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

[John Rigg]

On Tue, Apr 15, 2008 at 03:43:30PM +0100, James Courtier-Dutton wrote:
> 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?
 
In pro gear, levels are referenced to digital full scale in the A-D 
and D-A converters. The actual analogue signal voltage level that 
corresponds to this varies between interfaces. In most pro converters,
full scale corresponds to about +20dBu (about 11 volts peak)
but there is some variation (+/- 2 or 3dB) between manufacturers.
I'd expect the line in and line out levels on a consumer
sound card to be much lower than this, so obviously the same
standard can not be applied to both.

John