[pulseaudio-discuss] acceptable noise level for mic boost

Oleksij Rempel linux at rempel-privat.de
Fri Nov 22 09:34:06 PST 2013

Am 22.11.2013 17:48, schrieb Tanu Kaskinen:
> On Thu, 2013-11-21 at 16:42 +0100, Oleksij Rempel wrote:
>> Hello all,
>> i posted this email to ALSA, but probably PA devs have some ideas too.
>> ======================================================================
>> Hello Takashi,
>> what is acceptable noise level for mic boost?
>> My laptop has three levels of mic boost (+36dB). Level 3 is useless
>> since noise level will be about 0db. With level 1 i get -12dB noise, so
>> there is almost 12dB room for data... but it is not enough for normal
>> speech, which is about 20dB.
>> Are there any ALSA guide for HW designers, which will say some thing
>> like acceptable levels are 90-20dB?
>> There are some Skype and Microsoft prescriptions for certification.
>> For example M$ prescribe 18dB or more for speech to noise ration for
>> build in mics. It sound like good number to start.
>> In my case, i need to remove mic boost completely to guarantee minimum
>> 18dB SpNR. Suddenly i didn't found correct way to do it.
>> I will be thankful for some tip.
>> =====================================================================
>> So my question to PA devs. Do pulse audio need to know about noise level
>> of microphone? Will it help to make correct decisions? Especial for VoIP
>> and automatic gain control.
> We don't currently have any idea of the noise level, so we don't have
> code for adjusting our decisions based on that information either. So,
> I'd say that pulseaudio doesn't *need* to know about the noise level. Of
> course, if someone someday complains that pulseaudio does bad decisions
> because it doesn't take the noise level into account, then we would need
> that information.

Thank you for your response.
I have more questions:
Skype continuously trying to adjust record level. If there is a silence
it will try to push the input volume as close to 0dB (in pulseaudio
terms) as possible.
For my laptop 0dB(in PA) will mean some thing +68dB over base volume
(ALSA). The noise will reach 0dBFS.
Well, it wont kill any one, but it sound bad.

What 0dB mean in pulseaudio?
Why pulseaudio has capture volume range -130 - +11dB? Is it possible to
fit this gain to 16-bit PCM?

Now to my hardware. I can blacklist in the driver 2 "mic boost" steps
without introducing any regressions. Which will guaranty maximal noise
level = -12dBFS. But it is still not optimal. Best experience i had with
limit to -24dB. In this case i need to remove Mic Boost completely but
pulseaudio will fail to make a difference between internal and external mic.

What is best solution in this case? Fix pulse or leave at least one step
for mic boost?

The microphone on this laptop is a digital mic. Do it make sense to use
hardware amp? Or is it possible to get better results with software?


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