[alsa-devel] What does 0dB refer to? (Logitech USB Speakers)

[James Courtier-Dutton]

Lennart Poettering wrote:
> On Sat, 12.04.08 12:15, James Courtier-Dutton (James at superbug.co.uk) wrote:
>
>   
>>> I somehow expected that the magic "0dB" setting in ALSA would refer to
>>> some sane default, where the final output would have a sensible
>>> volume. In this case this is not so. I am thus wondering how ALSA
>>> actually defines 0dB? Is this a bug in the hardware that should be
>>> worked around in ALSA? Is this an ALSA bug?
>>>       
>
>   
>> The 0dB is a gain value.
>> So 0dB means that the input to the mixer element exactly equals the 
>> output from the mixer element.
>> So 0dB ensures no distortion will occur.
>> It is fine to set the 0dB for all output mixer controls except one.
>> The problem comes with the "Master" control, where 0dB generally means 
>> the maximum possible volume.
>>     
>
> In the case of the USB speakers there is no Master, only PCM. And it
> is that PCM volume that is the offender.
>
> My I thus assume that we can say that for all but the last element in
> the series 0db means avg input power equals avg output power, and for
> the last one 0db means max? That would fit on what you are saying but
> also match my usb speakers, not depending on the existance of a
> "Master".
>
>   
>> The correct setting of this is therefore very dependent on the type of 
>> amplification that is being done after the sound card master output.
>> Everyone's setup will vary. But, on my system a value between -40dB and 
>> -30dB seems about ok.
>> The user will hear sound, but it will not be too loud.
>> I suppose we will need to canvas people on the best value to use.
>>     
>
> With all hw I tested this scheme actually seems to work fine. (-40dB,
> Intel HDA, those Logitech speakers, Terratech Aureon 5.1.)
>
>   
>> Your report about anything over -41dB is way too loud does present a 
>> problem. Obviously, the controls for some sound cards are still 
>> calibrated wrong. Unfortunately, the calibration of USB sound cards is 
>> not left to ALSA, but in fact the sound card manufacturer sets up the 
>> calibration tables for us, so for your Logitech speakers, ALSA would 
>> have to add a specific quirk.
>>     
>
> Actually, it is fine with -40dB, not just -41dB. But then it gets
> rapidly too loud.
>
>   
>> Finally, does anyone else think -40dB for the Master, and 0dB for all 
>> other controls is too loud for the default or not?
>>     
>
> Works fine with my hardware at least ;-)
>
> My scheme in PA will thus now be:
>
> 1) Initialize Master to -40dB
> 2) If there is no master, initialize PCM to -40dB
> 3) If there is a master, initialize PCM to 0dB
>
> Does that sound reasonable?
>
> Lennart
>
>   

That sounds reasonable for now. Professional sound card users will maybe 
just have to remember to mute the master control before connecting their 
speakers.

I don't understand why your sound card is OK at -40dB but too loud at 
-41dB. It does not make any sense. There must be a bug there, because 
-41dB should be quieter than -40dB.


James