[Mp4-tech] Re: Mp4-tech Digest, Vol 36, Issue 21

[Glenn Connery]

Mohit,
Generally a receiver who sees a PCR in a packet with a discontinuity set will 
immediately load that PCR value into its clock register and will IMMEDIATELY 
be on the new time base.  However, even if the discontinuity is missed or 
dropped the receiver will still eventually end up on the new time base.  The 
receiver is assumed to be running a PLL which after enough PCRs go by on the 
new time base will eventually lock to the new time.  This could take quite a 
while though, certainly some seconds.  In the interval, as the time is 
drifting from the old time base to the new, various PTS and DTS values in the 
stream may be seen as invalid (before now, too far in the future causing a 
buffer overflow...), and as a result there might be no frames displayed, or 
macro-blocking, or some other result.
Regarding continuity counters, often there will be a continuity error at a 
discontinuity, as the discontinuity indicates that perhaps two different 
streams were glued together at that point.  So you can expect a discontinuity 
not only on the PCR PID but also on any other PIDs in the stream.
Glenn
On Friday 21 July 2023 7:56 am, mp4-tech-request lists.mpegif.org wrote:
> Thanks Song,
>
> I have a doubt regarding Continuity Counter(cc).
>
> Consider the following Base case and a variant mentioned at end of mail.
>
> <cc=0> <pcr_flag == 0, discontinuity_indicator = 0> // old system time
> <cc=1>   <pcr_flag == 0, discontinuity_indicator = 1> // new system time
> begin
> <cc=2> <pcr_flag == 1, discontinuity_indicator = 1> //  new PCR set.
> <cc=3> <pcr_flag == 0, discontinuity_indicator = 0>
>
>
>
> Acc to 2.4.3.5 of H.222.0 there can be a CC error in Case of PCR_PID Packet
> also,but
> This err will occur only in packet in which system discontinuity will
> occur.
>
> What I think this CC error is totally dependent on the Network.If there are
> some attenuations in the
> Network then only can a packet be dropped.This CC err can occur in any
> packet.
>
> Now consider the case when the Packet containing the new PCR set gets
> dropped.Then How will
> Demuxer will come to know of the new time base.Will it to wait till next
> discontinuity in PCR to occur,then new
> Time base to arrive or there is some way???
>
> <cc=0> <pcr_flag == 0, discontinuity_indicator = 0> // old system time
> <cc=1>   <pcr_flag == 0, discontinuity_indicator = 1> // new system time
> begin
> /////////////<cc=2> packet gets dropped
> <cc=3> <pcr_flag == 0, discontinuity_indicator = 0>
>
>
> Regards
> Mohit