[mythtv-users] Sound Syncronization Problems in Recording Playback ONLY

Steven Adeff adeffs.mythtv at gmail.com
Tue May 30 17:43:15 UTC 2006

On 5/30/06, Douglas Wagner <douglasw0 at gmail.com> wrote:
> Been a while since i've checked into this.
> Gonna start working on this again tomorrow evening so I decided to check up
> on the thread.
> Steve:
> I have a pcHDTV3000 Card, so i'm picking up a potload of OTA stations.  In
> my local area (which is the same for everyone elses of course) i'm getting
> ahold of both the Analog and Digital Stations (for instance 2 vs 2.1).  Now,
> in MythTV i've got the Analog Stations filtered out (i.e. They're not in my
> zap2it profile so they're not "valid" stations via Myth).  So ALL of the
> stations i'm displaying are in fact Digital Stations (HD, Non HD, I don't
> know, but they're all at minimum Digital).
> Here's where my issue comes in, when I tune to a station, say 9.3 (pbs) I
> get digital television displaying what PBS is displaying, sound and video
> seem to be synced up just fine.  When I then switch to 30.1 (ABC?  I think -
> Grey's Anatomy, Alias, Commander in Chief, etc.) I get video with audio out
> of sync...we're talking 200 - 300 ms out of sync...mid distance past the "is
> it really out of sync or am I just seeing things?" but certainly not into
> "OMG that's WAY wrong".  It's out of sync enough that it's noticeable and
> annoying without being unwatchable (other than the annoyance factor).
> Now, when I go to record shows:  When I record something on 9.3 (pbs) which
> plays synced up, it records just fine and is perfectly in sync.  When I play
> back something recorded on 30.1 the recording is about 2 seconds off the
> audio (people talk and then you hear the audio)...which is COMPLETELY
> unwatchable in any way at all.
> Now, here's where it gets wierd.  That SAME video (recorded to disk as an
> MPEG file by Myth v0.19) played through Xine syncs up video/audio PERFECTLY
> with absolutly no hitch what so ever.  This tells me a few things:
> 1) The Live TV out of sync problem is due either to my machine not being
> able to keep up in playing real time HDTV Streams (it's a 2.8G Pentium 4 HT
> Processor with 1G of memory, nothing else besides myth
> frontend/backend/mysql/httpd running) or MythTV isn't able to sync it up
> right for some reason.
> 2) The fact that the playback is farther off than the live show itself
> suggests that the ENTIRE problem is mythTV's issue, NOT the stations issue.
> That belief is exhaserbated by the fact that the videos play just fine on
> Xine outside of MythTV (if the show was recorded with sound/video off then
> it would be off played through xine as well), and my TV set with NO myth
> attached (just direct to TV) does just fine as well (there's no processing
> inside a TV that would sync up that audio/video stream, so it's GOT to be
> comming in synced up from the broadcast).
> The final "hum, that's interesting" note to this whole thing is that the
> recordings i'm doing on the various stations fit into two completely
> difering categories: 1) Kids shows recorded off of PBS running during the
> daytime and 2) "Parent" shows running in prime time recorded off of
> (generally) ABC(?).
> Now, I'm going to make the guess, and I think correctly, that the kids shows
> i'm recording are probably NOT being broadcast in HD with Dolby sound, but
> i'm pretty darn sure that Alias and Greys Anatomy sure as hell are.
> Tomorrow night i'm going to sit down and try to narrow the variables down
> alot more, see what I can find out.  I also have a few possible things to
> try:
> 1) could I be running into an issue where XvMC is actually hurting the
> performance or causing this issue?
> 2) Could Transcoding (which is supposed to fix audio sync issues) either be
> broken and making it worse or not working at all and not syncing up the
> audio?  How can I tell whether a recording has been transcoded and to what?
> Where's good documentation on the whole transcoding thing and how it
> works/what it does/how it's setup?
> Again, general and somewhat random thoughts to re-start this discussion.
> Now to your questions directly:
> ----------------------------------------------------------
> ok, let me make sure I understand. You've got an ATSC/QAM tuner card.
> you pick up a bunch of HD and digital SD stations. digital SD stations
> do not pose a problem with audio sync but *all* your HD stations do?
> -----------------------------------------------------------
> This is partly where my issue comes from, I can't tell which stations are HD
> and which are digital SD.  If you assume that PBS is a Digital SD while
> something like an ABC, CBS and NBC are all HD stations then you get farther
> with the discussion by saying yes to this.  I don't know which stations
> during prime time programming / HD broadcast all correctly record and which
> don't...that's a task I need to find out tomorrow.

All HD stations are 16:9, all SD stations are 4:3. There is an OTA PBS
that is HD, but they don't do kids shows yet from what I've seen, so
if your picking up a PBS with kids shows its most likely not HD.

> -------------------------------------------------------------
> If so, then lets assume whatever the issue is has to do with HD
> stations use of AC3 sound.
> Are you using digital out of your soundcard to a receiver or are you
> running analog output from the soundcard to your receiver?
> ---------------------------------------------------------------
> There is no digital sound in the setup other than what's comming in over the
> broadcast wire.  Once it hits the pcHDTV3000 card it is going out the PCI
> slot (I have tried with both the patch wire between pcHDTV3000 out and
> soundcard in and without it, same effect) to the Sound Card's Bus, to the
> sound card, and out through the ANALOG out through a Mini->RCA plug (single
> mini plug in the green analog out hole of the SB Live! Value Digital card I
> have, to the Red/White RCA audio in on the TV I have.  FYI the video is
> getting to the tv Via DVI->HDMI cable).  There is no subsidiary receiver at
> all in this setup, just the TV itself.

When you do your testing, try recording something straight off the
card outside of Myth and use MythVideo with the internal player to see
what happens with the audio. If possible put this on the internet so
someone can download it to see if they also get the audio sync issues
with your recordings.

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