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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