Quote:
Originally Posted by ppppenguin
The question of 3 vs 4 tube cameras was still an issue when the UK started its colour service in 1967. At least we had Plumbicon tubes rather than IOs. Hence no incentive to tray and match a single IO for Y with 3x vidicon for RGB. Or make a huge camera with 3x IO.
There were 3 cameras available in the UK in 1967. The Philips PC60 was 3 tube, the Marconi VIII and EMI 2001 were both 4 tube.
I think the Philips PC60 was sold in the US under the Norelco name. The Marconi VIII was widely sold internationally. The EMI 2001 saw little use outside the UK.
4 tube cameras gave better registration and grey scale. The problem of resolution on 3 tube camers was later solved by taking high frequencies largely from the green channel.
The other big argument was about constant luminance. Becuase all the RGB/YUV matrixing is performed on non-linear signals some Y travels in UV and vice versa. If Y and UV all had the same bandwidth this would hardly matter but they don't so you get various artefacts. 4 tube cameras got much closer to constant luminance than 3 tube.
In the early days in the UK, an EMI engineer called Ivan James was the main proponent of constnat luminance. I can see that I wrote almost the exact same post here in 2010: http://www.videokarma.org/archive/in.../t-248163.html
See also Poynton whose explanations are pretty good: https://poynton.ca/notes/video/Constant_luminance.html |
The problem with non-constant luminance is not fixed simpy by using the Y output from the fourth tube, since the missing luminance is only at the high frequencies that the UV channels cut off. So, for example, a red rose has generally the correct brightness and chroma over its broad area on a color receiver, but greatly reduced detail contrast. (It also appears too dark overall on a monochrome set, which does not produce the missing low-frequency luma that is carried in the UV channels.) Using the complete fourth tube signal for luma means that the low frequency luma becomes correct in a monochrome set but wrong in acolor set because it is carried in both the UV and in the Y. For a color set, you really would like to have the Y' produced by a three tube camera, augmented by added high frequencies from the fourth tube.
By the way, the proportioning of the luma signal from 30% R, 59% G, and 11% B was somewhat erroneously called constant luminance in some early texts, but this referred to it being nearly true for small chroma noise fluctuations on neutral or nearly neutral colors. An important principal of analog color TV, but not the issue of saturated colors having reduced detail contrast, which is the usual use of the term.