[mythtv-users] Coax splitters - how painful are they?

Brian Wood beww at beww.org
Sat Sep 29 15:21:16 UTC 2007

Jay R. Ashworth wrote:

> It's next to impossible to combine multiple wideband RF sources;
> you need a block converter to pull it off, and a lot of luck..
> Cable companies do it by pulling every channel down to baseband and
> then remodulate it to the channel you want it on, and feed it to a
> directional combiner tree.

Almost totally correct. Analog OA channels are usually handled by a
Hetrodyne Processor. The received channel is mixed down to IF (45.75Mhz
visual/41.25Mhz aural) and processed at that frequency (AGC, adjacent
channel filtering etc.), then hetrodyned to the desired output channel.
In a case where the input and output channels are the same, a single
oscillator can be used for mixing down and then back up, thus canceling
any frequency error in the oscillator. This method preserves the
original NTSC modulation from the station, eliminating the many problems
associated with demodulating and remodulating.

A signal received as baseband, such as from a satellite receiver or
terrestrial microwave, must obviously be modulated at the headend.

In an HRC system the local oscillators in the channel output modules are
all phase-locked to a central comb generator running at a nominal 6 Mhz.
(actually 6.001 Mhz. I believe, for some technical reasons I won't go
into here, but it has to do with interference to air navigation

> I'm thinking multiple tuner cards; why not?

I agree totally and said so a few messages back in the thread.


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