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In Reply to: RE: To Dither or Not to Dither? J. River/PonoMusic World posted by Ugly on April 20, 2016 at 20:20:01
These ADCs output 24 bits as a result of signal processing. The DSP creates low order bits, and generally quality of DSP will suffer if there isn't at least 48 bits of resolution in the signal processing. In addition, most recordings go through additional DSP in post production. The same situation happens during playback if a digital volume control is used in playback, whether done in the computer or in the DAC.
If dither is not used when bit depth is reduced (e.g. 48 down to 24) the result is signal dependent distortion. This is true even if the original analog signal is noisy. For example, if the ADC is only "good" to 22 bits, it might have noise at the 22 bit level added into the output. Or, there might even be distortion generated by the analog equipment when handling high level signals resulting in even greater errors. However, none of these errors will necessarily hide the errors involved when dither is omitted.
If you look at signal numbers that are applied to manufacturers' spec sheets and not how the mechanisms actually work you will not get a complete picture. The limitation in one dimension (noise or analog distortion) does not justify a limitation in another dimension. Each type of distortion can and will be perceived separately by critical listeners. In the case of 24 bit dither, most people would consider the benefit small and might not be willing to pay a large amount to eliminate what is a low level distortion. However, the cost of doing things correctly is essentially zero, being just a few instructions added to software.
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
Follow Ups:
" The limitation in one dimension (noise or analog distortion) does not justify a limitation in another dimension."
They may not always be the same point on the graph but I would argue that noise level in a transmitted signal must always set a practical limit for making distortion measurements. And every signal has noise.
My question isn't whether to use dither or not. I'll always follow he recipes DSP experts suggest since I can't be bothered to think about that stuff.
It is regarding the practical limits of detecting it's benefit. I don't know at what level a computer could detect distortion on a signal
"They may not always be the same point on the graph but I would argue that noise level in a transmitted signal must always set a practical limit for making distortion measurements. And every signal has noise."
This is not necessarily the case. If for example the transmitted signal consists of a low level sine wave that is buried in the noise then it will be possible to pull out distortion created by non-linearities, such as those caused by lack of dither. This can be done by doing a spectrum analysis to average out the noise and bring into view the signal and its harmonics. The practical limit to this operation depends on the stability of the clocks involved and the patience of the experimenter. Clock stability can be reduced or eliminated if it is possible to synchronize the clocks or run them off a common source. It may also be necessary to temperature control your entire apparatus, lest it begin to operate as an unwanted thermometer. :-)
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
from the Crookwood ADC page:
" With a good ADC, you can play and hear a sine wave, say at -20 or -30dB below the digital noise floor of the ADC."
I would have never guessed anywhere 20-30dB is possible. I wonder what the basis for this statement is, ie experimental evidence, wag, etc....
The depth you can go depends on the bandwidth over which the noise is spread. But these figures are approximately correct. Here the test would be to ABX several seconds of dither noise vs. several seconds of dither noise with a 1 kHz tone at the selected level. At the margin the tone is just barely perceptible by ear, as it fades in and fades out. If one is patient and listens carefully one can tell if it's present or not with some degree of reliability. If the tone is modulated with a random message using Morse code, I can decode the message reliably at about -15 dB, but the "bit error rate" is noticeable when the tone is further below the noise. At -20 dB I can tell something is happening, but I can't read the message. (This was at 44 kHz. More depth at higher sample rates.)If you capture the output of the signals on another computer using an ADC driven by a preamp that boosts the signal without adding more noise, ensuring that the ADC's own noise is not being tested, then you can use a very long FFT to go considerably lower than the broadband noise floor than you can do by straight listening. This is because the computer is averaging over a longer time period than your ear. What happens (assuming the clocks are stable) is that the noise is divided into more and more FFT bins as you increase the FFT size but the sine wave signal is always concentrated in only one bin, regardless of FFT size. (Here it may be on the edge and appear in two bins, this will mean each bin has half as much energy. Details depend on the FFT window. You can tweak this by changing the frequency of the test tone.)
All of these tests can be done with an audio editor that converts between bit depths with various dither algorithms and that has the ability to generate test tones and mix signals and that does FFT plots. I used Soundforge 10c for my tests.
Other fun and games include learning to hear the effects of various types of dither (or sample rates) on music. For more serious work, you have to write software that directly generates and processes the test signals involved and creates creates and analyzes WAV files that a sound card can use or generate.
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
Edits: 04/26/16
Kind of mind blowing to me to think that we/presumably other species can accomplish this much computational work with no apparent lag, ie we still get to hear everything we hear in real time. Nature still has tricks to teach us regarding the doing powerful information processing hardware the right way. I wont even try speculating on what processes our bodies must be using to accomplish this amazing amount of work.
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