[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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