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Music servers and other computer based digital audio technologies.

RE: Crookwood says 20-30dB into the noise is possible.

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


This post is made possible by the generous support of people like you and our sponsors:
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