[RTTY] The Problem with AFSK ...

[Kok Chen]

On Jan 19, 2013, at 10:51 AM, Joe Subich, W4TV wrote:

> Doesn't mean a thing if the level going into the mic jack is
> already too high for the microphone preamplifier. 

This is why I always advised against using the mic input for AFSK.  By the time you saturate the mic pre-amp, it is game over.  You are most likely to present a non-symmetrical signal to the balanced modulator, and we know what that does.

The other, less obvious (but equally important) reason is this: the cable between the sound card and the radio now has to carry a tiny signal.  This means that any noise and hum pickup is much larger by proportion.  

To do good AFSK, send as large a signal as possible on the cable to the radio and if needed, add two resistors to attenuate it at the radio itself.  But be sure not to saturate any transformer that is in the large signal.

On something like the Elecraft K3, as example, use as large a line level in the menu as your sound card will deliver cleanly.  Typically, I have found that -3 dB of the sound card full scale output is a good value to use.  You then adjust the line level menu in the rig to accept that.  Only back off the sound card output if that is still too much for the line level menu to compensate.

Even on something like the Kenwood TS-590 with its built in sound card, you should still use a good sized signal from the modem, just in case there is ground noise within the rig itself.

It is really not hard to produce good AFSK, as Andy K0SM's spectrum captures showed.  All those clean AFSK signals in Andy's spectra showed that it is not just theoretical possible, but it is quite practical to produce pleasant AFSK RTTY signals that are friendly to neighbors.  And quite a few contesters are using it.  With something like a K3 or a TS-590, there is almost no reason not to.

Like I have personally discussed with a few folks, the problem with FSK keyclicks is that we have to wait for the manufacturers to get off their behinds and do something about it (just look at how long it took Yaesu to remove CW keyclicks from their contest grade rigs).  It is not going to happen overnight, even though it is really simple to accomplish for rigs that generate FSK signals in their DSP stages.  In the meantime, we already have means today to generate a clean signal instead of waiting on manufacturers.  With CW keyclicks, responsible people also took matters in their own hands and modified the FT-1000s and other rigs to make them acceptable.

73
Chen, W7AY