[mythtv-users] Onboard sound full-duplex ?

Bruce Markey bjm at lvcm.com
Wed Oct 13 21:05:38 UTC 2004

Maarten wrote:
> On Wednesday 13 October 2004 20:28, Isaac Richards wrote:
>>On Wednesday 13 October 2004 02:11 pm, Maarten wrote:
>>>On Wednesday 13 October 2004 19:29, Tim Sailer wrote:
>>>>On Wed, Oct 13, 2004 at 07:03:18PM +0200, Maarten wrote:
>>>>Card 1 being the first capture card or the first sound card? You
>>>>specify the audio device for playback, so you can always play out
>>>>through the builtin device.
>>>With card_1 I meant the first capture card, not the first soundcard.  But
>>>I don't really think that makes any difference; you'll have two combos of
>>><<capture card + connected soundcard>>.  And if one of those combos lack
>>>full-duplex, it will not be able to playback (or -worse- it could fsck up
>>>the ongoing recording when selecting it for playback).
>>Why would you be using multiple soundcards for playback?
> Hm.  Okay, so maybe I'm confused,


>  but I need a soundcard each for each BT8x8 
> capture card, right ?  So either both those soundcards need to be able to do 
> full-duplex (otherwise live TV and playback will have no sound) OR one must

When you finish watching one recording and go on to watch another,
do you reach around behind the machine and unplug your speakers
from one card and plug them into the other? Of course not.
> have the possibility within mythtv to declare one card capable of recording 
> and playback, and the other capable of _only_ recording. (And mythtv reacting 
> accordingly...)  Is this assumption correct so far ?

It is an impossibility in myth to declare that a different audio
output should be used depending on the input card that was used
at the time of the recording. An assumption to the contrary is

The "Audio output device" is declared on the first page of the
frontend Setup->General. It is per host (your ramblings above
and below would suggest that you would never hear audio from
a slave host unless you turned the volume way up in the other
room ;-) and is independent of which type of capture card, which
DSP input is used for recording, or even if there are any capture
cards at all. This would still be the output device if you
used only MythMusic.

That's "> /dev/dsp". "< /dev/dsp", which is a completely different
matter, is configured in "setup" "2. Capture cards". If you
use the same device for both capture and output, there could
be a problem with duplex. If a device is used for recording
only then there should be no problem.

I deliberately do the following for dual tuner bttv systems. I
get a motherboard with whatever crappy onboard audio. I don't
give a rat's ass if it supports full duplex or has crummy output,
I'll never find out, I just want the DSP for audio encoding.

Next, I get a good PCI sound card with whatever features I'd
like such as digital out, DVD digital audio in, etc. I plug
one tuner card into the sound card and one into the onboard.
I configure the sound card as the first card (snd-card-0) and
the onboard as the second (snd-card-1).

I configure the PCI card for all it's inputs (CD, DVD, tuner)
and outputs (analog and digital to a receiver). The onboard
is only needed to capture from /dev/dsp1 so in "alsamixer -c 1"
I only need to mark "Line" for CAPTUR and run up the "Capture"
slider. That's it. Nothing to unmute, no master or PCM levels.
It's just capturing from /dev/dsp1 and nothing else so there
are no duplex issues.

> Note that I'm _not_ talking about PVR250 cards, but about BT8x8 cards.
> So I really think this is not an option.  Example: a recording on card_1 
> starts at time T. At time T+10 you select "watch TV", but the only free card 
> card_2 cannot playback sound simultaneous with recording. So that won't work, 
> ie. you get no sound then.  Isn't that correct ?

No, it dead wrong.

--  bjm

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