[pvrusb2] Audio problem: DC offset?

[isely at isely.net]

Jan:

Which mode is this happening in - digital or analog?

If this is digital, then the tuner doesn't do anything at all to 
separate or otherwise handle audio from video - it's just a bitstream 
from the transmitting station and that stream is passed via DVB over to 
MythTV.  So if there were a problem, then the entire stream should be 
disrupted not just the audio.

If this is analog, then the tuner indeed has separate paths for audio vs 
video.  The two paths are combined in the hardware at the point where 
they are encoded to an mpeg stream, which is then passed untouched 
through the pvrusb2 driver via V4L over to MythTV.

If this is analog and you're able to find a circumstance where the 
problem can be reproduced, then try to take MythTV out of the picture by 
directly capturing the stream into a file.  Then play that back with 
mplayer (or whatever) and see if (a) it pops, and (b) it pops exactly 
the same way if you repeatedly play the stream.

I agree that sure sounds like some kind of DC offset, though I have to 
think that at some point in the analog path there's some capacitive 
coupling to eliminate any chance for an offset.

Unloading / reloading the pvrusb2 driver also has the effect of 
reinitializing the hardware, as I'm sure you might imagine.

If you just power-cycle the affected tuner without reloading the pvrusb2 
driver, does that clear the problem?

You mentioned multiple tuners - does it always happen to the same tuner 
each time or is it a random distribution?

Did this problem possibly start after a kernel update?  (That would also 
entail a V4L update and for analog capture the audio is controlled via a 
module in V4L that could have changed...)

  -Mike


On Sun, 15 Jun 2014, Jan Ceuleers wrote:

> Dear list,
> 
> I have an occasional problem whereby "an event" occurs following which
> all recordings made using the affected tuner have no usable audio.
> 
> Tuner: HVR1900.
> 
> "Event": no clue. All recordings made using MythTV.
> 
> Audio symptoms: affected recordings, when starting or stopping playback
> (at any location within the recording) produce a loud popping noise
> followed by silence.
> 
> So in more detail:
> 
> - press Play: <POP> video starts, no audio
> - press Pause: <POP> video freezes
> - press Play again: <POP> video resumes, no audio
> - press Fast Forward: <POP> video fast-forwards
> - press Play: <POP> video resumes at normal speed, no audio
> - press Stop: <POP>
> 
> If this were an analogue system I'd suspect that audio is being recorded
> with a large DC offset that saturates the amp; does that ring any bells?
> 
> What fixes it (for subsequent recordings) is unloading and reloading the
> pvrusb2 module, like so:
> 
> ====
> #!/bin/bash
> /sbin/stop mythtv-backend
> /sbin/rmmod pvrusb2
> /bin/sleep 5s
> /sbin/modprobe pvrusb2
> 
> while true
> do
>   [ -h /dev/videop0 ] && [ -h /dev/videop1 ] && [ -h /dev/videop2 ] && [
> -h /dev/videop3 ] && break
>   /bin/sleep 5s
> done
> 
> /sbin/start mythtv-backend
> ====
> 
> (As you can see I have 4 tuners, and after reloading the module I have
> to wait for udev to recreate the persistent links to the tuners before
> restarting the Myth backend).
> 
> I have found and am using the "IVTV audio fix" [1] in my channel change
> script; this does not prevent the problem.
> 
> [1]: http://www.mythtv.org/pipermail/mythtv-users/2014-May/363737.html
> 
> This problem does not happen very often. When it does I'm afraid I
> rapidly delete the recording in hopes of catching a rerun. So I
> currently don't have one to share. If it helps I will resist rapidly
> deleting the next occurrence.
> 
> Version info:
> 
> root at zotac:~# lsb_release -a
> No LSB modules are available.
> Distributor ID:	Ubuntu
> Description:	Ubuntu 12.04.4 LTS
> Release:	12.04
> Codename:	precise
> root at zotac:~# uname -a
> Linux zotac 3.11.0-23-generic #40~precise1-Ubuntu SMP Wed Jun 4 22:06:36
> UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
> root at zotac:~# mythbackend --version
> Please attach all output as a file in bug reports.
> MythTV Version : v0.27.1-15-g050bf9d
> MythTV Branch : fixes/0.27
> Network Protocol : 77
> Library API : 0.27.20140520-1
> QT Version : 4.8.1
> Options compiled in:
>  linux profile use_hidesyms using_alsa using_oss using_pulse
> using_pulseoutput using_backend using_bindings_perl
> using_bindings_python using_bindings_php using_crystalhd using_dvb
> using_firewire using_frontend using_hdhomerun using_ceton using_hdpvr
> using_ivtv using_joystick_menu using_libcec using_libcrypto
> using_libdns_sd using_libfftw3 using_libxml2 using_lirc using_mheg
> using_opengl using_opengl_video using_qtwebkit using_qtscript
> using_qtdbus using_sdl using_taglib using_v4l2 using_x11 using_xrandr
> using_xv using_profiletype using_bindings_perl using_bindings_python
> using_bindings_php using_mythtranscode using_opengl using_vaapi
> using_vdpau using_ffmpeg_threads using_mheg using_libass using_libxml2
> 
> 
> Any hints would be greatly appreciated.
> 
> Thx, Jan
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-- 

Mike Isely
isely @ isely (dot) net
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