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

[Takashi Iwai]

On Mon, 15 Jan 2018 14:14:13 +0100,
Ricard Wanderlof wrote:
> 
> 
> On Fri, 12 Jan 2018, Takashi Iwai wrote:
> 
> > > The ALSA softvol plugin has a maximum gain of 50 dB (MAX_DB_UPPER_LIMIT is 
> > > fixed to 50 in pcm/pcm_softvol.c). Is there some technical reason for this 
> > > limit? These days when >16 bit audio is becoming common, it's not 
> > > unreasonable to have large gain figures, and that is in fact a usecase I 
> > > have right now.
> > > 
> > > I'm looking through the CONVERT_AREA and MULTI_DIV_foo macros to see if 
> > > there's something there that degenerates when the gain factor becomes too 
> > > large, but I'd thought I'd ask too rather than just do reverse 
> > > engineering.
> > 
> > There is no more reason than "some reasonable limit".
> 
> 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?


Takashi