On Mon, 15 Jan 2018 15:32:13 +0100, Ricard Wanderlof wrote: > > > On Mon, 15 Jan 2018, Takashi Iwai wrote: > > > > 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. > > No, I don't need 90 dB. The specific usecase I have calls for 55 dB, so I > could of course just change the maximum from 50 to 55 dB. Or perhaps with > a bit of margin, such as 60 dB or 70 dB. > > They way I see it though, softvol is just an amplifier component. It's up > to the user to use it for whatever purpose they want. And there is no > inharent danger in allowing large gains, as the signal will just clip at > full scale, it's not like having a power amplifier with infinite power > resources. So the upper limit should be bound by something technical, like > an internal processing stage overflowing, which is why I suggested 90 dB, > as it is at slightly more than 90 dB that a 16 bit signed short overflows, > 16-bit shorts being part of the current set of calculations in the > MULTI_DIV_ macros. So 90 dB seemed to me a limit that is related to the > technical limitations of the algorithm employed. > > Note that I'm not suggesting changing the default gain range which I > believe is -50 to 0 dB. In order to utilize the large gain, someone must > purposefully enter a 'max_dB' value in /etc/asound.conf and then set the > gain to that value, so there would seem little risk of setting a large > gain by mistake. Sure, the default shouldn't be changed, otherwise we'll hit many complaints :) 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. Takashi _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
