Comments form Peter, G3PLX the father of PSK31:                 18 Jan 2014
---------------------------------------------------------------------------

31.25Hz DETECTION:  I see that KA3WCA has picked-up on my suggestion of 
monitoring the 31.25Hz component of the uplink envelope in order to detect 
the presence of PSK31 activity in the uplink band. I did a lot of 
simulation work on this at the time and this was one of the features I 
thought would be very beneficial in a future transponder of this type. I am 
guessing that there is enough DSP capability in the design to be able to do 
this rather than build it in hardware. [WB4APR:  I hope to avoid DSP 
complexity on this mission]  I would be very keen to contribute ideas to 
this feature.

[WB4APR: KA3WCA's test showed severe 31 Hz fading] Yes in theory you could 
have two exactly equal-amplitude signals which have baudrates which differ 
by 0.01 Hz, and the detector would pull-in and drop-out in a 100 second 
cycle, but that's going to be a rare thing. Most of the time there will 
be a lump of narrow-band noise centred on 31.25 Hz if the uplink is active 
with PSK31 signals, and nothing otherwise. 

As you suggest, you detect the activity by a simply incoherent sum of the 
energy near 31.25Hz.  The 28MHz SSB receiver output will be AGC-controlled 
so it's r.m.s. level is constant (remember that I have suggested we do this 
to give the tx maximum mean deviation all the time) so this 31.25Hz detector
 should be very stable. If you want to experiment with this, I can write you 
a little program (or KA3WCA can) that will display the AM spectrum around 
31.25Hz of an SSB receiver output. You can leave it running on the 3 kHz slot 
where the PSK31 activity is on any HF band and see the effect, and compare it 
with how it looks when there is either only band noise or if there is some 
other signal present (e.g. a CB station or a fishing buoy). We could then add 
the detection algorithm and tweek it until we are happy and then hand it over 
to whoever will be writing the software for the on-board DSP chip.

[WB4APR: again, hoping for a hardware solution]

To shut the transmitter off out over an ocean where there is no activity,
a time-constant of several minutes would do that perfectly well. I suggest 
a detection bandwidth +/-0.25Hz would be enough. We can alert users to the 
need to check their soundcard samplerate calibration. Most PSK31 packages 
have an adjustment for this to take out the soundcard samplerate tolerance 
error. This not only makes sure the tone frequency readout is correct but 
sets the baudrate to exactly 31.25Hz.

73
Peter G3PLX