On Apr 11, 2017, at 20:56, Patrick <[email protected]> wrote:
> I've always been confused about dB, dBu, dBm, dBFS and possibly others.
‘dB’ is short for decibel. It’s merely a ratio between two different power (or
sometimes voltage) levels, expressed logarithmically. For the long explanation,
see:
https://en.wikipedia.org/wiki/Decibel
Since ‘dB’ all by itself is merely a ratio, to be meaningful it has to be
expressed in terms of some sort of standard signal. A couple of the common
ones used in audio engineering are:
dBm - Ratio against a 1 milliwatt signal into a 600 ohm load.
dBFS - “Full Scale” —e.g. the maximum level that a particular PCM
digital encoding can represent —e.g. +32768 / -32767 for PCM16.
The first has meaning only in analog contexts, while the second is meaningful
only in the digital realm. If you play Rivendell's standard reference tone, it
will show a ‘zero’ level on the meters, which corresponds to -16 dBFS, or
approximately 15.8 % of the full-scale amplitude of the digital signal (about
+/-5177 at the peaks). Likewise, on a properly operating analog ASI card with
the trim controls set to ‘zero’, that same tone should give you an analog
output level of +4 dBm (a common analog ‘reference’ level used by pro/broadcast
audio gear), while putting a signal of that amplitude into an analog input
should give a ‘zero’ indication on Rivendell’s meters.
>
> When I go to RDAdmin->ManageHost->AudioPorts I get options to set the Input
> and
> Output reference levels, which are all currently at 0 dB.
>
> Is that the trim you're talking about? If so, would increasing the input
> reference level make
> the input more or less sensitive?
Yes. Increasing the reference level will have the affect of making inputs less
sensitive while increasing the average level of outputs. Bear in mind that
these trims controls are not supported on all ASI cards, and where they are
they often have a very limited range. The spec sheet for the particular card
(most of them available at http://www.audioscience.com
<http://www.audioscience.com/>) should have the details.
Cheers!
|----------------------------------------------------------------------|
| Frederick F. Gleason, Jr. | Chief Developer |
| | Paravel Systems |
|----------------------------------------------------------------------|
| A room without books is like a body without a soul. |
| -- Cicero |
|----------------------------------------------------------------------|
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