[GreenKeys] Dtc

[Harold Hallikainen]

On Wed, March 16, 2022 10:46 pm, Gerry Block wrote:
> Harold
>   For your project you might look up the old patents held by Frederick
> Electronics regarding the design of the DTC known as the decision
> threshold computer. This was a rather complex rc circuit which tracked
> the peak (Dc) amplitudes of the mark and space channels and biased the
> output comparator (“slicer”) threshold to provide the most
> advantageous switching  point. In a bipolar design this would be 0v in
> perfect conditions.  But during selective fading it would move the
> threshold to as far as  .5 x the working channel peak amplitude.
> They did a lot of real research work in this.   Irv hoff and Keith
> Petersen took this design (which I think came from Vic Poor) into the TT/L
> design  and I think there are also some descriptions in issues if RTTT
> journal.
> Best
> Gerry
> AD6MC
>

Thanks! I'll do that. So far, my DSP TU works great on ITTY, but not that
great on over the air signals. I've recorded some over the air signal at
https://w6iwi.org/rtty/audio/ . I wonder how much selective fading there
is with 170 Hz shift. I'm going to calculate how far down one tone is when
a single reflection results in a complete null of the other tone. I also
plan on recording a two tone signal (mark and space simultaneously)
received on a web SDR and see how the tones track with fading. I'm also
going to add high speed UART output so I can get samples of various
signals inside the chip. I'll look at the patents on dynamic threshold
control. It SEEMS like perhaps the discriminator output could be run
through a low pass filter and divided by two to get a threshold. With hand
typed data, where the data spends most of its time in mark, the threshold
would tend to be the mark level divided by two. I did a closed captioning
system for movie theaters where I was transmitting 10 kbps data
continuously on FSK over IR. There, I used a couple diodes, resistors, and
capacitors to get the positive and negative extremes out of the
discriminator, took the center of those two voltages, and used that as the
threshold. That would be similar to the above LPF, though it used peak
positive and negative instead of the average the LPF would give, and the
continuous data was better than "hand typed."

Besides selective fading, there is also the transmit and receive bandpass.
This can result in the space tone having a lower amplitude.

I note also that my TU-170 uses several 2 pole BPFs in series. So far, my
DSP TU has just a single 2 pole BPF for each tone (with a Q of 50).

So, lots of stuff to play with!

Harold


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