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

[Takashi Iwai]

On Mon, 15 Jan 2018 14:49:38 +0100,
Ricard Wanderlof wrote:
> 
> 
> On Mon, 15 Jan 2018, Takashi Iwai wrote:
> 
> > > Ok, my suggestion would be set MAX_DB_UPPER_LIMIT to 90 dB. 90 dB comes 
> > > out as 31622, which still is manageable when shifted up by VOL_SCALE_SHIFT 
> > > (= 16), fitting in a 32-bit int. Thus it works in MULTI_DIV_short() too, 
> > > where it is multiplied with the 16-bit sample value yielding an int.
> > > 
> > > I'll try it out and prepare a patch.
> > 
> > Well, but +90dB gain doesn't look like a normal usage pattern. Why do 
> > you have to use softvol plugin for achieving that specific purpose?
> 
> The usecase we have actually calls for 55 dB of gain maximum, but I was 
> thinking that looking at the code the maths can actually handle 90 dB so 
> it would be good 'reasonable limit' - it's a nice round number and fits in 
> 16 bits (signed).
> 
> The usecase we have is using digital MEMS microphones ((2S connected, 
> with no integral amplification) and when the sound source is far away, a 
> fair amount of gain can be needed.

But why do you need 90dB?  "Because it can" is no good reason to
increase such a safe limit.


Takashi