[mythtv] LiveTV playback timing problems
Ed Wildgoose
lists at wildgooses.com
Sat Nov 27 17:47:09 UTC 2004
>Ok, I have managed to use /proc/asound to show that the soundcard is
>receiving and playing a 48khz signal so I do not think it is related
>to that.
>
>
Great. First step out of the way.
>My soundcard, as I said in my last e-mail, is a C-Meida CMI 8738 PCI
>(onboard) soundacard. I have also tried a usb sp/dif soundcard that I
>have and exactly the same thing happens.
>
>
OK, sorry I missed that point. I don't know what that is, but as long
as the driver works then fine.
Can you please try playing back one of the myth tv files in mplayer and
telling me whether that works ok? Please try using both alsa and oss
drivers in mplayer (use the "-ao" param). The code in mplayer is
"somewhat" similar to myth and so this gives a few clues.
Can you also confirm whether you ARE using an external digital
receiver? Can you try just using analogue out on the card.
Now, once you have done the above, try both the alsa and OSS drivers in
myth. OSS is done just by changing the output device to "/dev/dsp" or
whatever. To get alsa, just give it the device something like "
ALSA:plughw:0"
>I do not think that this is a problem related to sound/video hardware,
>I think that it is something to do with the linux timing services,
>like the RTC.
>
>
Perhaps. However, Myth works as follows:
1) Feed the audio to the card as fast as possible
2) Count the bytes going into the card and assume 48000 samples per
sec. When we have sent 48000 / 25 samples (=1920 samples) then it must
be time to draw the next frame (assuming 25 fps).
The RTC is just used to help us wait for the magic moment using a little
less CPU. The alternative is to simply use "usleep" or even count
sheep, which is less accurate and can consume more CPU. There are other
methods as well.
If you give myth an impossible audio device then it should error when
you start playback and offer to play without audio. This will then stop
feeding data to the card and should revert to simply using the clock for
video sync. Perhaps doing this will confirm if it's really an audio problem
In summary, feeding stufff down spdif has a few funny gremlins which can
bite with some dodgy drivers. Try the tests above and we can see if
it's your audio drivers or not.
Ed W
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