[opendtv] Re: CBS Color without a wheel
- From: Olivier Houot <olho_avatar_i@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Fri, 09 Dec 2011 23:09:30 +0100
Cliff Benham wrote:
> I should finally mention that today's DLP color sets are field
> sequential color at about 300 to 360 fields per second.
> I have a small DLP projector that I cannot see the flicker on.
>
DLP projectors are what first came to my mind when you evoked that color
wheel CBS standard. I like this technology but unfortunately i am
sensitive to the rainbow effect, and i suspect i would have the same
problem with the CBS system.
I also thought CBS would have been better off trying to perfect video
projection. Using a small bright monochrome tube, you can use optics to
create a rear projection TV set with a large picture, and put a
teeny-tiny wheel at a focal point in the optical path, that would make
no noise and consume next to nothing (just like in mono DLP devices).
Also a small tube like that has a small deviation angle, which would be
an advantage to produce high scanning rates.
Next, i wondered when the first video projecting devices made their
appearance, and your excellent lengthy article revealed that they were
already in use for the RCA demos. So it would have been possible.
So RCA managed to build the first color CRTs all by themselves, but
though focused on their color-wheel system and after trashing their own
plans for color, CBS nevertheless managed to independantly design a
better CRT than RCA for the very first year of NTSC commercial
broadcasts ? How remarkable.
Now one thing i did not understand in your sequential demo on a PC
monitor: how does it scan when you display from an NTSC source ?
Are you using interlaced display at a 180 Hz field rate ? Or 90 Hz
progressive display after deinterlacing?
I always found the challenge that the engineers of the time had set to
themselves to be a really awesome one : to be able to broadcast in color
at the same definition (vertically at least) using exactly the same
bandwidth, and furthermore in such a way that existing B&W sets could
make sense of the signal ! That must have sound like nonsense or mission
impossible at the time.
Yet your link suggests the idea came along gradually, together with the
means to achieve it. Obviously, RCA and its 4 carrier broadcasts with
reduced red and blue bandwidth and use of the green component for B&W
compatibility was groping in the right direction.
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