simple vga to composite video schematic needed

[neazoi]

Hello, I need a simple schematic (discrete components only) to convert a 640 resolution vga to color composite video. can this be done without ICs?

I have found a suitable driver for convert the VGA timings here https://reocities.com/SiliconValley/Horizon/7694/vgatv_pane.html so hopefully a simple circuit would work.
But I have not found any COLOR such circuit.

 

[chuckey]

Its a complicated task. You have to generate Y (luminance), by adding proportions of the RGB togather (easy). But you have to generate the colour sub carrier and modulate it with Y-R and another subcarrier at an angle of 90 degrees and modulate it with Y-G. The colour subcarriers must be locked to the incoming sync pulses.
can you use RGB outputs - that's easy Google "vga video".
Frank

 

[crutschow]

No, it's not simple to add color, since you need to add the Composite Video 3.58MHz color carrier signal and phase modulate that with the color components from the VGA signal. So doing that without ICs would require a very complex circuit. What do you have against ICs?

 

[neazoi]

Nothing bad about ICs, it is just that my modulator **broken link removed** uses 1 transistor plus 3 for the audio section (not shown in this page yet), so I thought an IC would be too much for this circuit, but since the task is so complicated then an IC has to be used for color I guess...
Composite video is needed, not RGB, to modulate this modulator.

Do you know, if I use the AD724, will I still need the driver software shown in post #1 or it can be directly connected to the VGA and output color composite video? Does it take care all the timings and screen resolution without the need for a driver?

 

[crutschow]

You don't need any driver software or other circuitry with the AD724. According to the data sheet it can directly convert an RGB (VGA) video signal to a Composite output that can directly drive a 75Ω reverse terminated (75Ω resistor in series with the output) video coax cable. One required addition is a 3.58MHz crystal to output NTSC Composite Video. Figure 15 on the data sheet shows the required parts and connections to go from VGA to Composite. You don't need the added amps to drive the RGB monitor or the S-Video output connector, if you aren't using those functions.

Note that the VGA output must be standard, interlaced video with a 60Hz frame (refresh) rate and a 15.74KHz horizontal scan rate. The chip can't convert one scan rate to another (that would require extensive added logic and circuitry).

It would be much simpler and likely cheaper to just buy a converter such as this.

 

[neazoi]

Note that the VGA output must be standard, interlaced video with a 60Hz frame (refresh) rate and a 15.74KHz horizontal scan rate. The chip can't convert one scan rate to another (that would require extensive added logic and circuitry).

It would be much simpler and likely cheaper to just buy a converter such as this.

I have a PC laptop with vga output, do you think it is the standard port or any way to check it out?
BTW, when I run the driver software mentioned before, the detection shows the attached settings. I think this program converts non-standard vga to vga that can drive the composite encoder directly, but I am not sure.
I would prefer to homebrew it, as you have already guessed

 

[crutschow]

I think you would need to have the NTSC TV button selected in the Video Standard column for proper operation of the converter (and I think you want to unselect the Exit to VGA Mode Option).

 

[betwixt]

You have a problem Neazoi, your country uses PAL with 720x576 pixel picture and the VGA standard is 640x480 pixels. If your TV/Monitor can use 525 line/ 60Hz NTSC the suggestions by Crutschow is good, if you can only use 625 line/50Hz PAL the subcarrier frequency is 4.433618MHZ instead of 3.579MHz but the other timings will be wrong as well so there is no guarantee the scan will lock or the burst timing will be satisfactory to allow it to produce color.

Brian.

 

[neazoi]

You have a problem Neazoi, your country uses PAL with 720x576 pixel picture and the VGA standard is 640x480 pixels. If your TV/Monitor can use 525 line/ 60Hz NTSC the suggestions by Crutschow is good, if you can only use 625 line/50Hz PAL the subcarrier frequency is 4.433618MHZ instead of 3.579MHz but the other timings will be wrong as well so there is no guarantee the scan will lock or the burst timing will be satisfactory to allow it to produce color.
Brian.

You mean either by using AD724 there will be a problem?
So there is no way I could take out a color composite out of the VGA in my case?
How about this program https://entechtaiwan.com/util/ps.shtm it claims you can set everything in software.

 

[crutschow]

The AD724 chip should also work to generate a PAL composite signal. Just use a 4.33MHz crystal for the color subcarrier and select PAL TV in the Video Standard box of your computer.

 

[betwixt]

You can already set everything in software, the VGA hardware is built upon it's predecessor the 6845 and is register compatible at the video generating level. If you look at the 6845 data sheet you will see all the settings the Entech program changes and you can experiment using 'debug' in a command window (if you are using Windows) to change the settings yourself. Note that although Entech are still selling their program, they have stopped development quite a time ago.

You might find this interesting too:
https://electronics.stackexchange.com/questions/22902/composite-rgb-video-from-simple-hardware

Brian.

 

[neazoi]

Thank you all for the help. I think I am going to try a ad724 with the simple software to see how it goes.