[alsa-devel] Maximum 50 dB gain in ALSA softvol plugin

[Ricard Wanderlof]

On Mon, 15 Jan 2018, Takashi Iwai wrote:

> OTOH, the amplification in softvol is really dumb, and such a high
> gain like 90dB is doubtful whether it's really useful.  As Jaroslav
> already suggested, we need a better setup to get more meaningful
> results.

Hi Takashi,

I didn't really get a definitive response to my elaboration last week 
regarding the maximum allowable gain value for the softvol plugin.

First of all, regarding the algorithm itself used, as it stands today it 
can handle a maximum of 90.3 dB gain without falling apart (as the gain 
factor is held in a signed integer). Changing the maximum allowable gain 
from 50 dB to 90 dB does not impact the algorithm or existing 
functionality in any way, it just changes the allowable maximum to 
correspond to the technical limit of the current algorithm. The limit is 
not visible to the user anyway until he tries to configure a max_dB that 
is above 50 (and then only in the shape of an error message). So from a 
purely technical viewpoint I cannot understand any opposition.

You mentioned that the amplification is really dumb, I'm not sure what the 
problem is, perhaps you could elaborate, essentially a gain is a 
multiplication with a gain factor, and that's what softvol does.

Another issue is if such a large gain is useful. I would say that is up to 
the user, and of course, if you have a weak signal to start with, with, 
say, only 8 bits of resolution, adding 90 dB of gain is going to mean 
there's a lot of noise bits there, but the signal quality is of course not 
worse than it was to start with, just louder, and that's how any signal 
(digital or analog) behaves when gain is added.

But the bottom line of course is that you are the maintainer. So what 
would you consider a reasonable maximum gain that's more than 50 dB. 70 
dB?

/Ricard
-- 
Ricard Wolf Wanderlöf                           ricardw(at)axis.com
Axis Communications AB, Lund, Sweden            www.axis.com
Phone +46 46 272 2016                           Fax +46 46 13 61 30