[mythtv-users] Lots of DVB continuity errors???

[Hamish Moffatt]

On Tue, Jun 29, 2004 at 07:07:49PM +0100, Chris Birkinshaw wrote:
> I have a Nova T PCI DVB card with mythtv 0.15 running, and am seeing lots of TS continuity
> errors in the logs, with most channels being almost unwatchable due to constant glitching. When
> using the windows software supplied with the card there are never any glitches at all.
> 
> Does anyone else have this problem? I'm sure it's not related to resources, since the disk being
> used as a buffer shows read speeds of 50MB/sec using "hdparm -t /dev/md1", and the system is
> running with an 800Mhz Duron with 768MB RAM. Does anyone know if mythtv has implemented the DVB
> inner and outer coding error correction, or if it just relies on the error correction within the
> MPEG stream?

I saw some behaviour like this when I was first setting up Myth. It
definitely seemed to be worse in MythTV than in Xine, for example.

I suspected that excessive CPU load is causing buffer overflows in the
DVB card, but I never proved it. I think your Duron 800 MHz is a bit
underpowered, though others will probably tell you it's just fine for
MPEG2 decoding. I played with setting PCI latency timers with setpci and
powertweak and it seemed to help a bit but I was never convinced. It may
be more important on your slower CPU; I was using an XP 2600+.

I don't have a simple answer for you. I was running ALSA and found that
sometimes a good DVB glitch would send Myth and sound specifically into
convulsions. (I can get the some convulsions just by changing channel in
live TV for a bit.) I switched to OSS, which seemed to help. I made some 
improvements to my antenna system and now the BER counts reported by
tzap or femon are very low. Finally I switched back to ALSA for various
reasons and haven't had any more trouble.

What sort of BER counts do you have? You can run femon (from the same
package as tzap) while Myth runs. Mine are typically 0x20 or so now.

Mind you, at one stage I had BER of 0x1000 to 0x8000 (faulty antenna
connection - actually the antenna wasn't even connected, just the cable
in the wall!) and it worked suprisingly well. Crashed a bit though!

I'm now using a "DPANDA" card (dpanda.com.au) (tda1004x + budget-ci) and
an Avermedia 761 (sp887x + dvb-bt8xx) with good results on both.

So ultimately I'm not sure how I solved this problem, looking back.
However my system is rock solid now.

BTW, you might find that the Windows software is just more forgiving.
Xine seemed more forgiving than Myth to me; mplayer was not robust at
all.

Hamish
-- 
Hamish Moffatt VK3SB <hamish at debian.org> <hamish at cloud.net.au>