Nigel Goodwin writes:
>For those who don't know, Britain used to have a 405 line B/W TV system
>originally using Band I for BBC only - this goes back to before the
>second world war. When a second channel appeared (ITV) there was no
>channel space available, so ITV was launched on Band III. Obviously at
>this point most TV receivers were Band I only - many early ones were TRF
>sets, made to work on one channel only!. This produced an outbreak of
>ITV convertors, down shifting Band III down to Band I.

        This is all quite interesting.  Some older sets in the United
States had IF chains that operated in the 21 MHZ range, but the vast
majority of sets that one runs across now or would have run across for
the last 30 or 40 years have an IF in the 41-47 MHZ range which is
exactly what Nigel is describing.  The interesting thing is that the
Band-I signals from the BBC were sometimes audible in North America
during peaks of high Sun spot activity.  The audio of those BBC
transmitters was AM and was found around 41.5 or so megahertz.  The
video carriers were around 45 or so megahertz.  Most television tuners
produce an IF which still follows that scheme even though NTSC
television signals always have the video 4.5 MHZ below the sound
carrier.  The sound has always been frequency-modulated for NTSC.
All the T.V. tuners I have run across seem to use high-side carrier
injection so the IF output is frequency-inverted from the true
relationship between the video and aural carriers.  That is my point
that the tuners today still produce their output on what would have
been the old BBC Band-I channel.

        This, by the way, would have been channel 1 in the United
states had we ever had a channel 1, but it was decided after World War
II to use 41-47 MHZ for two-way radio so lots of state police agencies
and plumbing companies, etc got to hear all that video junk floating
around any time the ionosphere opened up to England and France.:-)  In
other words, we had Channel 1 but lost it before it was ever used and
that's why your television tuner starts with 2 and not 1.

        I remember hearing BBC1 as late as 1982 or so.  The Solar
Cycle peaks roughly on an eleven-year period so by the time the Band-I
and Band-ii transmitters were shut off around 1985, it was not
possible to receive them from North America.  I kind of miss them, but
the lower VHF frequencies are really not suited well for television
because of Mother Nature's vagaries.  Even our channels 2-6 which run
from 54 through 88 megahertz are clobbered by Sporadic E to the point
of UN usability in mid Summer.

Martin McCormick WB5AGZ  Stillwater, OK
OSU Center for Computing and Information Services Data Communications Group