Re: AGC

[Fabien PELLET]

Hello,

After a long time of understanding all necessary points for creating an OOT (documentation is very very poor on some details like adding parameters, etc.), I manage to rewrite a new module AGC to test the solution proposed : compare tmp to 0 instead of "_gain" at line 141 of agc2.h.

Attack value works with this modification of the code. Maybe it could be added in a new version of gr-analog....

Best regards,

Fabien, F4CTZ.

Le 09/12/2021 à 14:52, Jeff Long a écrit :

Maybe "tmp" should be compared to 0, not to gain. It does look suspicious. Not sure what algorithm the author had in mind.

On Thu, Dec 9, 2021 at 8:41 AM Fabien PELLET <fpellet.f4ctz@free.fr> wrote:

Maybe I'm wrong but at line 141, it should be a division and not a difference ??? I'm telling that because in line 143, the result "tmp" is compared to the gain. There is a unit problem here I think : amplitude (result of the difference) can't be compared to a gain which do not have unit by definition....

I will try wider value for attack.

Le 09/12/2021 à 13:23, Jeff Long a écrit :

The change in gain is also proportional to the difference between the reference level and the signal level. So it probably is having some influence but it reacts more quickly. Try a smaller value for attack. Take a look at the source here

https://github.com/gnuradio/gnuradio/blob/35a242f1cd4b724be0c4708d8690f82c804aafd8/gr-analog/include/gnuradio/analog/agc2.h#L64

for more info. It may just not be a great algorithm, or may not work for the rates and changes in signal you have.

On Thu, Dec 9, 2021 at 7:07 AM Fabien PELLET <fpellet.f4ctz@free.fr> wrote:

Thanks for the explanation.

Do you have any idea on why the attack as no influence ?

Le 09/12/2021 à 12:58, Jeff Long a écrit :

Attack and decay are unitless per-sample multipliers, so 1.0e-2 gives you a time "on the order of" 100 samples. It's really an exponential attack and decay, but that gives you an idea. So, your observation is correct - a smaller value gives you a longer attack/decay.

On Thu, Dec 9, 2021 at 4:33 AM Fabien PELLET <fpellet.f4ctz@free.fr> wrote:

Hello,

I'm trying to work with the module AGC2 but I have some trouble using it.

First problem is : what is the unit of the parameters attack and decay ?
I thought initially that it was in second but after some test it is
clearly not in that unit. Indeed, if I increase the value of the decay
for example, the time of the increase of the gain at the end of a signal
will go down where it should go up.

Second problem (maybe in relation with the first) : the attack value
have absolutly no influence.... From 10-6 to 10-2, I can't see any
changes on the output signal.

Does someone have any explanation ?

Best regards,

Fabien, F4CTZ.