Hi!, On 2011-11-17 00:24, Rhialto wrote: > > Somewhat unrelated, but interesting nonetheless: for several Doctor Who > episodes from the 1970's that were made in colour, no colour video was > archived. At best, there are black and white telerecordings: basically a > film camera pointed at (and (hopefully in phase) synchronised with) a > black and white monitor. > > Now it turns out that the resolution was so good, that the colour signal > is visible in the luminance. And some bright people have worked on > restoring the colour information from that. Unfortunately, the > information I could find about it is a few years old and it seems no > more recent developments happened. See http://www.techmind.org/colrec/ . That's very neat indeed! (I've just read the article. One would think, an attempt like that would need a film scanner of very high resolution at the first place, in order to oversample the image so that adjacent rows and geometry could be restored with as little hassle as possible... Seems like they just had to fight with the additional obstacle of not having such digitizer, which must have made things much more difficult). Thinking that over, there's yet something more that affects the picture quality of the VIC-II, or, better said, the C64 and probably the C128 too. (I'm only speaking about PAL machines, because I don't (yet) have an NTSC model so I could never see it in action, yet). The composite color signal was originally designed in a way that luma and chroma affect each other as little as possible, even though the spectrums of the two components just overlap each other. (Better said: chroma is embedded into the spectrum of luma, with the color subcarrier freq purposely selected so that at the end, real "overlap" of particular spectral lines would be minimal). In theory, this would ensure that even though luma and chroma are transmitted in the same 5-6MHz band mixed together, they can be separated with no loss of bandwidth of the components (especially luma, which is practically image sharpness) and with minimal artifacts and crosstalk. (In practice, the chroma signal would result in a constantly "interlacing" pattern on top of luma, which, with the arrival of better picture tubes, became more serious of a problem, so manufacturers started to invent techniques to get rid of it ie. filter chroma remnants out of luma before the signal would be displayed. On the downside, this usually results in a loss of bandwidth for luma ie. a more blurry luma image. At least the tv processors I know of, TDA836* and the like, specify an output (RGB) bandwidth of only 3.5MHz for PAL composite input, and even less, 2.8MHz for NTSC). Even with fully conforming to the original specification, there are special cases when the formula, err, fails seriously. Typical example is the man in the suit :-). Some suits are made of textile with sharp, regular, frequently alternating patterns. Recording and broadcasting those just fools the idea of minimal artifacts of luma/chroma separation, and the result is massive color moire and artifacts above the respective surfaces. Our computers are more special, ie. conform to the original spec is generally a "don't care" bit as long as the construction provides colors, and second, the pattern "transmitted" by the machine is regular (because it's basically a matrix of pixels, replayed at a constant sampling frequency). That'd alone seriously call for keeping luma and chroma apart at the first place, if composite video is to be generated - ie. keeping luma's band top below the subcarrier freq + bandwidth of color, in order to avoid artifacts. The C64 design has some aspects that violate those rules. First, luma is AFAIK not filtered before mixing with chroma. Second, the pixel clock of the C64 is quite high, and is also special. People who use C64s with decent CRT displays and either RF or composite connection have for long noticed the color artifacts that generally appear around sharp luma edges. This is a consequence of above - more specifically, the consequence of luma's overlapping to the color signal's band at the point of sharp luma transitions (that are then decoded as color transitions in the display). Also, if someone creates a pattern of black and white stripes on the screen, one pixel of width each, he'll see a color gradient on top of the stripes on composite displays... As explanation, the pixel clock of the PAL C64 is 16/9 the color subcarrier frequency. A series of black and white pixels is therefore a square signal whose base frequency is half that, in other words 8/9 of the color subcarrier frequency - very close, almost equal. This signal will definitely be catched by the chroma separator in the display, and get displayed as color. As this signal'd be constantly shifting in reference to the PAL burst (since it is "slower" than that of PAL's nominal frequency), the result is a gradient of constantly changing color. From the proportion, we could also conclude that the gradient is periodic for 16 pixels. ...Now, imagine how that'd look like if the PAL reference phase was unknown, or constantly shifting... :-DDD Best regards, Levente Message was sent through the cbm-hackers mailing listReceived on 2011-11-17 11:00:03
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