No, this page isn’t about jazz musicians getting together after hours to play for their own pleasure and enlightenment far into the night. It’s something more sinister!
It’s to do with how you block out an undesired radio transmission. You can’t actually ‘neutralise’ it, as such… what you have to do is transmit another powerful signal on the same frequency from as near as possible to the enemy transmitter, which will overlay and render the original signal incomprehensible. Assuming this original signal was speech, your success in obscuring it is very much enhanced by what you transmit. If you were to transmit a single tone for example, people might be able to ignore it & still hear the original signal, albeit less satisfactorily.
No; what we need is some broad-band noise or variable-tone signal to truly make a mess of what is underneath. I daresay there’s quite a bit of ‘Jamming’ still going on now, but the four samples of it you can hear on this page are quite old, as they date from the 1940s, and probably during & after WW2 (1939 - 1945). To hear them just click on the picture above, and you’ll hear a 15-second extract of that disc. The sound is uniform throughout the disc. They were almost certainly recorded off the air; the discs are 10” (25cm) shellac pressings.
There doesn’t seem to be any programme material underneath… this would make sense; if you wanted to study the jamming signal, you would want it ‘by itself’, not jumbled and garbled together with whatever it was supposed to be blotting out. So these discs were presumably recorded after the underlying transmission had ceased… or of course, before it had started. Any information as to date, source, frequencies they were used on, would be much appreciated and will of course be added here.
How do I come to have these two examples of radio arcana? The answer is prosaic in the extreme: I found them in a junk shop here in Birmingham, some time in the 1960s!
To see a larger picture of the labels, click the links below….
Re-formatted 20th December 2015.