[Elecraft] [K3] AGC White Paper

[Jim Brown]

Wes and Ed,

It's important to realize that in the K3 and later Elecraft rigs, all of 
this is happening in the digital domain. One thing we learned about DSP 
in pro audio is that dynamics processing burns a LOT processor cycles. 
DSP SIMULATES the behavior of analog circuitry, but depending on the 
code and the processor, the quality of the emulation can be varying 
degrees of ideal.

ALSO -- those of us who have worked in broadcast audio know that 
dynamics processing can be a VERY complex business, especially when 
you're trying to do a lot of it, and even with a budget far larger than 
in a ham rig. Even in analog, it's tricky to do a LOT of gain change and 
still sound good. A handful of designers were good at it, and made a lot 
of money for themselves, or their employer, or both.

And I'll repeat my previous observation that a signal path with gain 
that varies with time or with the signal amplitude is NOT a linear 
device, and non-linearity => distortion.

73, Jim K9YC

On Tue,3/7/2017 1:53 PM, Wes Stewart wrote:
>  AGC adjusts the receiver gain, compared to the audio bits, rather 
> slowly and DOES NOT change the "dynamic range."
>
> If you're listening to a signal and it's too loud for your ears and 
> you turn the audio gain down 10 dB, does that change the amplitude 
> ratio of any signal you're listening to?  No, it lowers everything by 
> 10 dB.  AGC is no different, it just operates at another location in 
> the radio.
>
> IMHO, this thinking is the result of looking at too many ACG slope, 
> threshold, and phase of the moon charts and assuming that bigger 
> signals have their gain reduced more that smaller ones at the same 
> instant in time.
>
> Wes  N7WS
>
> On 3/7/2017 2:19 PM, Ed Muns wrote:
>> Decoders operate on the audio stream presented to them from the 
>> receiver.
>> The decoding algorithms use the difference in audio level at each 
>> instant in
>> time to make bit decisions.  AGC reduces those differences and thus
>> increases the bit error rate.
>>
>> Of course, it is important to adjust the absolute audio levels so 
>> that the
>> dynamic range of the signal(s) is placed within the dynamic range of the
>> decoder.  But, compressing the dynamic range, i.e., AGC, increases 
>> the error
>> rate of the decoder.