Reply To: Noise shaped dither – Does it cause issues with DSP-based volume controls?

March 30, 2023 at 1:59 pm #5621

Yes, the standard one.

that’s good.

We also have options to under-dither with TPDF of somewhat lower amplitude.

Oh, you shouldn’t do that.  If you reduce the amplitude of the TPDF from 2Δ width, then it’s like changing it to another PDF and you lose the statistical property of fully decoupling the mean and variance of the quantization error from the signal value.

But noise-shaping all of that up into the stratosphere, where I can’t hear it, is very smart.  And then when it’s 28 dB louder, it’s still there after the undithered gain reduction, even down by 10 or 15 dB.  Very smart.


You do have an ~4 dB boost at lower frequencies.  Like below about 500 Hz.  So, if I do my accounting right, white TPDF unshaped dither has the noise floor of 96.33-4.77=91.56 dB below full scale. (I am assuming the signal is the same uniform pdf.  If it’s a sine wave brought up to the rails, add another 1.76 dB, but I don’t do that.)  Now, your 4 dB of noise over the white dither puts the noise floor at about -88 or -87 dBFS, right?  So there would be a little rumble down there below 500 Hz, right?  (But it would be a friendly rumble with with its mean and variance completely decoupled from anything else.)

Am I correct so far?

What’s cool is that from 2 kHz up to the “noise wall”, you’re better than 10 dB below the standard white TPDF noise floor.  You gotta CD with 16-bit words and you have better than a -102 dBFS noise floor at the frequencies that, when we’re young, are the most sensitive (according to the Fletcher-Munson curve).

Last thing is, even if the feedback filter is FIR, it’s in a loop.  You had to be pretty creative making a filter of order 60 that’s in a loop (so the whole thang is an IIR and them mothers can go unstable especially at high order).  I would ask you more details, but I don’t wanna put you on the spot where you have to decline exposing any recipe to the secret sauce.