Is there such a thing as a green screen generator?

[pandy71]

This circuit use operational amplifiers so resistors can be pretty high value - in passive matrix you need use different values.
Assuming that impedance load on RGB input will be 75ohm then you must calculate resistors as reciprocal i.e. for Green (1/0.59)*75 = 127 Ohm, Red (1/0.3)*75 = 250 Ohm and Blue (1/0.11)*75 = 682 Ohm. Combined signal from those three resistors will give you grayscale signal that can be feed to Green input of your display.
hope that math is correct - calculated this ad hoc. Of course if you wish you can use different resistor values and you may need also active circuit if necessary - nowadays life is so easy - there is plenty of nice circuits to do such things.
Forget to mention that those resistors value are not highly agnostic - small variations are acceptable (so 130, 240 and 680 from E24 5% will work OK).

Above resistor values seem to be correct however signal loss need to be compensated by using amplifier with gain = 2.
Seem pure passive should be quite OK with resistors values divided by 8 (they will be pretty low values but this is good for bandwidth) R=33 Ohm, G=16 Ohm, B=86 ohm.

[Stedy]

Originally Posted by Starglider 2

Aha! So just take the full color output and connect it to the green input and it will look green and contain all the picture data? I thought we were overthinking it.

My switch box is actually an XRGB Mini. The RGB input is essentially SCART. It's going to be used by my Amiga.

Therefore I'll need a cable that goes from the composite out of the Apple IIe, to SCART, wired to Green in the SCART. Or basically with the R & B pins removed from the SCART plug.

Then a SCART splitter cable between Amiga and Apple. All doable! Something like this?:

Apple IIe Composite RCA ---> SCART Green pin only ---> SCART Y-cable ---> XRGB Mini ---> TV

Any reason that wouldn't work? Might the picture be too bright if all the signals are pushed into green?

Thanks!, Starglider
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Hi,

The Apple IIe PAL/NTSC output has the colour carrier signal, controlled by UB8 in the schematics. If you feed this into a video chip expecting a non composite video signal, you will have a speckly display, caused by the high frequency components of the colour information in the signal.

If you wanted greyscale video, it's possible to filter out the colourburst but you will lose some video information, with a crude RC filter.

[A4000Bear]

Originally Posted by Stedy

Hi,

The attached circuit will convert RGB to YPbPr but if you use the Y part, circled in Blue, you'll have a lumuinance signal. I grabbed the circuit from here: http://cds.linear.com/docs/en/applic...ote/an57fa.pdf

If you play about with the 1070R, 549R and 2940R resistors, you can tweak the RGB levels to get the best green level. Feed the output of this to the SCART/RGB to HDMI converter and depending on the converter type, either use the reduced CSync signal (680R in series) if it's SCART input or the composite video signal from the Amiga. Then it should all work.

Ian

Actually its easier than that. Simply disconnect the red and blue inputs to the circuit, while leaving all component outputs connected. If you want to simulate an amber monitor, disconnect the blue input only.

[Starglider 2]

Originally Posted by A4000Bear

Actually its easier than that. Simply disconnect the red and blue inputs to the circuit, while leaving all component outputs connected. If you want to simulate an amber monitor, disconnect the blue input only.

Thanks. Did you see the reply above from Stedy?

[pandy71]

Originally Posted by A4000Bear

Actually its easier than that. Simply disconnect the red and blue inputs to the circuit, while leaving all component outputs connected. If you want to simulate an amber monitor, disconnect the blue input only.

Then you will loose information - for example red and blue... Old CRT's don't loose information even if there is particular phosphor color involved - it is like grayscale seen trough translucent color.