On Dec 14, 2011, at 5:31 PM, Tony Duell wrote:
AFAIK from reading hte manuals, the US CoCOs have
a VHF modulator
switchable (esternal swithc on the rear of the machine) to channel 3 or
4.. The outptu is an RCA phone socket.
This was pretty common practice over here in the '80s; Most game
consoles, computers which could connect over RF and VCRs had a choice of
In most cases the modulator was a bought-in module (one popular brand, at
least over here was Astec). IIRC there was a pin on the modulator to
select channel 3 of 4 (it was tied to one of the supply lines for one of
the channels, left open for the other I think), It's not supring that
many devices that used such modualtors also had the swithc connected to
this pin.
Over here, the common modualtors did not have a channel select input.
They were noramlly pre-tuned for our UHF channel 36,. It was possible to
tweak them onto other channels by adjusting one of the cores.
This is OK for comptuer and the like, where the computer is the only
thing conencted to the TV aerial input at the tiem ,but VCRs, of course,
also pass on the bradcast channels. So the output of a VCR had to be
tunable to avoid interfering with said boradcast channels, often there
was n external tweaker for this. THe later VCRs ,adn things like DVD
recorders (if they support RF output) have this configurable on a setup
menu.
channel 3 or 4. The RCA jack usually cabled to a
coupling box with an
F-connector input and output to pass the other signals through; my
I sem ot rememebr that some US TVs have a balanced 300 ohm aerial input,
and that this boc is also a Balun (the modulator output is always
unbalanced, 75 Ohm).
Our VHF channels go from 2 to 13 (I don't remember
the story behind
why channel 1 is missing) and UHF covers 4 to... 68? Most of time the 3
major networks ran on VHF, because it reached further; we always had a
harder time receiving the UHF channels at home when I was growing up.
We used ot haev VHF television channels, they were used for the alas
long-dead System A 405 line system. Channels 1-5 were 'band 1' and were
used for the BBC transmitters (there was only one BBC programme at a
given tume, all the transmitters transmitted the smae thing). Channels 6
to 13 (IRIC) were Band 3, used for the independant television (again,
only one programme -- that's right, you had a choice of 2 things to
wath).
Band 2 was the FM radio broadcast band (88 to 108MHz ro so).
The UHF channels are (soon to be 'were') used for the 625 lin Sysmte I
transm,issions. Originally, the only thing here weas a second BBC channel
(different programmes), the 405 line programems were not duplciared here.
This led to 'dual standard' TV sets, whcih could be switched ot receive
either. On the groudns that the line frequency was differnet, the
transmisison frequency was differnet (UHF or VHF), the video moulation
polarity was reversed, and that the sound o nthe VHF transmissions was AM
and on the UHF transmisison FM, this meant a lot of switching!
LAter on, the 405 line tranmssions were duplciated on 625 lines, so all
you needed asa 625 line set.
Ther never was colour on the 405 line broadcasts. This meand that for a
time, only BBC2 was in colour. Of course osme of the early colour TVs
could receive 405 line transmissions (in monochrome, of course). Those
sets were nto simple...
Somewher I ahve a list of the UK channel numbers and their frequencies...
-tony