[pvrusb2] Jumpy Video in Mythtv

[Mike Isely]

On Thu, 28 Jun 2007, Mark Goldberg wrote:

> I'm still working on the audio issues.

I meant to comment on this before and forget.  Sorry...

The sysfs controls for audio directly manipulate the current audio gain 
in the device.  However so do the V4L controls and therefore also the 
mythtv "recording volume".  So if you use sysfs to attempt an audio gain 
adjustment it will only stick around until MythTV overrides it.  It's 
just one volume control knob and if multiple fingers are trying to twist 
it...  :-)

It is a known fact that if the audio is set to full scale on a PVR USB2 
device then you *will* get distortion.  This is actually what the V4L 
developers prefer, i.e. "full scale" in the V4L interface is supposed to 
set "full scale" in the hardware.  I happen to disagree with this and 
think that it is better for the driver to scale the volume to a uniform 
standard since the point of distortion is going to depend on the 
specific hardware combination -- and only the driver really can really 
understand that detail.  There's no way an app could know that for 
example if it's a PVR USB2 then "max" is really say 59000 but if it is a 
WinTV-Go then "max" should be 64000.  Drivers try to hint this by having 
a default volume that should be where the max is, but that is a really 
poor subsitute.  But it is what it is - and I'll step off my soapbox 
here.

One good way to try to debug the audio issue here is to divide and 
conquer.  Try operating the device just with mplayer, using sysfs for 
the volume control.  Figure out what the right maximum gain is in that 
case.  Note the value.  Now go back to MythTV and get into the state 
again where you are recording audio with distortion.  Go back into the 
sysfs interface now - while it is recording - and _read_ the audio 
volume again.  Has it changed?  (Realize that this is a single volume 
knob and both sysfs and V4L can write _and_ read the value so if it's 
changed via the V4L interface you can read back the changed value 
through the sysfs interface.)  If it has changed then MythTV is 
erroneously pushing the recording volume up and you should check what 
you have set there for the recording volume.  If it has not changed then 
the distortion is being introduced somewhere else that's MythTV specific 
or at least downstream of the PVR USB2 device, in which case I'd 
probably look closer at your playback settings and/or your soundcard 
settings.  Though if you are not getting distortion for ATSC playback 
then this would seem to rule out everything downstream of the PVR USB2 
device.

  -Mike


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