[mythtv-users] Watch TV - Then nothing
beww at beww.org
Mon Mar 27 04:47:17 UTC 2006
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 ??
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