[mythtv-users] HD hardware: what next?

Daniel Kristjansson danielk at cuymedia.net
Sun Jun 25 16:33:46 UTC 2006


On Sun, 2006-06-25 at 09:09 -0600, Greg Woods wrote:        
> So now I need to change some hardware. I am risking throwing good money
> after bad if there's something on this system that just is known not to
> work, but if HD display works, the system can handle it. So on to the
> questions:
> 
> FIREWIRE FROM MOTOROLA DCT-6412 CABLE BOX        
> Just flat out won't work. I have tried powering off the cable box,
> unplugging the firewire cable, and rebooting the system countless times,
This is probably a firmware problem. In most setups the firmware gets
downloaded from your cable provider's head end and it needs to be
setup properly. I have the same problem with RCN in New York with
the DCT-6400, in that case I have talked with their technical people
and they do have working firmware. The problem is that when they
load that firmware my DCT-6400 starts working, and the DCT-6400 used
by 90% of their customers without firewire stops working.

> HD3000
> It "sort of" works. I can load the DVB modules and access the card, I
> channels are there and the signal is good. But I can't get a good
> picture from the HD3000. It looks like it's all broken up and
> hesitating, and the sound is garbled. I've been told that this is likely
> some sort of signal interference but I don't know how to determine what
> the source of the interference is or how to correct the problem. 
I had this kind of problem twice. The first time it was due to a low
signal. Then after replacing the cable and the splitters it started
working. But then later on I put this same card in a new motherboard
and I got these problems again. But this time it wasn't interference
but instead PCI timeouts causing the problem. In that case I needed
to play with the PCI latencies to get things to work. You should be
able to google for instructions for this.

> Questions: first, is there any chance that the interference wouldn't
> bother OTA signals?
Yep, QAM-256 modulation is inherently more susceptible to noise
than 8-VSB and cable systems usually stick the digital channels
at higher frequencies (which attenuate more quickly in cables
and splitters). If you are using any 45-980 Mhz splitters,
replace them with 5-2400 Mhz splitters (or at least 45-2000 Mhz),
and replace any RG-59 cable with RG-6, and use good connectors.

A 1->2 splitter usually loses 7 dB in it's rated frequency range
and much more outside of it's rated frequency range. A 1->3 looses
about 11 dB. Each connector looses about 3 dB, and the cable looses
a set number of dB per foot (the are good RG-6's and bad, but they
all meet a minimum standard). Each 3dB of loss is halving your
signal. It is best too lose as few dB as possible. Even if you
add a signal amplifier that counteracts all the dB lost, it is
also adding it's own noise and amplifying any noise the signal
picked up while the desired signal was low.

> Second, are there any other HD/QAM cards available that would be less
> sensitive to the interference?
It depends on the type of interference. For QAM, I don't think so.

> Third, is there any way to determine where the interference is really
> coming from? I heard one person say that he fixed this problem by
> replacing his power supply, but that's a financial and major labor
> expense, not something I would want to do based on one piece of
> anecdotal evidence.
I would go with the cables first. I've used HDTV cards from
various manufacturers with three different computers, and only
once had a bogus powersupply cause problems. That powersupply
blew up with sparks and flames two months later..

-- Daniel



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