[PD] db and pd

Marc Lavallée marc at hacklava.net
Sun May 22 02:42:26 CEST 2005


Le 21 Mai 2005 13:54, IOhannes m zmoelnig a écrit :
> Phillip Stearns wrote:
> > 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.

I believe it has more to do with 16 bits digital audio than MIDI.
http://en.wikipedia.org/wiki/Decibels#6_dB_per_bit:
-----
6 dB per bit
In digital audio, each bit offered by the system doubles the (voltage) 
resolution, corresponding to a 6 dB ratio. For instance, a 16-bit (linear) 
audio format offers a theoretical maximum of (16 x 6) = 96 dB, meaning that 
the maximum signal (see 0dBFS, above) is 96 dB above the quantization 
noise.
-----
Since 96 db is near 100 dB, using 100dB as a maximum means that 4 dB would 
be the level of the quantization noise for 16 bits audio.

Earlier, I should have mentionned this reference from Miller's book:
http://www.crca.ucsd.edu/~msp/techniques/latest/book-html/node6.html
The first equation, for 16 bits resolution: 20 * log(1/65536) = -96.33 dB
But, as Miller explains: "In digital audio a convenient choice of reference, 
assuming the hardware has a maximum amplitude of one, is
a0 = 10^-5 = 0.00001 so that the maximum amplitude possible is 100 dB, and 0 
dB is likely to be inaudibly quiet at any reasonable listening level. 
Conveniently enough, the dynamic range of human hearing--the ratio between 
a damagingly loud sound and an inaudibly quiet one- is about 100 dB."

--
Marc




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