[mythtv-users] Weird audio bleed-over with analog recordings

Bob Sully rcs at malibyte.net
Wed Mar 28 05:33:31 UTC 2007

Rod Smith wrote:

> The mixer programs just make available the mixer channels provided by your
> hardware. If there's no Duplicate Front channel in your mixers, that's
> because there's nothing with that label for your hardware. There may be
> something with the same function but a different name, though, so you
> might
> want to try muting everything except what you're positive should be left
> unmuted until you find the right one (if you do -- I make no promises you
> will).
> Also, the alsamixer program separates audio channels into two groups:
> Playback
> and Capture. On my systems, it comes up showing the Playback group only.
> Be
> sure to check both groups by pressing the F3 and F4 keys to toggle between
> them.


alsamixer -V capture brings up several inputs, only two of which are are
being used (any value > 0 ) - PCM Capture and Mic.  I have the sound from
the STB routed into the Mic input.

Tried dropping the PCM Capture level to zero while watching live TV on the
card afflicted with the echo.  That solved the echo problem.  The audio
remained in sync after pause/restart.

Unfortunately, doing just about anything (even simply switching tuners)
jumps the PCM Capture level back up to 94...which just happens to be where
the PCM level is in Setup -> General.  However, if I drop that down, I get
no volume at all when I fire up the analog tuner again.  Myth setup
appears to be setting up bot the PCM capture AND playback - I need the
capture level to be at zero and the playback level up around 90 or so.

> As a side comment, this is part of what's frustrating about configuring
> sound
> on MythTV: There's little standardization on the names for mixer functions
> across different sound hardware. This makes it difficult to have sensible
> conversations about what mixer settings work and what ones don't. (This
> isn't
> a MythTV issue per se; it's an issue with the Linux drivers.)


>> In Alsamixer, there is a PCM capture, which I
>> can lower but not mute - that one seems to be the culprit, but when I
>> bring up mythfrontend again, the PCM Capture level is back up to where
>> it
>> was previously (yes, I do save the setting on exit)...I don't know why
>> it
>> won't stay down.
> You said that the problem occurs when watching live TV, but I don't recall
> if
> you said it occurred when recording. If not, then my guess is that you
> should
> *NOT* be muting the PCM Capture channel but you SHOULD be muting something
> else. I'd expect the PCM Capture channel to be the one used to record
> audio
> inputs, and as MythTV always records, you'd want to record this. With the
> Capture channel set low, try pausing your live TV. If the audio continues
> to
> play while the recording is paused, thus getting out of sync when you
> start
> playing again, then you've got the wrong channel. If the audio pauses
> along
> with the video and stays in sync when you start up again, then you've got
> the
> right channel.

See above.

The echo does not occur when I watch a recorded show later - only if I
watch live TV, and only on the analog or s-video inout on the tuner card
which has the STB audio input.

The PCM capture channel is the problem.  If I mute it while I'm watching
TV by using Alsamixer, it's fine (the echo resolves), but Myth's settings
override this and reset the capture input anytime either of the tuner
cards is used.

Now...I can fix this by disabling "use internal volume controls" in Setup
- if I do this, I can set the PCM playback up and the capture to zero in
Alsamixer, and they stay there (and the echo is gone) - but then I can't
use the keyboard (or Myth remote) to control volume - I have to use the TV
or receiver's remote.

Is there any way to set JUST the PCM **record** to zero and leave playback
levels alone in Myth's setup (or is there a setting I can tweak without
using the Setup GUI)?

> If I'm wrong and you've located the correct channel, then you might look
> at
> the MythTV recording options. Some of these relate to audio volume, and
> it's
> likely that one of these is adjusting the PCM Capture channel, probably
> thinking that it's the channel from which MythTV is recording audio.

See above - it is, but it's controlling both the playback and record

> One other possibility occurs to me: Some sound cards can be configured to
> provide audio input devices for Linux. I don't believe my pcHDTV 3000 is
> one
> of them, but I don't know about the pcHDTV 5500. If you've got a Linux
> device
> file for your pcHDTV 5500's audio channel, then it's conceivable that
> MythTV
> is recording audio from that and that, independent of MythTV, your mixer
> is
> configured to pass through audio from the sound card's inputs to the
> speakers. If this is the case, then unplugging the cable from the pcHDTV
> 5500
> to your sound card should fix the echo while leaving recordings working.
> There is one problem with this hypothesis, though: If it were true, I'd
> expect you to hear audio from that card at all times, or at least whenever
> you're recording from the card. Perhaps there's some reason this isn't
> happening, though, and given that you just need to unplug that one cable
> to
> test the hypothesis, it's worth checking.

The pcHDTV 5500 cards do have an audio device (Sigmatel).  It doesn't seem
to matter wheter I mute it or not.  If I use it as the sound device for
the tuner in question, I get sound on the analog but not s-video...so for
that tuner, I have to use /dev/dsp (the Audigy) for the sound device on
that card, not /dev/dsp2 (the internal audio device).

Unplugging the cable from the involved tuner card to the line-in on the
Audigy causes that **all** of the sound to go away (but it takes several
seconds for all the echoes to fade away fully).

Bob Sully - Simi Valley, California, USA

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