[Bug 787085] tsdemux: Sudden jump in PTS when there is a backward jump in PCR

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=787085

--- Comment #12 from parithi <parithi at gmail.com> ---
Thanks Edward for your suggestions.

One more observation on this issue:

UDP input buffer has 7 TS packers and two of them are having PCRs.
one PCR is going backward and the next one is having proper value.

Consider PTS=12:00:00, base_pcr = 12:00:00 and offset=00:00:00
current_pcr=10:00:00.

For explanation purpose consider skew=0 and base_time=0.

When the backward jump happens (by 02:00:00) 
In calculate_skew() function following changes will happen
offset = 02:00:00
base_pcr = 12:00:00
last_pcr = 12:00:00 -> current_pcr + offset


In function mpegts_packetizer_pts_to_ts()
out_PTS = in_PTS + offset = 12:00:00 + 02:00:00 = 14:00:00

    if (G_UNLIKELY (pcr_pid != 0x1fff &&
            ABSDIFF (res, pcrtable->last_pcrtime) > 15 * GST_SECOND))
      res = GST_CLOCK_TIME_NONE;
    else {
      GstClockTime tmp = pcrtable->base_time + pcrtable->skew;
      if (tmp + res > pcrtable->base_pcrtime)
        res += tmp - pcrtable->base_pcrtime;
      else
        res = GST_CLOCK_TIME_NONE;

According to above code first if condition will be satisfied and
GST_CLOCK_TIME_NONE will be returned. Here there is *no issue*.

Now we have a proper PCR (current_pcr=12:00:00). As per calculate_skew
function, first add offset with current_pcr

current_pcr = 12:00:00 + 02:00:00 = 14:00:00
Then this value is assigned to last_pcr.
last_pcr = 14:00:00
offset = 02:00:00

Since both PCR buffers are present in same UDP buffer, both are having same
time stamps. So the following condition will be satisfied and base_pcr time
will not be updated in calculate_skew(). So now we have base_pcr=12:00:00

  /* Ignore packets received at 100% the same time (i.e. from the same input
buffer) */
  if (G_UNLIKELY (time == pcr->prev_in_time
          && GST_CLOCK_TIME_IS_VALID (pcr->prev_in_time)))
    goto no_skew;



Now in function mpegts_packetizer_pts_to_ts()

out_PTS = in_PTS + offset = 12:00:00 + 02:00:00 = 14:00:00

As per above code snippet of mpegts_packetizer_pts_to_ts(), the if condition
will fail because out_PTS == last_pcr and in else part following code will be
executed.

     if (tmp + res > pcrtable->base_pcrtime)
        res += tmp - pcrtable->base_pcrtime;

So the out_PTS = out_PTS - base_pcr = 14:00:00 - 12:00:00 = 02:00:00.
This is the *issue*.

If we are completely ignoring the second PCR buffer in calculate_skew() as
mentioned in comments there won't be any issue. But we updating last_pcr, that
leads to this issue.

-- 
You are receiving this mail because:
You are the QA Contact for the bug.
You are the assignee for the bug.