[opendtv] Re: Precision

At 11:43 PM -0400 5/24/07, Mark Schubin wrote:
I think there is a fundamental misunderstanding of digitization in your question. When analog signals are properly digitized, the only difference between 8-bit, 10-bit, or 12-bit coding is signal-to-noise ratio, not "graduations." 10-bit systems have about 12 dB better SNR than 8-bit, and 12-bit systems have about 12 dB more than 10-bit.
To properly digitize an analog signal, there must be at least one-half of the least-significant bit of level uncertainty (call it noise or dither). If there is, then the necessary quantization error will be uncorrelated noise; if there isn't, the necessary quantization error will be correlated distortion.
Consider a one-bit PCM audio system. Without at least 1/2-LSB dither, it functions as a gate: Anything above the threshold gets through; anything below doesn't. It sounds unintelligible. With 1/2-LSB of dither, there will be a fairly loud, constant hiss (or some other noise, depending on the spectral shape of the dither), but the audio can be heard reasonably clearly through the noise. Add a bit, and the SNR increases by 6 dB.
So your question should be how much SNR we want, not how many gradations we need.
Here is an excellent case in point for what Mark is talking about. I suspect that what i am about to describe also extends into thew realm of LCD TV displays.
I just bought a new MacBook Pro, which is advertised as supporting "millions of colors" or 8 bit RGB. Last week a class action lawsuit was filed against Apple, claiming that Apple is misrepresenting the capabilities of the displays. 

http://arstechnica.com/journals/apple.ars/2007/05/18/lawsuit-over-mac-book-mac-book-pro-displays
According to various stories the displays used on Apple's laptops only support 6 bits per RGB component (18 bit or about 262,000 colors). Other stories suggest that no manufacturer offers a true 8 bit beer component laptop display - that they are ALL 6 bit.
To deal with the presentation of "millions of colors" Apple uses a dithering algorithm that supposedly fools the eye into seeing the proper colors. This is exactly what Mark is talking about - the samples are dithered (i.e. "noise" is added) to create the illusion of more colors. To my eyes it works fairly well, although it is possible to see some gradients if you are really looking for them.
To make all of this even more interesting, it appears that Microsoft has a better dithering algorithm than Apple. Several people reported that when the MacBooks are booted using Windows the image quality is better. 

Perhaps Apple will offer a software upgrade that offers better "noise."
This makes one wonder what the actual bit depths are for most of the LCD panels being used for television displays. My guess is that many are build using panels with less than 8 bits per color component. 

Regards
Craig


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