In message ,
Terry Casey writes
In article ,
says...

In message , Arny Krueger
writes

"Terry Casey" wrote in message
...


A couple of questions regarding that list:

Why is the HRC channel spacing offset[1] by 300Hz - 6.0003MHz instead of
6MHz?

I don't know.

I recall once specially tweaking a UK 8MHz HRC harmonic comb generator
(to which all of the TV channels were locked). It was a little above (or
was it below?) 8MHz. There was a reason for this, but at the moment, I
can't remember what it was.


One system where this was done was the old BT Westminster system -
probably very useful in an area where I would expect a lot of off-air
reception problems.

Same lot, in the land of the concrete cows and a thousand roundabouts.
;o))

I don't know what offset they used but, as an example, if you alter the
comb to 7.990963855MHz, channel E45 is bang on (663.25MHz) so, if you
centred the five off-airs around this using E41, E43, E45, E47 and E49,
the worst case error will be +/-36kHz from the nominal frequency.

Ah yes. That's certainly one of the reasons for using a weird reference
frequency. I recall that certain systems insisted that the four set-top
bypass channels had to be close to the standard off-air broadcast
channels, because some TV sets would not tune to anything but these.
Almost certainly this is what I was involved in.

But wasn't it at the same place which ingeniously used a not-quite-8MHz
comb reference which was actually derived from one of the UHF off-air
channels? As the headend equipment was largely supplied by the 'other'
company, I doubt if I would have been involved with tinkering with it
(although I'm pretty sure that I did swap one or two of the modulator
SAW filters because of the problems which arose when NICAM started).
Maybe 'my' comb generator was a replacement.
--
Ian