[GreenKeys] FSK Intercept equpment (was something about FRF)

[Duncan Brown]

On 09-Feb-14 20:24, Jim Haynes wrote:

> Mention of sending messages on two different tone frequencies - doesn't
> the U.S. MD-522 do something like that in one of its modes?
>
The MD-522 (used in the AN/GRC142 & AN/GRC-122 HF RTTY systems starting 
about 1968) was basically a single channel FSK converter, operating at 
85 or 850cps shifts. Unlike previous military FSK converters, it was 
AFSK and designed for an audio interface with a SSB radio.  However it 
had a couple of special modes.  One was voice plus TTY, were the TTY was 
sent with 85 cps shift in the high end of the audio band, around 2800 hz 
and there was a LPF or notch filter to keep the AFSK out of the voice 
audio.  The other was a frequency diversity system within the audio 
passband. This used 85hz shift, one around 2800hz and the other lower - 
around 1000hz?

The Russian DFSK system used four tones, but only two were on  at a 
time. It was sort of like two normal FSK signals on top of one another, 
but using different tones. So one transmitter could send two RTTY 
signals at once. (I don't know what, if any, advantage there was over 
sending multiple AFSK signals in the sidebands of an ISB radio like the 
US did.)

Duncan
K2OEQ


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