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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