Dither and Jitter

Robert Katz Leave a Comment

From: David H

I do not want to take advantage of your help, but… I’m also trying to get a better understanding of dither. The “Secrets of dither” was excellent, it really cleared up a lot for me. However, in trying to get a grasp on all these digital theory concepts, I try to summarize the concept so I keep it straight in my head. In a nutshell, how does dither increase apparent dynamic range. How does it make soft sounds more pleasing? Is there an analogy that can be made to analog tape machines?
Also, could you please provide simplified answers to the following questions: What are the effects of jitter on signal reliability and noise floor. How do you reduce jitter? What benefits are there to using a master word clock generator (such as the Aardvark Aardsync) to synchronize an entire digital studio? Thank you for all the time and effort you put forth in educating the masses! I promise these are the last questions I’ll bug you with. 🙂 I will of course keep reading and re-reading your informative site!

Dither: Increases dynamic range by adding a small amount of random noise to the long word source BEFORE truncating to a lower wordlength. The low level material rides on this noise and produces the same average level that any low level material would on an analog noise floor. For example, in a totally analog system, let’s say that tape hiss at a certain moment in time is 1 millivolt in level, positive going. And that the audio signal at that same moment in time is 2 millivolts, positive going. The sum will be 3 millivolts, positive going.

If you perform the same operation in digital, creating a random noise and add that to your signal BEFORE truncating, then the sum at that moment in time will still be 3 millivolts. And that can be encoded to the lower wordlength. At other moments in time, of course, some low level signals added to the noise at a given moment in time will be below the minimum level of the lower wordlength. BUT ON THE AVERAGE, the dithered result will be the same as it would have been in an analog system with the same noise floor.

In my book, Mastering Audio, I diagram this in detail and a picture is worth a thousand more words  :-).

Jitter, in short:
If you already have made a conversion (A/D) and have it stored digitally, then jitter in your system only affects the listening. The apparent noise floor is higher during signal, it can sound harsher, and less deep (spaciality). This is a function of monitoring in the D/A converter.

On playback and mixdown, the best way to reduce jitter is NOT TO REDUCE it, but to obtain a D/A converter which is immune to its effects! Such as a Prism converter, which is known to be extremely immune.

During recording, jitter can permanently affect the recorded signal, and the best way to keep it down is to use an A/D converter on Internal sync.

The benefits of word clock are to have a continuous, stable, timed signal feeding all the gear that can be slaved. However, the central “generator” should be your A/D converter on INTERNAL sync. For word clock distribution, let the A/D be the master, then have its AES or WC output feed the word clock distribution, and everything else slave to the WC distribution.

Again, I refer you to the book for a more detailed explanation with diagrams to help.

Best wishes,

Bob

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