[PD] db and pd

[IOhannes m zmoelnig]

Phillip Stearns wrote:
> Ok so I have been curious about this ever since I've
> been messing around with PD:
> 
> What does the dB represent?
> 
> What is its reference?
> 
> Why not use the dBfs scale (0dB is the clipping
> ceiling and everything below is measured negative) if
> we're running digital?

the most important reason to not use technical dB but dB with an offset
is (i think) because of MIDI: MIDI gives us values between 0..127; it is
very hard to express negatice numbers with MIDI-values.
pd is a child of max (on next-computers) which was the first
patcher-language and which was a lot about MIDI (please forgive me...);
to stay compatible with old patches (*.pat loader) and (more important)
an established paradigm, pd still uses "midi-dB"

another important thing is, that "0" midi-dB means "-infinity" technical
dB (this is: absolute silence); "-infinity" is very impractical within a
technical system.
"100" midi-dB equals "0" technical dB (unity gain), and is a very easy
to remember value (something like "100%")
(and to talk about real MIDI-values again: you still have a headroom of
+27dB for amplification of low signals)

in musical applications -100dB can normally considered to be silent,
thus the narrowing of the scale does not really matter.


the conversion is:

            { 100 - dB(midi) : dB(midi)>0
dB(techn) = {
            { -inf           : dB(midi)<=0

           { 100 + dB(techn) : -100<dB(techn)
dB(midi) = {
           { 0               : -100>=dB(techn)


mfg.a.sdr
IOhannes