On 2/8/2013 7:48 AM, Tauno Voipio wrote:
On 7.2.13 11:35 , Ian Field wrote:


"Tilmann Reh" wrote in message
...
Michael A. Terrell schrieb:

80's vintage German printing equipment (offset press industry) uses
a video
plug-in card (made by the manufacturer of this equipment) to generate
parameter display for the operator. The display is a standard
baseband video
tube monitor. (It is possible, being German and sold in the USA
market, that
the video may be NTSC or PAL.)

If it's monochrome, we don't need to talk about NTSC or PAL and their
particular color carrier frequencies...


In PAL & NTSC; the colour carrier was a multiple of the line rate.


Not quite.

In PAL, the subcarrier is 4.433618 MHz and the line rate is 15625 Hz,
the ratio is 283.75512, not an exact multiple.

As far as I know it is
283.75 fH + 1/2 fV = 283.75 fH + 25 Hz = 4433618.75 Hz

I don't recall why. But Google has the answer:
http://www.db0anf.de/app/bbs/messages/show-460135PA2RHB
To make the dot pattern that results from the colour subcarrier almost
invisible, we need to satisfy this equation:

4*fc - 2*fr fl = line frequency (15625 Hz)
fl = ----------- fc = colour subcarrier frequency
n fr = frame rate (50 Hz)

This will ensure that dark and light dots cancel each other as much as
possible between alternating lines and between alternating frames.

The number n must be odd, and high enough to get a high enough colour
frequency. It was chosen to be 1135.

We get 4*fc - 2*fr = 1135 * 15625
fc = 1135 * 15625/4 + 25 = 4433593.75 + 25 = 4433618.75 Hz


Note: when colour television was first on the air, we did not have the
25 Hz offset yet. And although the dot pattern should have cancelled over
the screen, it was visible and you could tell, from watching your old
black and white screen, that a colour transmission was on.
After adding in the 25 Hz "time compensating" offset, this was no more.

The "integration time" for the screen is 4 frames, or 80 milliseconds. If
you could photograph the screen with that as the exposure time, the dot
pattern would be absolutely invisible.
If you took a picture with an exposure time of 2 frames (40 ms), or in
other words exactly one complete screen, you could see the residual dot
pattern.