[RTTY] Bad AFSK

[Kok Chen]

Joe,

Referencing your original P3 capture (http://www.subich.com/BAD_AFSK.jpg), you can see a perfect 1 kHz relationship from the Mark tone at 14.095 MHz to the 2nd harmonic of the Mark tone at 14.096 MHz to the 3rd harmonic at 14.097 MHz.  It is quite obvious to someone with an ounce of engineering eye.

Unless you made up that spectrum maliciously (by using Photoshop, for example), there is absolutely no mathematical way for a receiver or spectrum analyzer, however poor its IMD is (or even if your gutters are rectifying the RF) to produce a perfectly aligned 1 kHz progression from an RF signal at 14.095 kHz. In German, one would probably say "unmöglich."

Since this is not yet April, I will preclude the "Photoshop" suggestion :-).

The only way to see what you had recorded is for the fault to come from the original audio chain at the transmitter, and if W1AW were using a 1000 Hz Mark tone.  Since the harmonics are on the high side of 14.095, I further postulate that they are using USB and thus using an AFSK Space tone of 830 Hz.   

They probably chose this questionable tone pair so they can also use a 1000 Hz PSK31 tone and not have to move their VFO when switching between digital modes.  

If I were a technical advisor, I would have suggested that they move their PSK31 offset instead to 2295 Hz, and use 2295 Mark/2125 Space USB, so any possible harmonics from RTTY and PSK31 would fall outside the SSB transmitter's filter. 

They could even use LSB (with 2125 Mark and 2295 Space) and moving PSK31 to 2125 Hz offset to match the RTTY Mark, since BPSK31 works equally well with USB or LSB with no software change whatsoever.

We will see tomorrow if W1AW has fixed this problem.  It would have been the third transmission for today and RTTY would be skipped it if there is no time to transmit in all three digital modes.  Tomorrow, it follows the MFSK16 transmission.

Thanks for raising the problem to W1AW, Joe.  They need to fix a problem, period.

73
Chen, W7AY