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In Reply to: Re: A lot of people don't understand dither posted by Christine Tham on March 2, 2007 at 14:30:28:
A good mic, mic pre amp and analog console have dynamic range of around 130dB. A good ADC will have dynamic range slightly exceeding 20 bits.So, the noise floor of electronics alone are too low to produce self dithering when truncating from 24 to 20.
Let's consider the case of "self dithering" caused by ambient background noise during recording. As far as I know, this is a "myth" and I'm not sure many engineers still believe in this. Let me explain why.
Typical peak levels in a live recording: 120dB SPL. Typical ambient noise in a live recording (assuming a concert hall) - around 40-50dB SPL. I have measured far worse, often due to air conditioning.
Typical peak levels in a studio: 100-110dB SPL. Typical ambient noise in a quiet studio: 20-30dB SPL. Again, I have measured studios that are worse (well, mine for instance :-))
These are all numbers based on my experience. Feel free to validate them yourself.
Assuming that we set the levels such that peak is close to 0dBFS, then typical noise level in a recording: around -70-80dB.
You can actually validate this yourself by doing an analysis of commercial recordings. The majority of them will exhibit ambient noise levels at these levels.
These levels are clearly too high to produce a self-dithering effect when truncating from 24 to 20. Particularly when you analyze the spectrum of the ambient noise and realize it is not random.
And in any case you can prove it to yourself. Take a recording with ambient noise at say -70dB. You will find you can easily hear a reverb tail even when it's amplitude is below -70dB.
Even in the best case scenario, noise levels may be at -90-100dB. This may potentially produce a "self dithering" effect for a 16-bit recording, but it won't for a 24-bit recording (with between 20-22 bits effective resolution).
You don't have to believe anything I say, but the numbers kind of speak for themselves, and they are numbers you can easily validate yourself.
Follow Ups:
"Even in the best case scenario, noise levels may be at -90-100dB. This may potentially produce a "self dithering" effect for a 16-bit recording, but it won't for a 24-bit recording (with between 20-22 bits effective resolution)."I think you meant to swap the numbers: This may potentially produce a "self dithering" effect for a 24-bit recording, but it won't for a 16-bit recording.
Note that nobody is denying that applying dither noise *is* required if a high-resolution audio signal is truncated to 16 bits. But I don't think adding noise is necessary if the same high-resolution signal is truncated to 20 bits.
And comparing dither needs of audio signals with the dither needs of video signals is comparing apples with oranges. Note that video color is not something that is hurt by dynamic range, since the color range is finite. Low rez video color deals with wider spacing between adjacent colors. Which is a lot more benign than such application applied to audio signals.
*** I think you meant to swap the numbers ***No, read it again. The statement is as intended.
*** But I don't think adding noise is necessary if the same high-resolution signal is truncated to 20 bits. ***
It doesn't matter what you think, in reality dithering at 20 bits will make a difference, just like dithering at 16 bits. It may be harder for our ears to hear the difference though.
It's probably a moot point though because I can't think of a valid reason why someone would want to truncate a 24-bit recording to 20-bits. Do you?
*** Note that video color is not something that is hurt by dynamic range, since the color range is finite. ***
I'm not sure what you mean here. Try comparing an image with 8 bit colour vs 16 bit colour vs 24 bit colour and tell me whether it's "hurt" by dynamic range or not.
*** Which is a lot more benign than such application applied to audio signals. ***
Assuming that we set the levels such that peak is close to 0dBFS, then typical noise level in a recording: around -70-80dB.You can actually validate this yourself by doing an analysis of commercial recordings. The majority of them will exhibit ambient noise levels at these levels.
These levels are clearly too high to produce a self-dithering effect when truncating from 24 to 20.
The noise level does not have to be near the quantization noise floor. It only has to be _above_ the quantization noise floor. Dither levels are deliberately set near the quantization noise floor so the dither doesn't reduce S/N too much.
Particularly when you analyze the spectrum of the ambient noise and realize it is not random.It doesn't have to be random. It merely has to be uncorrelated with the musical signal and above the quantization noise floor in level from 0 to fs/2.
*** The noise level does not have to be near the quantization noise floor. It only has to be _above_ the quantization noise floor. ***The point is that there are NO effective dithering sources above 20 bits.
We already know that there is a benefit from dithering when truncating from 24 to 16 bits. Not only is it possible to hear the difference between dithering and non dithering, but it's possible to hear the differences between dithering algorithms (see for example the results of the Great Dither Shootout).
The reason I made a point about background noise typically being at the -70-80dB level is that this is ABOVE the quantisation noise floor of a 16 bit recording. Therefore, if it does not eliminate the benefit of dithering at 16 bits, it also does not eliminate the benefit of dithering at 20 bits.
Otherwise there would be no benefit recording at a resolution higher than about 14 bits.
*** It doesn't have to be random. ***
Note: just because I pointed about that ambient noise is not random, doesn't mean that I am asserting that an effective dithering source has to be random. In any case it is easy to demonstrate that we can hear well below the so called "noise floor" of a room.
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