[mythtv-users] Watch TV - Then nothing

[Brian Wood]

On Mar 26, 2006, at 8:28 PM, <jborn at charter.net> wrote:

>
>
> video source is called zap2it and have the channel frequency table  
> set to us-cable (NOT us-cable-hrc? what is the difference?)

Harmonically Related (or "coherent") Carriers was/is a system of  
allocating the channel frequencies on a cable system such that each  
carrier is precisely 6 Mhz. from the ones adjacent to it (actually  
they are at multiples of 6.001). A harmonic comb generator is used to  
phase lock the modulators/processors to ensure that each frequency is  
a precise harmonic of the base generator.

The theory was that all the composite beat products would fall  
precisely on top of other carriers, where they would not create  
perceived interference. The main effect on the subscriber is that the  
channels are no longer in their "correct" places, broadcast channels  
being offset as much as 1.25Mhz. from "normal" (even more in the  
cases of channels 5 and 6).

IMHO the system was not very successful. It might work in theory if  
all that was being transmitted was unmodulated carriers, but since  
the sideband energy was not "coherent" (unless you had the same video  
on all channels, and all genlocked with each other) you would still  
get beat products that would show in in the pictures, and the FM  
aural carriers were obviously not coherent, even if they were 12-17  
db. down they could still cause beats.

Note also that the improvement was a "perceived" one and not directly  
measurable. Manufacturers just love extremely expensive options whose  
performance can't be measured or proved.

The system also meant that you could no longer phase lock the  
channels being used "off air" in your market to minimize ingress  
interference, as they were no longer on the same frequency.

I believe that the only effect of using an HRC system was to vastly  
increase the profits of General Instrument, Scientific Atlanta et al.  
The only real benefit to cable operators was to reduce the  
requirements for monitoring the system for leakage.

>
>
> I might have the same problem with xawtv as myth becuase I get no  
> video with xawtv.  When I start it up via the following command:
> xawtv -device /dev/video0

Xawtv was designed to work with basic capture cards that capture raw  
video, not with cards that encode the signal to mpeg-2 like a PVR. It  
would not be expected to work with your PVR-500.


Are you sure you have your capture cards set up as MPEG-2 cards and  
not as basic raw video cards ?? What does your log show, does your  
frontend give you any messages ??