[mythtv-users] What does ATSC poor signal look like? or is the PC not up to it?

Andre Newman mythtv-list at dinkum.org.uk
Thu Apr 16 09:56:00 UTC 2009


HI,

I've set up a MythTV system for a friend in Boston MA, he is seeing  
some strange effects and I'm not familiar enough with ATSC to know if  
there is a problem with his system or if he just needs a better antenna.

The system is Ubuntu 8.10 with the mythbuntu weekly build 0.21-fixes,  
the system is borderline usable for MythTV but it's all he's happy to  
commit to MythTV until he gets a feel for how useful it might be. The  
CPU is a 650Mhz P3with 320MB ram and two  UDMA/66 disks one ext3 for  
the system and one XFS for recordings. The tuner is a HVR 950 plugged  
into a USB2 PCI card as the system only has USB 1.1 on board, he's  
using a indoor antenna but he's only 4 miles from the transmitter and  
there's nothing much in between.

Initially I couldn't get the HVR 950 to tune within MythTV but we  
added some ram (from 256MB to 320MB) and it started working fine. The  
system is only used as a backend, he has a mac laptop and a PS3 for  
playback.

Ok the problem is recordings have sections missing, everything seems  
fine for a minute or two to maybe even 10 minutes, then the picture  
freezes with lots of blocks then carries on with what seems like a  
second or so of programme missed.

I have seen similar on my UK based system here when my server is very  
very busy so I have enlarged the HD ringbuffer to 13500kB, this took  
him from missing sections several times a minute to the current every  
few minutes chunks missing.
I got him to move the USB2 card to a different slot so it wasn't  
sharing interrupts with anything.

I think it's system load related but the box doesn't seem heavily  
loaded ever and I'm not sure if I'm fooling myself, difficult to know  
when the box is only a ssh session to me! Unfortunately the HVR 950  
doesn't seem to report BER or any other meaningful measure of signal  
quality, he tells me that if he uses the HVR 950 on the laptop the  
signal is good for the local stations and I can see that the more  
remote stations are unusable, that's fair.

I'm a DVBT and DVBS person, I've not seen what an ATSC TV does when  
the signal is on the edge so maybe what it's doing is normal for ATSC,  
I know DVBT behaves differently to this as I had a marginal DVBT  
signal for some channels until just recently. So some "local"  
knowledge and experience would be much appreciated, either from others  
who know what happens when ATSC is marginal or others who have old low  
powered hardware for their backend systems.

Many Thanks

Andre


  


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