[RTTY] AFSK Splatter

[Richard Ferch]

W3NR wrote:


>Then is it better to set up the sound card level first or the rig. ?

I would suggest neither. They should both be set so as to distribute the 
gain to avoid overdriving (audio harmonics or IMD) or excessive 
noise-to-signal ratio in any one stage. Don't run your sound card's gain 
all the way up (or down), and don't run your radio's mic gain all the way 
up (or down). This may require you to go back and forth between the two 
(and the attenuator setting in your interface, if you have one).

Avoiding non-linearity throughout the entire signal chain (not just in the 
ALC) is essential for multi-tone modes like PSK in order to avoid 
intermodulation distortion. In the case of RTTY, IMD is usually less of a 
problem than for PSK, but spurs due to harmonic distortion are just as much 
of a problem in RTTY as they are in PSK (BTW, I have seen PSK signals with 
exactly the same kinds of spurs as people are talking about here). Avoiding 
non-linearity helps prevent harmonics as well as IMD.

There is a mitigation technique that is generally ineffective against audio 
IMD, but can help to reduce harmonics introduced in an earlier stage. This 
technique is filtering. The simplest way a user can make use of this is to 
use "high" audio tones (2125/2295 Hz), so that the radio's IF filters will 
attenuate audio harmonics.

BTW, FSK is *not* automatically better than AFSK. AFSK does indeed give the 
operator a lot of opportunities to screw up. Especially using low tones, 
you can generate harmonics which the radio does not filter out, by 
overdriving in the sound card or in the radio's microphone amplifier. 
However, AFSK also allows the software designer to use digital techniques 
for filtering and shaping to resolve problems over which the FSK user has 
little or no control.

For example, if the radio's design is inadequate, it can generate keying 
clicks. One of the most popular contesting radios available today generates 
significant key clicks in CW, and the same kind of fault is in principle 
possible in FSK. Of course, AFSK software can generate keying clicks too, 
but it's my impression that the writers of amateur radio software are much 
more amenable to making changes to remove this kind of design fault than 
the designers of radio hardware have shown themselves to be.

73,
Rich VE3IAY





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