[GreenKeys] Understanding the Dovetron demodulator

[Jeffrey D Angus]

On 2/13/2013 10:18 AM, Jim Haynes wrote:
> at least the FSK portion of the set.  A good place to start is with
> the patent, 4,013,965.
In short, it decodes only the leading edge of the separate mark
and space tones, and uses those edges to change the state of
the output.

I remember running 850 Hz shift and watching the "Selective
fading" of the two different tones. At 170 Hz shift, the effect
is less pronounced as both tones tend to fade equally.

"Automatic threshold control" was a means to counter the
effects of fading, both dual and selective.

leading edge decoding of the individual mark and space tones
was the "next step" in removing distortion from the recovered
signals.

An interesting effect. The "stretching" of the mark or space
signals due to propagation effects. In essence you have times
that both the mark and space signal are present at the decoder.

Back in the early '70s when talking about spread spectrum
could get you Federal prison time, having ALL the coded bits
arrive at the same time was a critical factor in putting them
all back together.

The measurement of the systems involved was called "Group
Delay" where, hopefully, all bits would be delayed by the same
amount and remain "in phase" with one another.

Thanks Jim for posting this.

Jeff-1.0
wa6fwi