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

March 24, 2023 at 4:31 pm #5567
Bob Katz

    Dear Phil: Let me confirm your assumptions, trying to put some numbers on it.

    You wrote:

    “I can’t imagine a volume control, digital or analog, that would affect dither or its effect on what is heard.”

    To elaborate: Let’s say the listener wants to turn up his monitor control. If the original was noise shaped 16 bit dithered, with very strong HF dither at an extreme of say -71 dBFS around 20 kHz, and audio at an integrated loudness of -20 LUFS, an extreme case… maybe representing a piece of very dynamic classical music. While dBFS and LUFS are different measures, for purposes of discussion, let’s assume they are close, which they are likely, within a few dB. These are nominal calculations, to get us in the ballpark.

    Now, regardless of whether they have an analog or a digital monitor control, let’s say they want to adjust their monitor to reach, say, an SPL of nominally 80 dB average SPL — for that integrated loudness. OK, using simple arithmetic,  then the 20 kHz dither noise, would end up, if  -20 LUFS == 80 dB, so nominally, -71 dBFS would be +29 dB SPL at, say 20 kHz. I doubt that would be audible, it’s way too low for my ears!

    OK, now are you worried about cumulative dither? Let’s squelch that question: If it’s a 24-bit dithered digital monitor controller, its dither would be at -141 dBFS. So, loosely and nominally, if -20 LUFS is 80 dB SPL, then the dither noise of the monitor controller would come to -121 dB SPL! That’s MINUS -121 DB SPL! Even if you add the noise of the DAC, the noise floor of the digital monitor controller would still be in the negative SPL!!!!

    If it’s a pure analog monitor controller, let’s say its noise floor is -80 dBu, so if 0 dBu == 80 dB SPL, then its output noise floor is 0 dB SPL or thereabouts, still quite inaudible. No matter how  you compute or fudge the meaning of “nominally“.

    So, I agree with you, Phil, looking at the numbers everything comes up roses.