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CD Level Variations
Kega writes:
chung wrote: Kega (myself) wrote: Note also that I have not in my reasing involved dithering and other techniques to increase the quality in a 16 bit PCM at low signal values. But you HAVE to consider dithering, which is a key part of PCM systems. Dithering effectively transforms the quantization errors, from being correlated to the signal, to noise. So you can't say that at lower input levels, the harmonic distortion increases because the step sizes are now relatively large. In a properly dithered system, you do not see the harmonic distortion terms. The system is linear, with a slightly higher noise floor. Now I'm getting curious. You see back in 1977 when I studied PCM (and Adaptive PCM, Delta Modulation etc..) they (the teachers) never mentioned dithering. It was first after CD arrives on the scene I heard about it. Kent, The basic university professor may not have been aware of it back then. From what I have been able gather, Robert Wannamaker's PhD thesis is a landmark paper on the entire subject. Of course it gets deep, but he does a pretty good job of explaining things before he gets into the depth. He also has an excellent section on the history of quantization in chapter one. Essentially he credits L.G. Roberts as the first one to use dither (in video) back in 1962. The thesis is freely available at: http://audiolab.uwaterloo.ca/~rob/phd.html I know slightly what it is (to add a kind of low level noise to trigg the A/D-converter to differ between 2 samples that has close values). That is about as intuitive as it gets, and is right on. But how does it work? Do you have different dither spectrum and level depending on the level of the signal? (or depending of the spectrum of the signal itself, etc...) For instans when level is sufficient high you don't need any dithering, do you. To answer your immediate question, yes, you do even for high-level signals. To see what happens if you don't dither, write a quick Matlab script that generates a perfect (at least to double-precision floating-point) sine wave, quantize it to 16 bits, and look at the resulting spectrum. You'll see lots of nasty spurs. The business of examining dither involves the study of random signals (AKA random processes or stochastic processes). There are two main properties of a random process: 1) the amount of correlation from one sample to the next (or between one time t1 and another t2 for a continuous random process), and 2) the distribution (pdf, or probability density function) of the process. It is property 1 we are describing when we call a dither signal "white." Rob shows that an nRPDF (the sum of n rectangular PDFs) white dither will decorrelate the first n moments of the quantization error spectrum from the input signal. In practice, n = 2, and 2RPDF is also known as TPDF, or triangular PDF. So that means the first moment, or the DC correlation, is removed, and also the second moment, which is the so-called noise power modulation. Hope this sparks some understanding/interest. -- Randy Yates Sony Ericsson Mobile Communications Research Triangle Park, NC, USA , 919-472-1124 |
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