> In November of 1970, I heard BBC1 TV audio in the 41.5 MHZ range for
> the first time here in the very center of the contiguous 48 United
> States.
> A few other notes are in order. The BBC wasn't the only television
> service audible during those exciting times. The French also had a
> Band-I service whose audio was at 41.250 MHZ plus or minus offsets.
> More often than not, both the British and French television audio were
> simultaneously receivable so it is good that they weren't on the same
> exact frequencies.
> If you consider the distance between Oklahoma in the middle of the
> continental US and the UK or France, one would expect the propagation
> to be almost identical, but the path tended to favor the French
> system. Either their transmitters were run at a higher power level or
> the angle of propagation for the signals was more favorable to France
> than to England. Also, various transmitters in the UK would be
> booming in loud and clear while others were barely audible.
I have no idea about the power levels of the French transmitters on
these frequencies, but the Crystal Palace (London) BBC transmitter ran
with a vision ERP of around 200kW, and Divis (Northern Ireland) was
listed for 35kW ERP.
If I recall correctly, the sound transmitters were generally run about
10dB below vision, so you would have been looking at about 20kW ERP
from London and 3.5kW from N. Ireland on the 41.5MHz frequency.
Crystal Palace also used vertical polarization while Divis used
vertical.
There were several other BBC transmitters on ch. 1, although at much
lower power levels.
Mike Brown's transmitters webpage has a full list of the old
VHF/405-line British transmitters:
http://tx.mb21.co.uk/info/index.asp
> The audio, by the way, for both the British and French systems was AM
> or amplitude modulated. The video for at least the British system was
> 405 lines at 50 fields per second. A few hobiests in the US actually
> cobbled together modified monochrome television sets and tuners and
> were able to get scratchy images. I was told that without modification
> to the video circuits, the images were reverse polarity because the
> 405-line system used the opposite signal levels for black and white
> than do modern PAL or NTSC systems.
The 405-line system did indeed use positive video modulation. The
French at that time still had their 819-line system, with very wide
channel bandwidths. In fact the French channels had a degree of
overlap to fit them all in the band, with alternate channels having
the sound carrier above and below the video carrier. They used
positive vision modulation too, as indeed they still do with their
modern 625-line system.
Paul Coxwell
Eccles on Sea
Norfolk, England.
Paul Coxwell (paulcoxwell@tiscali.co.uk)