Dithering when Gain Changing 

Robert Katz Leave a Comment

From: John La Grou

HI Bob 
You are on Len Moskowitz’s WWW page, so I guess I’m writing this through the back door.. Your MIX article is terrific. I think it will open a lot of eyes. Did you say that “all” digital gain reductions require a certain amount of dither?

John La Grou

To be very exact, all digital gain reductions (and most other non-trivial DSP operations) require a certain amount of dither if the signal is to be truncated to a shorter wordlength (there is only one correct minimum amount of dither). If the signal is going to be transmitted or broadcast at its original wordlength, then redithering will not be necessary. For example, I start with a 20 bit recording, and I drop the gain by 4.5 dB. This becomes a long word (48 bits or more) within the DSP (for more information on this, see the dither article). Internally, in the processor, this long word should be dithered up to 24 bits. Then you receive it at 24 bits. If you can use or transmit these 24 bits, then so much the better. Otherwise, when you reduce the word to 16 for storage or transmission, you will have to dither.

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