80-Bus News

  

January–March 1982, Volume 1, Issue 1











Page 39 of 55











CB

Breaking Computers D. Re Hunt

MELAS SS A ES

Somewhere in this issue Dr. Dark mentions the use of CB for data transmission. The editor pointed this out to me, knowing my interest, and suggested I might like to write a reply. I, like a mug, said I would, as it would be the only words from my Nascom in this issue, part 6 of the Dodo’s guide being only half written at this stage. No, I haven’t bowed out because I committed a ricket in the last Dodo’s Guide, that one was a major disater. If you’re one of the 5,000 odd people who DIDN’T write to me to tell me, you’d better read issue 5 again. Anyway, I set out to examine the practicalities (or otherwise) of the idea of using CB for data transmission.

Well first of all lets look at the snags. The major snag to the idea is that under the terms of the CB licence, it’s illegal!! Paragraph 6 of the expensive piece of blue paper says:

"6. All transmissions other than selective calling signals or digital transmissions designed solely to identify the transmitter, shall be in plain speech only." ‘

If being outside the law doesn’t dampen the enthusiasm let’s find a few more areas of trouble. Effective range, I haven’t done any calculations to find out what would be a tolerable noise level, but for acceptable data recovery without the use of clever and complicated equipment, the received transmission must be at least reasonably free from interference. Now with the legal FM mode of CB transmission employed in this country this is slightly less of a problem than it would have been with an AM system, as the FM capture ratio ensures that if a signal is a minimum of 3dB better than any interfering signal then the required signal will be received to the reasonable exclusion of the interfering signal. Comforting news, so where’s the problem? Well around Dr. Dark’s ‘twenty’ (location or area for those uninitiated in CB slang) this may well not be a problem. I have heard reports of interference free reception over fifteen miles out in the sticks, but around North West London, fifteen metres is nearer the mark. Competition for channel space is both fierce and ruthless to the exclusion of good radio operating practice and good manners. With the exception of channel 9, the voluntary ‘Emergency channel’ (which round our way remains remarkably and inexplicably free), each channel may have anything up to four Simultaneous transmissions at various receivable ranges going on at any one time throughout the 24 hours.. This limits the practical range, bearing in mind the effective capture ratio effect, to about two miles radius. I know of about seven Nascom owners in my area which would be within acceptable range but I think my situation would be the exception rather than the rule.

Now we come to the ungentlemanly behaviour of a minority of the local “breakers” (CB users). I have to say ‘minority’ or I’’1l be drummed out of the Lima Bravo Breakers Club. (The LB club members, myself included, consider ourselves ai cut above the rest, being firstly ‘more mature in age” -and secondly, being mainly engineers, bona fide radio amateurs, and the like.) In my opinion, round our way the ‘minority” is about 70% of the whole. I don’t need to try it to surmise that any attempt at data transmission round my way would cause the more antisocial elements within the fraternity to commence jamming operations within minutes, or at the very least, it would result in numerous well intentioned but uninformed ‘breakers on the side’ (someone trying to get in on the transmission) coming up trying to find out what the heck was going on. In either instance, with it’s attendant effect on the degradation of the signal.

So, for the determined, how may these problems be tackled. Well the first obstacle is the legality, plainly it would be illegal, and far be it from me to incite anyone to break the law, so what follows are my thoughts, AND ONLY MY THOUGHTS on how the other obstacles may be overcome, sadly, without due regard to the law. It’s obvious from the above lots of power would be an advantage and that some disguise must be adopted to avoid deliberate interference.

Apart from the illegality of sending data over the air, it’s plainly obvious that other conditions of the CB licence are being openly flouted round my way, judging


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Page 39 of 55