# [PD] db and pd

IOhannes m zmoelnig zmoelnig at iem.at
Sun May 22 21:22:42 CEST 2005

Marc Lavallée wrote:
> Le 22 Mai 2005 05:14, IOhannes m zmölnig a écrit :
>
>
> For those not using MIDI (which I believe is the case of most PD users),
> midi-dB is not that intuitive... And I don't see any reference about

hmm: i guess a lot of pd-users have no idea about technical dB too.
so for them it should be just an abstract "unit", with 100 ("%" ??)
being unity gain.
i (normally) do not use MIDI and i don't have problems with it; however,
i agree that it is confusing, especially if you have some knowledge
about technical dB, and try to figure out whether to use "real" dB,
"midi" dB or rms.

> midi-dB in Miller's book. I can believe that midi-dB is handy and make
> sense, but I used MIDI and I don't understand your explanation (and the
> conversion pseudo code).

the pseudo code says:
MIDIdB == dBfs + 100
that is all.
the only thing that complicates the pseudo-code was, that i tried to
express that "0" midi-dB (which is "-100" dBfs) is set to 0.0 rms (which
is "-infinity" dBfs) by definition!

>
> If 128 values are used to control a 0-100 dB range, it means that the
> midi-dB resolution is 0.78 dB. For 16 bits audio, the range is 0-96 dB, so
> the midi-dB resolution is 0.75. For 24 bits audio, the range is 0-144 dB,
> so the midi-dB resolution is 1.125 dB. Is that correct?

WHY ?

dB is a relative measure. +1dBfs == +1dB(midi) == +1dB(16bit) ==
+1dB(32bit) == ~ *1.12 (rms)

128 midi-values are *not* mapped to a 0..100 dB range; instead they are
mapped to a -100..+28 dBfs range.
this is what i meant by "28dB headroom" for amplifying low signals.

you cannot just map one range to another range.

mfg.asd.r
IOhannes

PS: if someone can explain what i mean in a better english i would be
greatful.