[linux-audio-dev] ALSA features.

[Dan Mills]

On Saturday 03 September 2005 20:30, Jussi Laako wrote:
> On Sat, 2005-09-03 at 19:19 +0100, Dan Mills wrote:
> > > annoying. IMO, all volume adjustments should be in dB (or dBu or some
> >
> > db, as this is a gain change (which is unitless).
>
> It could also be calibrated, thus with unit.

I don't think this is right, a signal **level** can be measured against a 
known reference level, and for metering it is important to know what this is, 
but a **gain** value is **always** unitless (You are multiplying a signal 
which has units by a scalar value).

Consider that +6db gain is double the amplitude at a signal LEVEL of -60dbu, 
+40dbKW, or anything else, it is always double the amplitude (4* the power).  

Note that some gear has for example switchable 'gain' of -10dbV or +4dbu, what 
they actually mean is switchable reference level of....

Controls which switch the nominal reference level should be marked in terms of 
what that reference IS, but  general gain controls should be marked in just 
db.

Meters should be referenced to some specified level (Ideally in terms of real 
world voltages where analogue outputs are concerned), in the case of digital 
outputs where the signal can only be referenced to full scale (whatever that 
means),  it is important to document any offsets. For example common UK 
practice is to line up so that 0dbu = -20dbFS, now that is not your problem, 
but if you are implementing a (say) VU meter, please document how many db 
below 0dbFS your 0db mark is!

(Sorry, Hot button issue atm having spent far too much time trying to sort out 
a problem with this stuff...).

Regards, Dan.