bender@oobleck.Eng.Sun.COM (Michael Bender) (01/09/91)
My video swticher, (Panasonic WJ-MX12) processes the video signals along two seperate paths (Y and C) internally. I'm modifying the keyer circuit to do a different type of keying (described later), but I'm stumped at one point in the design of my modification - does anyone know the format of the chroma signal? Specifically, how is hue determined? By the amplitude or the phase of the chroma signal? OK, here's what I'm doing, and when I get it done I'll post my mod to the net so that anyone with an MX-12 that wants to implement the mod can do so. The MX-12 has 3 video input sources - video 1, video 2 and an external key input. The way the current keyer works is that when the Y (luminance) value of the key video input is within a certain user-settable window, the keyer will insert one of 6 (or 8) fixed colors, determined by the background color control rotary switch. The keyer has two color options - use the background color, or use white. If you select "white", the keyer gets fed a white Y/C signal from a dedicated white generator in the MX-12. This is a seperate buttone from the "background color" button, and the seperate white generator is used so that you can still have a selectable background color from the background color generator for other things. I'm modifying the keyer so that if you select the "white" key color button, instead of supplying the signal from the while generator, the video 2 Y and C signals will be keyed into the output signal, so that for example if you have an external key camera pointed at a large letter "A" the outside of the letter "A" will be the video 1 source and the inside will be the video 2 source, rather than a color. The problem that I'm having is that I can't get the video 2 colors quite right when video 2 is keyed in (as evidenced by a top/bottom comparison of the bars from my color bar generator). The video chroma signal path in the MX-12 is seperate for each of the video 1 and 2 sources until it gets to the mix/wipe stage, at which point it goes into a bandpass filter, what I believe is a current mirror (an NJM1469M) and then into an inverter and buffer. I've decided to tap the chroma for video 2 off before the bandpass filter/current mirror/inverter/buffer and feed that into the point where the keying circuit would normally expect the chroma signal from the white color generator, but the colors are off hue; then, I built up a circuit containing the bandpass filter and the inverter/buffer, and tried that, but still the colors were off; close, but not quite on. The only piece of the normal chroma path that I didn't build was the current mirror. With all that explaination, what I'm asking I guess is what does a current mirror do (if that's indeed what this chip, the NJM1469M, is), and what determines the hue of a signal? thanks, mike -- Won't look like rain, Won't look like snow, | DOD #000007 Won't look like fog, That's all we know! | AMA #511250 We just can't tell you anymore, We've never made oobleck before! | MSC #298726