[mythtv-users] DVB(S) tuner sensitivity (and my other) experiences

Dave dave at 0bits.com
Sun Feb 18 11:20:27 UTC 2007


thought i'd post so people will realise that often it's not myth's fault.

I built a mythtv system with a variety of components in a relatively 
small form factor box (thermaltake vc7001sns) and a Technotrend S1500 
budget dvb card. Everything went well, but for the life of me i couldn't 
watch a decent picture from the satellite as i was getting constant

[mpeg2video @ 0x86b3238]ac-tex damaged at 13 26
[mpeg2video @ 0x86b3238]concealing errors

errors and stuttering blocky video images and pip and squeak audio every 
minute or so. I was so depressed as the WAF (wife approval factor) was 
tending to zero having parted with my dreambox only recently.

I had 3 satellites with a diseqc switch and the same signal which worked 
(i borrowed another humax box) on a satellite box was unstable on my 
mythtv on every single satellite LNB and channel.

Everyone and everything i asked/read implied i had a weak signal, 
clashing interrupts, slow disk, insufficient CPU, crap onboard audio, etc..

So my experience is as follows to narrow the problem.

1. If you think it is bad cable feed from you satellite(s), try with a 
old satellite set-top-box. If your signal is strong, and no dropouts, 
rule that out.

2. If you think it is a disk issue (since all mythtv watch tv is 
actually a recording), disable mythfrontend,  tune to a channel and use 
'xine' or 'mplayer' to watch a program. It the problem persists, it's 
not a disk issue.

3. If you think it is sound, do as in #2 above and disable audio with 
'xine' or 'mplayer' and check again if you have a picture without blips 
and blocking.

4. If you think it is an interrupt problem, boot linux with the one of 
the options 'noapic' or 'irqpoll' and your system should rejuggle the 
interrups. If the problem persists, it's not that either (check your DVB 
card is not sharing with your video card or disk interrupts by looking 
at /proc/interrupts). Also if you have a roomy pc - without the 
limitations my small-form-factor thermaltake chassis has, you should 
shuffle your PCI cards around in different slots.

5. If you think you're having a cpu problem, look at 'vmstat' and see 
what system and idling time is at and if you're waiting for io.

So what was my problem in the end ?? Well, none of the above. I had 
ordered 2 identical (well there's no such thing) DVB cards and sold one 
to a friend who was also building a mythbox. His card worked fine with 
no stuttering at all. Mine was faulty. By changing *nothing* else but 
replacing with a Skystar 2 DVBS card i now have a perfect picture and 
stronger signal and more channels. So remember, sometimes there is dodgy 
h/w still out there. Do process of elimination diagnosis before blaming 

Other random suggestions:

a. Stop all non essential processes on your box (do you really need to 
run sendmail as a daemon ? Who will send email to this box ?)
b. Stop wasting memory - you do not need *any* window manager to run 
c. Use a transparent mouse cursor (with xsetroot).
d. start X11 early in the boot process.
e. Disable all non essential kernel modules

This was a public service announcement

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