[Elecraft] Setting TX Levels for Digital Modes

[EricJ]

I've always suspected the computer sound card was a limiting factor in 
receiving digital modes. I recently bought the recommended EMU0204 for 
use with LP-PAN. While waiting for the parts to bring out the K2 IF, I 
tried the EMU 0204 in place of the computer sound card for JT65-HF. 
There was about a 3-6 db improvement in received signals. I could decode 
that much deeper into the mud than I could before.

I'm going to try your advice on the TX side now.

Eric
KE6US


On 6/27/2013 7:01 AM, Jim Brown wrote:
> 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
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