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