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

[Jonathan Rogers]

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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.

Jonathan Rogers
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