Topband: PSK-31

[Jim Brown]

On Tue, 27 Apr 2004 09:22:40 -0400, Joe Subich, K4IK wrote:

>Folks would be better served by keeping the sound card output down 
>by one third to one half and using a smaller pad (ca. 10 dB).  

As the old saying goes, "it depends." In this case, it depends on where
the sound card clips/distorts, how much RF you might have floating
around your shack, and providing immunity to hum/buzz that might be
flowing on the shield of the cable between your PC and your rig.  

3% THD corresponds to the harmonics being 30 dB below the fundamental.
Most sound cards ought to be at least that good unless they're at clip,
especially if there's no load on them, and a rig is a high impedance
load. 

I work in the pro audio world, and one the most common problems I see
is folks running their systems 20 dB below clip and wasting headroom.
The general rule is that you want system gains to be set so that
everything hits clip at the same time, and then run just below clip,
even on the hottest peaks. Since a PSK signal is essentially a constant
level, you don't have to waste headroom allowing for peaks. By
contrast, speech needs 14 dB or more between average and peak levels. 

If you run the PC 10 dB hotter and it's still clean, you've gained 10
dB with respect to hum, buzz, and RF feedback.  Granted not all sound
cards are. :) 

RF immunity is a consideration for me -- my 160 antenna puts a current
node in my shack. 

Jim K9YC