[RTTY] Some basic RTTY radio questions

[Kok Chen]

On Mar 7, 2016, at 3:49 AM, Bill Turner wrote:

> Isn't constant better? Are you saying a decoder does a better job when
> the tone amplitude is varying rapidly? Hard to believe but I am
> willing to listen to arguments. 

If you are willing to manually ride the RF/IF gain controls, "AGC off" is best.

As David G3YYD has pointed out, you need the "gains" of the Mark and Space tones to be perfectly equal.  Under poor SNR but good propagation conditions, 0.5 dB of imbalance will cause noticeable harm in the error rates.

Basically, you want the gains between the mark (M) and space (S) bits to be constant.  The strength of the composite signal (M+S) need not be constant.

Together with proper filters (narrow enough to avoid QRM while adding no intersymbol interference), slicing (deciding whether mark or space has arrived) is an equally important aspect of FSK demodulator design.  You can easily make the case that the slicer becomes more important when conditions are poorer.

The slicer decides whether the mark signal or the space signal is greater at each bit period.

Good demodulators take care of slicer imbalances by the use of "automatic threshold correction" (ATC) circuits or software code.  You can also use FM techniques to get around mark/space imbalance, but that creates more problems that it solves -- that is why good demodulators nowadays use two individual "AM" demodulators.

It is always best to present to the demodulator with a signal that has as little possible tone imbalance so that the ATC has the least amount of work to do.  

This way, you minimize the problems that the demodulator has to overcome.  

Thus, you would rather have AGC that does not keep the amplitude perfectly constant, as long as the two tones have the same amplitudes.   Remember, the key is to have no imbalance.  The two tones must fluctuate by the same amount. 

Good A/D converters (sound cards) provide dozens of dB worth of dynamic range to handle fading.  Just keep remembering that RTTY demodulation depends on SNR and not on signal strength.  Receiver requirements are very different from voice or CW modes.

The ATC circuit has to work really, really hard (and fails often) when the AGC is fast enough to be affected by the tone amplitudes fluctuating independently.  The AGC time constant must therefore be much longer than a bit period.  Even an AGC time constant that is around 176 ms (character period of RTTY) already pose problems.  

Thus "AGC off" is the best, and if you are not willing to constantly ride the RF gain control, the slowest AGC time constant possible is the next best choice.

Use a A/D converter with good dynamic range, and let the demodulator designers handle the rest for you instead of depending on the receiver designers and their AGC circuits (few of them are designed with RTTY in mind).

There really should be two channels from a receiver -- one that uses no AGC, and is fed to the demodulator.  The other is a channel with AGC that goes to the human ears.  That is how I embed an RTTY demodulator into my own SDR program.  With floating point arithmetic, the channel that is fed to the demodulator has practically unlimited dynamic range. 

73
Chen, W7AY