.DITHER


Word lengths and Dithering

Dither is perhaps the most difficult concept for audio engineers to grasp. If this were a 24-bit world, with perfect 24-bit converters and 24-bit storage devices, there would be much less need for dither, and most of the dithering would go on behind the scenes. But until then (and the audio world is heading in that direction), you must apply dither whenever wordlength is reduced. The fine details of dithering are beyond the scope of this booklet; learn more by consulting the references in the appendix.


Here are some basic rules and examples:

1 When reducing wordlength you must add dither. Example: From a 24-bit processor to a 16-bit DAT.
2 Avoid dithering to 16 bits more than once on any project. Example: Use 24-bit intermediate storage, do not store interpediate work on 16-bit recorders.
3 Wordlength increases with almost any DSP calculation. Example: The outputs of digital recording consoles and processors like the Finalizer will be 24-bit even if you start with a 16-bit DAT or 16-bit multitrack.
4 Every "flavor" of dither and noise-shaping type sounds different. It is necessary to audition any "flavor" of dither to determine which is more appropriate for a given type of music.
5 When bouncing tracks with a digital console to a digital multitrack, dither the mix bus to the wordlength of the multitrack. If the multitrack is 16-bit digital, then you're violating rule #2 above, so try to avoid bounces unless you have a 20-bit (or better) digital multitrack. Example: You have four tracks of guitars on tracks 5 through 8, which you want to bounce in stereo to tracks 9 and 10. You have a 20-bit digital multitrack.You must dither the console outputs 9/10 to 20 bits. If you want to insert a processor (like the Finalizer) directly patched to tracks 9 and 10, don't dither the console, just dither the Finalizer to 20 bits. The Finalizer's ADAT interface makes this process relatively painless. One complication: The ADAT chips on certain console interface cards are limited to only 20 bits. Consult your console manufacturer. Although the Finalizer's ADAT interface carries a true 24 bits, if the console's ADAT interface is limited to 20, you need to dither the console feed to the Finalizer to 20 bits and once again dither the Finalizer output to 20 bits to feed the multitrack.


Quelle: Bob Katz