[mythtv] HDTV Random Blockiness

[Hamish Moffatt]

On Sun, Feb 13, 2005 at 01:11:46AM -0700, Brandon Beattie wrote:
> > I don't think I see the same thing occurring on my HD-3000 card, but it
> > might just be because that's the only card in a different backend server.
> > The HD-2000 is in my master backend, which is also where my /myth/store
> > (live TV ringbuffer) is shared out of, while the HD-3000 is on a slave
> > backend that I only seldom use.  Could the extra DMA activity of the HDD's
> > be exacerbating this?
> 
> The HD-3000 is slightly better/nicer with surviving longer when there
> are limited pci resources availible for it to transfer data.  You bet
> any PCI card, HDD, or anything that uses the system bus will cause more
> buffer overrun errors -- Regardless of it being an HD-2000 or HD-3000.
> Motherboards and CPU speed also play a minor role.  .. If you don't have
> enough resources to send data, you see overruns.  In the future if the
> driver can be modified to work better, hopefully we'll see less or none
> of the overruns.  Don't get stuck on them though.  The amount of data
> loss typically is very very minor, for most people.

I disagree. I spent many hours trying to improve the driver for the
Avermedia 761 card to avoid buffer overruns. That card uses the BT878
chip which is the same as used on the HD-2000.

I made driver changes and I fiddled with PCI bus latencies. For me the
problem got much worse when I started to use the on-board SATA
controller which is on the same logical PCI bus as the slots. Whenever
the BT878 was active, I could get buffer overruns (thousands) simply by 
running 'hdparm -t /dev/sda'. Recording to that disk in Myth led to
continuous overflow errors.

In the end I concluded that the BT878 was inadequate for this purpose;
it isn't designed for it and so the FIFO is too small. It's provisioned
for audio data at up to a megabit, not ATSC/DVB data at 20 Mbit or
above.

I replaced the card with one based on the cx23883 (as used on the
HD-3000 also) and haven't had a problem since. The older card was
completely unusable once I added the SATA disk.


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