[Milsurplus] overheard telephone conversations

[Al Klase]

OK.  It's time to read the SIGSALY article on Wikipedia: 
https://en.wikipedia.org/wiki/SIGSALY

They mention that the existing voice security on international 
radio-telephone circuits was "A-3."  Do any of you have a handle on A-3?

Other clues from an old phone company radio guy I know:

     The rearranged analog sub-band scheme may be a Lenkurt product.

     AT&T may have used frequency inversion on some channels, making 
them unintelligible to the casual eavesdropper.

Interesting thread.

Al

Al Klase – N3FRQ
Jersey City, NJ
http://www.skywaves.ar88.net/

On 5/31/2016 2:06 AM, AKLDGUY . wrote:
> I'm highly skeptical. The only way frequency slicing like that
> could have been done in 1937 was by translating the entire
> audio band up to some much higher frequency, then using
> highly selective crystal filters to pick out each slice (say 3 of
> them), then translating each one to a segment of another
> baseband, then translating that baseband back to audio.
>
> Given the state of crystal filter design in the 1930s, I think
> even the telecomm industry would have balked at that level
> of complexity.
>
> 73 de Neil ZL1ANM
>
> On 5/31/16, Rob Flory <farmer.rob.flory at gmail.com> wrote:
>>   "I don't know whether SSB was
>> used for those 1937 radio telephone links, but if it was,
>> an enthusiast may have figured out that a BFO would
>> allow reception."
>>
>> You can be sure an enthusiast would have intercepted the signals if they
>> were simple SSB without encoding, and no one would have counted on the
>> thousands of enthusiasts, never mind the commercial and political
>> interests, from figuring that out.
>>
>> If I recall correctly, the baseband audio was broken up into several slices
>> that were shuffled, as in 0-600Hz was re-inserted at 1200-1800 etc. and
>> that shuffle was changed frequently according to a pseudorandom sequence
>> based on "one time pads" known only at TX and RX locations.
>>
>> The transmitting station was just a few miles from here.
>>
>> RF
>>
>>
>>
>>
>> On Mon, May 30, 2016 at 9:21 AM, AKLDGUY . <neilb0627 at gmail.com> wrote:
>>
>>> How were they encoded?
>>>
>>> Be careful how you answer. You're addressing a former
>>> telecommunications technician, 1968-1984.
>>>
>>> When I was transferred into the long distance transmission
>>> division in 1975, there was a very old Western Electric
>>> system that had been in use back in the 1930s and was still
>>> being kept running as a standby in case of emergency.
>>> It was junked not long after, because by that stage we had
>>> enough diversity with multiple routes such as coaxial cable
>>> and microwave links out of Auckland.
>>>
>>> It was an industry-standard 60-108 KHz carrier system of
>>> 12 channels at 4 KHz spacing designed for open wire 600
>>> ohm line.
>>>
>>> Essentially it was an upper sideband broadband system,
>>> with crystal filters for each direction of speech, 24 filters
>>> in all. Each filter was in a sealed (soldered) copper case
>>> about a foot long, 4" wide, 5" high. I can't remember what
>>> tubes it used, but they were big glass ones like in radios
>>> of the early 30s.
>>>
>>> That was probably the state of telecommunications
>>> technology in the 1930s. I don't know whether SSB was
>>> used for those 1937 radio telephone links, but if it was,
>>> an enthusiast may have figured out that a BFO would
>>> allow reception.
>>>
>>> So I'm interested to hear what kind of encoding you
>>> think was used.
>>>
>>> 73 de Neil ZL1ANM
>>>
>>> On 5/30/16, Rob Flory <farmer.rob.flory at gmail.com> wrote:
>>>>   "Radio telephone communications opened in 1927 on a 24 hour
>>>>   basis, US-UK. By 1949, there were 70 radio telephone circuits for
>>>>   all 5 continents."
>>>>
>>>> They were encoded, so an enthusiast would not have been able to
>>>> overhear
>>>> them.
>>>>
>>>> RF
>>>>
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