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

[James Courtier-Dutton]

John Rigg wrote:
>> On Monday 14 April 2008 23:07:22 James Courtier-Dutton wrote:
>>     
>>> The aim of the dB values in the ALSA mixer is this.
>>>
>>> For Playback:
>>> Set all the mixer controls to 0dB.
>>> Send a sample digital signal from the CPU to the sound card.
>>> The measured analog signal on the line-out should be the same for ALL
>>> sound cards.
>>>       
>
> The dB setting in a mixer refers to gain, not level.
> Expecting the same absolute level from pro and consumer
> sound cards at the same gain setting is a little ridiculous.
> If I set a control at 0dB I want just that: 0dB gain.
>
> John
>   
But surely it is a bit more than that.
If I work with Pro gear and I capture some sound via line-in.
I then send the digital sound file produced to another person who also 
has pro gear.
When they output that digital sound file to their line-out, it should be 
identical, level wise, to the captured line-in was so long as both 
people have the same dB gain levels set.
In this case, we would need some way to convert Volts from the line-in 
to specific digital samples in the sound file.
I have not been able to find out what this conversion should be.
E.g. a 16bit signed digital sample with value +5000 equals X Volts 
output from line-out.
What is that X value?

James