[Elecraft] Setting TX Levels for Digital Modes

[Jim Brown]

On 6/27/2013 3:42 AM, Joel Black wrote:
> Splattering *usually* (I say usually because *always* is too definite) 
> has to do with TX audio gain. 

Actually, distortion can be created almost ANYWHERE in the signal chain. 
With digital modes, that starts with the COMPUTER.  In general, the 
sound quality of computer sound cards is mediocre at best. In a GOOD 
audio stage, distortion should be low until the stage clips -- that is, 
until the signal approaches the DC supply "rails" -- but in many 
computer sound cards, distortion rises with signal levels of only half 
of clip level. Putting numbers to it, typical sound cards clip at around 
1 volt, but their distortion begins to rise at 0.5 volt or even lower.  
I measured distortion at -40 dB with 0.5 volt out of my Thinkpads, and 
-30 dB at a level just below clip. That difference produces sidebands 
that are almost 2 S-units stronger!

It's pretty easy to set audio levels, but it's VERY important to do that 
right.  You can do it with a scope or a voltmeter on the output of your 
sound card, or you can do it with your ears listening to headphones 
plugged into the sound card.  With the voltmeter, simply increase the 
sound card output until it no longer produces more voltage, then reduce 
the computer output to a bit less than half that voltage. So if max out 
is 1 volt, reduce it to about 0.4 volts.

With headphones and your ears, increase the computer output until you 
hear the sound of the digital tones start to get raspy, or harsh -- that 
is distortion.  Back the computer gain down slightly until the 
raspyiness goes away, then reduce it further until the tones SOUND half 
as loud to you. This works because we humans hear a change of 6-10 dB as 
being half as loud.

Once you've got the computer as clean as it is going to get, simply 
follow the instructions in the K3 or KX3 manual to set levels in the radio.

Another observation.  A few years ago, I surveyed low cost USB sound 
cards sold for use in the home studio and DJ market, found several that 
looked promising, and bought two of them. Both made significant 
improvements in the DECODING of digital signals, and also cleaned up the 
computer's TX audio.  These were NOT expensive units -- a little Numark 
unit ($35 at B&H Photo) worked just as well as a $75 Tascam. And, unlike 
the more expensive SignalLink, which has Pin One Problems that cause 
RFI, the Numark and the Tascam are free of these problems.

73, Jim K9YC