[Milsurplus] overheard telephone conversations

[John Vendely]

On 5/31/2016 6:50 PM, AKLDGUY . wrote:
> The consensus seems to be that WW2 necessitated encryption
> and spurred on its development, but there's nothing to suggest
> that 1937 radio telephone calls were made unintelligible to the
> casual HF listener.
Those circuits using the aforementioned A-3 would have been 
unintelligible to and virtually unbreakable by the casual listener. A-3 
was commonly used
by Bell for commercial traffic, not just military.  So far as I can 
determine, in the late thirties Bell was using both SSB and AM on 
transcontinental HF telephone circuits.
I suspect they used mainly SSB for their own circuits, and may have used 
full carrier DSB AM where needed to be interoperable with other systems, 
though I'm speculating here.

Needless to say, Sigsaly mainly used SSB; it probably was almost a 
necessity, as this system operated with essentially zero margin. It's 
really remarkable that it worked as
well as it did.  I seem to remember reading somewhere that on a few 
Sigsaly circuits DSB AM was used as an expedient (perhaps on the 
shipborne systems?), but I can't be certain.  I've seen very little 
written about the radio links themselves.  I wonder if anyone else has 
found anything on this?

I do recall hearing some conventional AM international telephone 
circuits on HF into the late 1960s and early 70s, though most everything 
was SSB by then.
One AM system in particular was said to be between Cuba and the USSR, 
and used some kind of speech inversion technique.  I remember hearing 
this system frequently
from the mid 1960s, and upon occasion maybe as late as 1975.  It had a 
peculiar channel marker mode in which an odd little electronically 
synthesized melody with a reedy
sound reminiscent of bagpipes played endlessly in the absence of 
traffic.  The carrier would drop momentarily between repetitions of the 
melody.  Any one else remember
hearing this thing?

73,

John K9WT


>
> It doesn't even seem clear whether SSB was used. Terrestrial
> telephone links certainly used upper sideband in broadband
> Frequency Division Multiplex systems pre-war, as I pointed out
> in an earlier post where I described the Western Electric crystal
> filters. I just now recalled that those filters had the ring modulator
> or demodulator (depending on speech direction) built-in, together
> with impedance matching transformers.
>
> 73 de Neil ZL1ANM
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