Ian Jackson wrote:

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. However, I have a feeling the reason was a
bit of a red herring. I also remember tweaking another so that one of
UHF cable channels was carefully offset from a local off-air in order to
minimise the visibility of interference patterning (essentially the same
fix as discussed below).

Why are the IRC channels offset from broadcast channels (where they
exist) by 12.5kHz?

If memory sserves, two transmitters that are interferring just a little,
produce nasty herringbones if they are running at the same frequency, but
move them apart a tad, and the artifacts are far less objectionable.

That is almost certainly the reason. Running IRC channels exactly on
frequency can result in unacceptable beat patterns of 0.75 and 1.25MHz
(at least, it did on one European system I was involved with). Moving
all 65 channels HF by 25kHz worked wonders. [This is close to the 5/3 x
15.625kHz offset broadcasters use for off-air 625-line TV.] 12.5kHz will
probably also be a good offset.

[1] UK cable systems mostly use HRC at 8MHz spacing but this is
sometimes varied by a carefully calculated amount so that one block of
UHF channels coincides almost exactly with the broadcast frequencies.
This is done on systems with a by-pass facility to allow a few channels
- usually the local off-airs - to be fed directly to the TV giving the
subscriber direct access from the TV without needing an aerial.

Obviously this block of channels has to be chosen so as not to conflict
with local transmitters, so the offset will vary from system to system

US cable systems ran on some of the same channels as local broadcasters.

These days, UK cable TV systems don't seem to avoid clashing with (or,
at least, partially overlapping) the off-air TV channels (which are all
UHF). Obviously, to prevent interference problems caused by
ingress/egress, sufficient attention has to be paid to the RF-tightness
of the network.


HRC = Harmonically Related Channels. That means that of the carriers
are all multiples of an integer to reduce IMD. I designed and built
some headends in the early '80s.


--
You can't have a sense of humor, if you have no sense.