[Elecraft] Setting TX Levels for Digital Modes

[Lance Collister, W7GJ]

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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-- 
Lance Collister, W7GJ
(ex WA3GPL, WA1JXN, WA1JXN/C6A, ZF2OC/ZF8, E51SIX, 3D2LR, 5W0GJ, E6M, TX5K)
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