[mythtvnz] DVB-T recording artefacts after upgrade to Mythbuntu 11.10 with HVR2200

[Stephen Worthington]

On Mon, 28 Nov 2011 14:42:49 +1300, you wrote:

>> Hi,
>>
>> I use a DVB-T Hauppauge HVR-2200 to record FreeviewHD and play back with
>> VDPAU. All was working fine with Mythbuntu 11.04 and then I upgraded to
>> 11.10. (My first attempt was updating through Update Manager and that
>> resulted in my system not booting any more, probably to do with some
>> LightDM setting, but I ended up reinstalling from scratch.)
>>
>> Since we upgraded, we are getting intermittent strange visual artefacts
>> in the recordings, where blockiness gets worse until it is bad enough
>> that minutes of a program will be completely unwatchable and sound is
>> distorted to the point that we can't understand it.
>>
>> The new features aren't that compelling, so if nobody has any ideas I'll
>> just roll it back to the old Clonezilla image of 11.04 (I learnt that
>> trick a while ago!!!)
>>
>> Thanks in advance,
>>
>> Aaron
>>
>> _______________________________________________
>> mythtvnz mailing list
>> mythtvnz at lists.linuxnut.co.nz
>> http://lists.ourshack.com/mailman/listinfo/mythtvnz
>> Archives http://www.gossamer-threads.com/lists/mythtv/mythtvnz/
>>
>
>Hmmm, this is interesting.  I'm getting exactly the same behaviour with my
>HVR-2200.  I'm in Auckland and have line of sight to SkyTower and
>Waiatarua. I get the same symptoms on all 3 HD channels with my aerial
>pointing to either transmitter.  Reception through my TV is perfect and
>signal/quality are both 8+/10.  So I'm thinking it's HVR-2200 driver or
>firmware or graphics related.
>
>The symptoms I see are exactly as you describe.  2-3 mins of progressively
>degrading video then audio.  Then it snaps back to perfect picture and
>audio.  This pattern reoccurs at random intervals of 20-60 mins.
>
>If you flick back to your Clonezilla image of 11.04 and the problem goes
>away, I'd be very interested in knowing which combination of HVR-2200
>driver/firmware and graphics card your running.
>
>I'm running 0.24.1+fixes from the Debian Multimedia Stable repository on
>top of Debian Squeeze with a stock 2.6.32 kernel.  VDPAU via a GT210 (bit
>underpowered I know)
>
>Anyone else seeing same behaviour?
>
>Dave

Those symptoms are of temporary bad reception causing a damaged
recording.  I had that for a while until I installed an aerial
amplifier.  If it is a damaged recording, then playing the damaged
spot again will cause exactly the same symptoms.  If the problem is in
the graphics drivers, I would expect that recordings made before the
upgrade would suffer from the same problems as new recordings.  I have
not had any problems playing files that were recorded on my main Myth
box (11.04) on my laptop (11.10), so I expect that the graphics
drivers are not the problem.  I do not use my laptop to record, so I
do not know if I have any problems with recording on 11.10.

One thing to try with a damaged file - when you have played past the
damaged point in the file a bit, try skipping backwards a little bit.
I have found that this can clear up the video and audio problems, as
long as the place you go back to is after the damage has ended.  It
looks as though Mythfrontend does not handle damage particularly well
and can let it cause more visible and audible problems well past the
actual damage, but going backwards causes it to recalculate things and
get it right.

It is likely that the point everything comes right again after damage
is the point that there is a key frame in the file.  Key frames
contain all the data needed to display a frame.  Non-key frames have
to use the data from the previous frame(s) plus the current frame data
to display a frame, so they can be contaminated by bad data from
previous frames.  I am not sure how often key frames occur in TV H.264
files, but it is fairly typical in generating compressed video files
to force a key frame every one to ten seconds.  But I have seen them
as much as one minute apart.