[RTTY] AFSK to FSK converter

[Kok Chen]

On Sep 17, 2012, at 1:06 PM, Doug Faunt wrote:

> And he may have been unaware of the OOK to FSK hack (which would work with AFC on) and should be trivial to implement, just set one of the AFSK output frequencies to zero.

Hah!  I never thought of that -- turning AFSK into OOK by turning one frequency of the tone pair to 0 Hz.

There could be just a small side effect: if the transmit AFSK tones are generated in a phase continuous manner, the digital output to the sound card will not be zero, but instead some constant DC level, until the OOK tone resumes.  

Sound cards don't put out DC (thus the "DC hump" problem with SDRs) and who knows what they do with the DC during those OOK transitions.  The OOK decoder may end up with a small random jitter that varies from bit to bit.

There is another potential problem.  If the tones are being generated digitally by some DDA (digital differential analyzer) fashion, the delta-T value for 0 Hz would cause an overflow.  I guess this can be mitigated with the use of 1 Hz instead of 0 Hz, and let the sound card suppress the 1 Hz signal.

73
Chen, W7AY