[Elecraft] ALC implementation

[Jim Brown]

Right.  Almost ten years ago, I used an audio FFT to measure the 
harmonics in a Thinkpad's line level (also used for headphones) output. 
The distortion decreased by 10 dB when the output was first set just 
below clip and then reduced by 6 dB (half the output voltage).  Just 
below clip, the second harmonic was -30 dB; at half that voltage out, it 
was -40 dB. To put this in perspective, Thinkpads are better than 
average laptops.

This is the basis of advice I first published around that time to first 
set the output level of sound cards 6-10 dB below full output and then 
set input level in the rig. There are app notes on my website that 
describe three different ways to do this, depending on your available 
tools, one being our ears!

The "ears" method is this.  Plug headphones into the computer output 
jack, have the computer generate the tone(s) your going to transmit, and 
increase the output level until you hear the tone(s) sound a bit 
rougher, then back off the output of the computer until the tone(s) 
sound half as loud. This works because our ear/brain perceives a change 
in loudness of 6-10 dB as "half (or twice) as loud."

73, Jim K9YC

On 6/28/2019 2:29 PM, Joe Subich, W4TV wrote:
>
> Among other issues, this article is incorrect when calling for the
> "Device" (Master) volume level in Windows to be set to 100%. Nearly
> every sound card for which I've seen test results has increased
> distortion above the 70 - 80% level (-1 to -2 dB or -3 to -6 dB
> depending on the driver calibration) and some "inexpensive" devices
> have significant distortion above 50% (-3 or -10 dB depending on
> driver calibration).
>
> One will have a cleaner signal, all other things being the same, by
> setting the Windows "Device" (Master) slider down a bit and increasing
> the transceiver mic gain (Line In, DATA In, etc.) slightly.  In most
> cases, with the sound card "Master" at 70 - 80%, the transceiver mic
> gain will still be relatively low.
>
> 73,
>
>    ... Joe, W4TV
>
>
> On 2019-06-28 10:18 AM, Andy Durbin wrote:
>> Does anyone know of any rig that uses "compression" rather than 
>> linear gain control to implement ALC?
>>
>> ref - https://www.dropbox.com/s/f4uaop5tqwzaweu/FT8Noise6.docx?dl=1
>>
>> 73,
>> Andy, k3wyc
>
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