[GreenKeys] Comparing TU demodulators

[Harold Hallikainen]

As previously mentioned, I've been playing with making a DSP terminal
unit. Since I am most familiar with programming PIC32 microcontrollers, I
am using a PIC32MZ. So far, I have duplicated a Flesher TU-170 demodulator
in software (close,anyway).

The DSP TU looks like this:

1. 12 bit ADC sampling at 8 kHz

2. Convert 12 bit integer to double with range of -1.0 to +1.0.

3. Clip or limit the signal using the copysign function. If the input
negative, the output is -1.0. If positive, it is +1.0.

4. Limiter to mark and space tone filters, each 3 stages of biquad BPF
with an overall bandwidth of about 83 Hz.

5. Full wave rectify tone filter outputs using the absolute value function.

6. Rectified tones to data low pass filters with a cutoff of 45 Hz.

7. Loop is keyed Mark if the MarkLpf-SpaceLpf>0, space otherwise.

I compared this DSP TU to the Flesher TU-170 by recording 100 lines of the
Quick Brown Fox at the KFS SDR, then running it through each demodulator
and counting how many lines are perfect. The received audio has fading,
selective fading, noise bursts, and QRM.

I've summarized the results along with some more filter data at
https://w6iwi.org/rtty/TuComparison/ .

It would be great if people could test this on other TUs and give me a
count of how many perfect lines of text there are. Also, details of other
TU demodulators would be interesting (limiter or not, AGC, BPF in front of
limiter, tone filter details or other demodulation methods, etc.).

My rather disorganized notes on the DSP TU are at
https://w6iwi.org/rtty/DspTU/ .

THANKS!

Harold
https://w6iwi.org