Hi Eric,
Also see what the software is looking for in a sound card. I know in WSJT, you
should enter in the sound card correction factors shown in the lower left corner of
the screen. Also, on WSJT, the software was written for a sampling rate of 11025
Hz. So for best results, you should select a sound card sampling rate of 11025, or
some multiple thereof (such as 44,100 Hz). GL and VY 73, Lance
On 6/27/2013 5:14 PM, EricJ wrote:
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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