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

Tue May 30 16:24:22 UTC 2006

I don't have any answers but will try to provide a sanity check on where I
have identical problems and possible provide some clarification if it helps.

On 5/30/06, Douglas Wagner <douglasw0 at gmail.com> wrote:

> 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).
>

Same setup here except I am using Dvico Fusion Gold card.  I also filtered
out all sdtv stations (typically with channel numbers with the format
of X.2rather than
X.1)

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).
>

Exactly identical to problems i see.  The easy way to define whether a show
is HDTV is whether it is 16:9 or 4:3.  I would say all my 16:9 shows are
200-300mS off and most, if not all, 4:3 programs are synced up well.

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.
>

I do not have this problem.  My recorded shows have the same sync has live
tv.  Well, actually I never watch live tv but basically all my 16:9, HDTV
shows have audio 200mS behind video.

> 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.
>

I have  a 3.2Ghz pentium 4 HT that is lightly loaded even with HDTV.

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:
>

I assume this goes back to what shows are HDTV and what is SDTV.  Again, I
would say I have problems on all 16:9 shows and no problems with 4:3
shows.   That does coincidentally line up with kids shows and parent's shows
for the most part.

1) could I be running into an issue where XvMC is actually hurting the
> performance or causing this issue?
>

I don't use XVMC.

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?
>

I have never tried transcoding.

Again, general and somewhat random thoughts to re-start this discussion.
>

My random thought-
-I once read that a weak signal can hurt A/V sync.  I do indeed live in the
fringe zone so that is possible for me.  How strong is your signal?  That
would not explain your 2-3 second problem but it might the 200-300mS on live
tv.

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.
>
> -------------------------------------------------------------
>
> 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.
>

I have used both spdif and analog connections.  Alsa and just normal old
default device.   Same problem no matter how I hook it up.

--Doug
>
>

Doug-  You said you were going to post something on the developers list.  I
am not subscribed there.  Did you get any responses or learn anything new
there?
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