[GreenKeys] anyone using a two tone TU ?

[jhaynes@alumni.uark.edu]

Well you just triggered off an old-war-story event.

Back around 1960 I was reading about synchronous systems and how they
were a few db superior to asynchronous systems because the detector knows
when to expect the bit boundaries and thus can accumulate all the energy
in a signal pulse.  At the time we had pretty rigid FCC standards for
amateur RTTY, so I couldn't see any way to apply this practically.
(Collins had a thing called "predicted-wave" radioteletype operation
that was synchronous.)

By the mid 60s - I'll admit to being a little slow sometimes - I realized
we could transmit synchronously in 7.0 unit code, and it would be legal 
and copyable on an ordinary TTY.  The trick was just to receive the
7.42 code from the TTY machine, pump the characters out at 7.00 code,
and stick in a fill character whenever there was not a message character
waiting.  This still wasn't something I wanted to build out of discrete
transistor flipflops. A name for it is "compatible synchronous" since it
is a synchronous transmission compatible with ordinary async TTY machines.

Also about the same time I remember exchanging 
mail with Vic Poor when he and Irv Hoff were working with what became
the TT/L and TT/L-2.  We were talking about limiterless detectors and
that sort of thing.  In fact the idea of a "slideback" detector had
come up among the RTTY community and had been published in some T.U.
designs.

Around 1967 I put together on a breadboard a compatible synchronous
transmitter made of Motorola RTL integrated circuits, which at the time
were the cheapest kind you could get.  This worked, but I never used it
operationally because wires kept falling out of the breadboard and
it was very susceptible to RF from the transmitter.

Around 1970 we could get TTL integrated circuits that were pretty cheap,
so I built up a synchronous tranmitting converter out of them and put it
in a metal box and started using it on the air.  Bill Carver K6OLG was
going to build a receiver that would lock to the bit and character rates
and use synchronous detectors for the tones.  I didn't recall that he had
ever finished it; but he recently told me that he did and was able to
achive a synchronous lock.  Meanwhile I was using this thing on the air
and the people with slideback detectors noted that it made limiterless
operation work a lot better than random hand-sent keying.  The reason was
simply that there was activity in both the mark and space channels all 
the time, which allowed the detectors to set their thresholds properly.
With hand-sent keying the mark detector would work fine because there was
always some signal to set the threshold; but the space detector would
forget what the signal strength was and start detecting noise.  And
maybe some people liked the sound of the signal with characters happening
all the time, rather than the long lulls when the sender paused to think.

Meanwhile Vic Poor and Irv Hoff had discovered a patented circuit called
a "decision threshold computer".  This mitigated the problems of slideback
detectors somewhat by leaking some of the average level of the mark
signal into the space channel detector threshold.

Time went on, and we started to get electronic terminals and then
computer-generated RTTY.  Lots of these included "diddle" as an option,
not because of synchronous reception but just because some people
liked the sound of it and because it seemed to improve the copy if
the receiver was using a two-tone detector.  Besides the ST-6 and its
progeny there was a T.U. called the DT-600.  I used one of those for
quite a while, and still have it.  It is switchable for limiter or 
limiterless operation.  It seemed like some people liked "diddle" and
some found it annoying, so it was usually optional whether to send it.

Then in 1996 we got K6STIs RITTY software, perhaps the first of the
software that uses a PC and sound card to do detection using DSP.
At the time Brian published a couple of papers, one explaining why
diddle is a good thing (he can detect synchronously) and why wide shift
is a good thing (frequency diversity with limiterless reception).
I was really tickled to see this, because it does what I always wanted
to do with synchronous operation.  And since it's software he doesn't
need a strict 7.0 unit code; he can sync to anything.  I've been using
that, alternating with the newer MMTTY, and a few other things such as
FSK in the HAL PCI-4000 DSP modem.

My impression is that the limiterless detection, whether in hardware
or DSP, is better than limiter detection under certain conditions; but
under those conditions copy is pretty rough all around.  What I haven't
done, for lack of opportunity, is to try wide shift versus narrow shift
with the different kinds of detectors.

Of historical interest, there was a World War II-era machine using an
electronic timer to supply artificial start and stop pulses, after
detection.  This assumes the sender is sending from tape so that the
character rate is pretty synchronous.  I keep wishing I would run into
some WW-II vet who could tell me how much good the thing did in practice.



-- 

jhaynes at alumni dot uark dot edu