[opendtv] Re: ATSC and Lip Sync

If the ad were inserted in the compressed domain it could assert a
discontinuity indicator (although any compressed splicer that I ever
worked on went to great lengths to avoid anything in the bitstream that
could be used to identify ads). On seeing a DI, it would be reasonable
for a receiver to reset its internal timebase from the stream.
 
Another gotcha for compressed domain ad insertion is that AC3 frames
(30msec) do not align with video frames (33msec) so there is likely to
be an inconsistancy in the audio PTS values. Again a receiver may see
this as an error.
 
Of course none of this applies when ads are inserted prior to
compression.
 
gary
 
Gary Hughes
Distinguished Member, Technical Staff
Motorola On Demand Video, MA34
80 Central St.
Boxborough, MA  01719
Email: ghughes@xxxxxxxxxxxx
Office: 978 266 7269
Mobile: 978 339 3615
Fax: 978 264 9108
 
 


________________________________

        From: opendtv-bounce@xxxxxxxxxxxxx
[mailto:opendtv-bounce@xxxxxxxxxxxxx] On Behalf Of Mark Schubin
        Sent: Tuesday, May 26, 2009 7:00 PM
        To: opendtv@xxxxxxxxxxxxx
        Subject: [opendtv] Re: ATSC and Lip Sync
        
        
        As far as a broadcaster is concerned, a commercial (unless it
has some change in AFD or dialnorm comparable to a scene change in a
program) is just more of the program stream.  It is continuous audio and
video into the encoders.
        
        TTFN,
        Mark
        
        
        Tom Barry wrote: 

                Mark -
                I thought there were some sort of discontinuity flags in
the digital
                audio at the start of each new section of material which
could cause a
                reset/recalc of time stamps.  Is this not true?
                
                - Tom
                
                Mark Schubin wrote:
                  

                        As far as the receiver is concerned, the
commercials are just a continuation of 
                        the programming.
                        
                        TTFN,
                        Mark
                        
                        
                        dan.grimes@xxxxxxxx wrote:
                            

                                I, too, have noticed out-of-sync
commercials but not nearly as frequent as 
                                regular programming.  Could it be that
since commercials are usually 30 
                                seconds or less that they have less time
to get out-of-sync than the longer 
                                program lengths?
                                
                                Dan
                                -------------------------
                                Date: Sun, 24 May 2009 09:29:21 -0400
                                From: Mark Schubin
<TVMark@xxxxxxxxxxxxx> <mailto:TVMark@xxxxxxxxxxxxx> 
                                Subject: [opendtv] Re: ATSC and Lip Sync
                                
                                Richard C. Ramsden wrote:
                                      

                                BUT!
                                have you noticed that the commercials
are never out of lip sync?
                                        

                                No, I have not.  I've seen many
out-of-sync commercials.  But, if I
                                quickly change channels and return, they
come back into sync (as do the
                                program segments).
                                
                                TTFN,
                                Mark
                                      

        
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