachment.pgp>

More information about the ffmpeg-devel mailing list