[mythtv-users] WYCC-DT, MythTV choosing wrong audio track

[Anthony Messina]

On Monday 01 October 2007 04:05:01 pm Jonathan Rogers wrote:
> Anthony Messina wrote:
> > On Monday 01 October 2007 10:40:51 am Brian Wood wrote:
> >> Anthony Messina wrote:
> >>> On Monday 01 October 2007 09:07:59 am Anthony Messina wrote:
> >>>> When using my HD-5500 to tune to channel 20-1, WYCC-DT, I get good
> >>>> signal for video and two audio tracks. For some reason, MythTV chooses
> >>>> the "second" audio track which does not tune properly and is very
> >>>> static-y. When I manually change the track to the "first" one, the
> >>>> audio is perfect.
> >>>>
> >>>> I'm not sure how to address this issue.  How do I get MythTV to select
> >>>> the right audio track at the start of the tuning process so I don't
> >>>> get static for audio when I record something in the middle of the
> >>>> night?
> >>>
> >>> Further investigation reveals that the audio track that MythTV chooses
> >>> is AC3 5.1, which is probably correct (for MythTV).  That is the audio
> >>> that is scratchy.  The AC3 2 channel audio is perfectly clear--the one
> >>> I have to switch to manually.
> >>>
> >>> Strange thing is, I get perfect AC3 5.1 on all the other OTA digital
> >>> channels except this one (WYCC-DT Chicago).  Is there a place to adjust
> >>> the audio frequency "tuning" parameters for a specific channel?
> >>
> >> There is no such thing as the "audio frequency" on a digital channel,
> >> it's all embedded in the same digital stream with the video.
> >>
> >> Start at the source, can you get decent audio on the "bad" channel by
> >> going directly into a receiver?
> >
> > thanks for helping.  i am not sure what you mean "directly into a
> > receiver."
>
> Brian Wood probably meant that you should try receiving the channel with
> a different hardware tuner/receiver, such as the one built into a TV. Of
> course, you may not have one available. The only ATSC receivers I have
> are the two PCI cards in my MythTV machine, since my LCD TV only has an
> NTSC receiver, though it has a native resolution of 1366x768.
>
> An alternative to try would be to have MythTV decode the audio by
> disabling the AC3 passthrough. Or, you could try a different TV viewer
> application to see if the problem is related to MythTV. Unfortunately, I
> can't recommend one of those, since I've only successfully used MythTV
> for ATSC.
>
> > i have myth doing audio out to my stereo via optical cable.  when i tune
> > to 20-1 (WYCC-DT) in 5.1 AC3, the stereo receiver shows that it's
> > receiving 5.1 digital surround on the display, but there is tons of
> > static. if i switch over to 2 channel audio, it still gets to the
> > receiver using the optical cable, and the stereo receiver shows that it's
> > getting 2 channel digital audio.
> >
> > no static with the 2 channel audio,
> > lots-o-static with the 5.1 channel audio
>
> My guess is that the static is coming from the station, but that's
> something you could verify by using a different receiver. I've
> experienced various audio and video problems on ATSC broadcasts, such as
> noisy or distorted audio, weird echoes on 5.1 tracks, and undersized and
> noisy video.
>
> > no issues with any other tv channel using either 2 or 5.1 channel audio.
> >
> > i've tried screwing with the antenna placement, etc. to see if it's a
> > reception issue, but no matter where i place the antenna, the audio never
> > changes (for better or worse).  i live in the city so i get plenty of
> > good signal.
>
> As others have pointed out, if the video and some audio tracks of a
> channel are received well, it's extremely unlikely that one audio track
> wouldn't be received well, since the entire logical channel (actually
> usually multiple logical channels from the same station) are transmitted
> in a single bitstream modulated onto one radio frequency band. Since the
> video takes a far greater percentage of the bitstream than the audio,
> one tends to see video reception problems before hearing audio reception
> problems. Also, audio reception problems usually sound like dropouts or
> chopiness rather than static.
>
> Choosing an audio track to play from an ATSC broadcast happens at
> playback, since mythbackend just records all audio tracks with the video
> track of a logical channel. I'm not aware of a way to force mythfrontend
> to choose a particular audio track based on channel, but there may be one.

thanks for your help.  i too, do not have another tuner i can use.  i do have 
a hdhomerun on the way, so i'll try with that.  however, i did try disabling 
the ac3 passthrough in an effort to have myth decode the audio stream with no 
difference--the 5.1 channel audio is still mostly static.

is there a possibility that there is a problem with the broadcast itself? 
probably not, but this does seems strange that it only occurs with the 5.1 
channel audio on this one atsc channel and i'm 2.3 miles from the tower!

-- 
Anthony -  http://messinet.com - http://messinet.com/~amessina/gallery
8F89 5E72 8DF0 BCF0 10BE 9967 92DC 35DC B001 4A4E
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