chill@cc.curtin.edu.au (05/30/91)
I've been listening to tapes from the 91 Apple Worldwide Developers Conference in the area of audio-video integration, and I'm floundering soemwhat with some of the new jargon terms. Can someone explain in words of few syllables what: quantization and dithering are please? Forgive me if the terms are pedestrian to you, but they're new to me. Thanks Ian Hill
johna@gold.gvg.tek.com (John Abt) (05/30/91)
In article <1991May30.093848.8487@cc.curtin.edu.au> chill@cc.curtin.edu.au writes: >I've been listening to tapes from the 91 Apple Worldwide Developers Conference >in the area of audio-video integration, and I'm floundering soemwhat with >some of the new jargon terms. >Can someone explain in words of few syllables what: >quantization and dithering are please? Quantization is a process which produces finite numerical values at a given resolution from inputs of arbitrary (but greater) resolution. Rounding-off or truncation are examples of quantizing, as is analog- to-digital conversion. A problem with quantization is correlated error. When a correlated input is quantized, the error resulting from the quantization operation is also correlated. Correlated error tends to "stand-out" and is therefore objectionable. Contour lines in an image gradient are a good example of correlated error. "Dithering" is a process which attempts to "un-correlate" quantization error. Essentially, a "dither" is a noise signal which is added prior to quantizing. This has the effect of "breaking-up" the patterns of correlated error. In the image example above, a contour line exists because pixels "make-the-jump" along a distinct spatial line. Dithering causes the line to be less distinct by moving it around a little. Although dithering is the subject of considerable research and development, the concept certainly is not new. A good treatment can be found in "Television by Pulse Code Modulation", W.M. Goodall, Bell System Technical Journal, pp. 33-49, January 1951. John Abt Grass Valley Group
alves@calvin.usc.edu (William Alves) (06/04/91)
In article <1991May30.093848.8487@cc.curtin.edu.au> chill@cc.curtin.edu.au writes: >I've been listening to tapes from the 91 Apple Worldwide Developers Conference >in the area of audio-video integration, and I'm floundering soemwhat with >some of the new jargon terms. > >Can someone explain in words of few syllables what: > >quantization and dithering are please? > As another poster said, quantization is simply assigning discrete numbers to analog values. It's usually used in reference to analog-to-digital conversion. There are a number of problems (or "artifacts" in the jargon) which occur as a byproduct of quantization and sampling (assigning these numbers at regular points in space or time). In spatial sampling a common artifact is jagged edges, in color visible contours or "bands," and in time jerkiness of motion. These artifacts are all analogous in the ways that our senses recognize repetitive patterns and are even drawn to them. In signal processing language, these are results of localized high frequen- cies. Our eyes will not notice them as much if they are filtered out and their energy distributed over a wider spectrum. Dither is a relatively easy "filter" of this type to implement. A small amount of "noise" or randomization is injected into the signal before quantization. For example, if color of pixels in the area around a sudden change of color is slightly randomized, the eye will tend to "average" the colors and see a smoother transition (and less jagged edges and visi- ble contours). The first important investigations of dither were in television. Some references include: L. G. Roberts, "Picture Coding Using Pseudorandom Noise," IRE Transactions in Information Theory IT-8, Feb. 1962, 145-154. L. Schuchman, "Dither Signals and Their Effect on Quantization," IEEE Transactions on Commcation Theory, COM-12 (Dec. 1962), 162-165. J.F. Jarvis, N. Judice, and W.H. Ninke, "A Survey of Techniques for the Display of Continuous Tone Pictures on Bilevel Displays," Computer Graphics and Image Processing 5, No. 1 (1976), 13-40. Dither is also used in digital audio. Analogously to the eye's attraction to repetitive structures, the ear tends to find harmonic distortion much more objectionable than the equivalent amount of noise. But at very low amplitudes, where the signal is represented only by a bit or two, the sig- nal can be greatly distorted. However, if a small amount of noise is in- jected into the signal before the analog-to-digital conversion, the energy of the objectionable harmonics are in effect spread over the entire spectrum. For references here, see: Ken C. Pohlmann, Principles of Digital Audio, (Indianapolis, IN: Howard W. Sams & Co., 1985), 58-59. J. Vanderkooy and S.P. Lipschitz, "Resolution Below the Least Significant Bit in Digital Audio Systems with Dither," Journal of the Audio Engineering Society, March 1984. Barry A. Blesser, "Digitization of Audio: A Comprehensive Examination of Theory, Implementation, and Current Practice," Journal of the Audio Engineering Society, 26, No. 10 (Oct. 1978) 739-771. Often, one also sees dither used to refer to similar non-random processes. For example, "dither" on some scanner software I have used actually puts a patterned alternation between two colors to simulate a color in between. Because of the regular pattern, this is not really dither, at least as I have defined it. While it will help simulate extra colors, the repetition of the pattern introduces a new artifact which catches the eye. I hope I didn't use too many long words, and I hope this helps. Bill Alves