[linux-audio-dev] ALSA features.
dmills at spamblock.demon.co.uk
Sat Sep 3 16:14:01 EDT 2005
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
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...).
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