[mythtv] DVB recording issues

[Lincoln Dale]

Jason,

some suggestions for troubleshooting:

At 01:40 AM 24/03/2005, Jason W wrote:
[..]
>The signal from the antenna cannot be too bad as a Teac DV-B300 STB runs 
>perfect on it. I have an amplifier on the line at the moment which I need 
>for SBS. Attenuating the VHF frequencies or removing the amp all together 
>do not affect anything besides my ability to receive SBS at all. (i.e. 
>same number of signal errors).

this is a very subjective thing.
while you're recording, can you run 
'./linuxtv-dvb-apps-1.1.0/util/szap/femon' in the background?  (ok - just 
femon - but that is where i keep it...)

signal/snr/ber/unc are polled per-second and can be seen if they're 
incrementing.

the fact that a STB shows a "perfect picture" doesn't mean a lot as its 
clearly different physical cabling and well away from your PC that is 
likely a huge EMI source.
how shielded is everything around your PC case?  not one of these new 
kool-dude cases with perspex windows everywhere which do squat to eliminate 
interference?


>Right now I am doing a test by recording 3 different channels at once. 
>Initially, watching the recordings as they are in progress on a remote 
>frontend, it seems fine without any visible errors. Within a few minutes I 
>am having buffering problems. Load average is up to 18 and peaked at 23 
>before I canceled the recordings. my myth log shows a lot of
>
>2005-03-24 00:28:02.644 IOBOUND - blocking in ThreadedFileWriter::Write()
>
>as well as the continuity errors from above occurring on all cards. 
>Nothing different in syslog (i.e. still Unknown EII protocol messages).
>
>This seems to be wrecking havoc on my recordings. Does anyone have any 
>thoughts on this? The load average skyrocketing is disturbing. When not 
>recording my load average is almost always under 1.
>
>It certainly looks like the audio problem is to to with corrupted 
>transport streams.
>
>Any hints/tips people can provide would be most appreciated.

(1) can you run "vmstat 1" while you do this recording test?
i wouldn't expect 3 SD recordings to push a disk to its physical limits 
(its really only ~4-5 MB/s) -- so my guess is that your box is starting to 
swap.
its also likely that in the first few minutes, you get so much buffered 
data for write the buffer-cache fills up then the system needs to do some 
large writes to try to keep up .. and doesn't.

(2) can you also confirm that your PATA controller is not running in PIO 
mode - i.e. its running in udma mode.
         [root at spam src]# hdparm -v /dev/hd[abd] | grep using_dma
          using_dma    =  1 (on)
          using_dma    =  1 (on)
          using_dma    =  1 (on)

if yout system is too busy issuing writes then its likely that you'll start 
to get overflows in the ring-buffer with the DVB cards - causing bad 
picture/audio, wash, rinse, dry, repeat ...


cheers,

lincoln.