The RGB signals in VGA and the RGB signals found in a SCART connector are electrically compatable, but the sync signals aren't. In VGA the vertical sync and horizontal sync are carried on separate wires, but in SCART they are both combined into a composite sync signal carried along a single wire. This circuit converts the component sync signals into a composite sync compatable with SCART. The red, green and blue signals can be connected through as-is.
NOTE:
- If your TV doesn't support RGB inputs then this isn't going to work for you. (Although you could probably feed the signal from this circuit into the RGB -> S-Video converter circuit and get S-Video and composite outputs but I haven't tried it).
- VCRs generally don't support RGB inputs.
- You need to be able to tell your graphics card to use your own resolutions and timings - if you're running Windows you're probably stuffed. See below for suitable modelines.
- Your graphics card needs to support interlacing - some don't, notably some versions of the official nVidia drivers for Linux are broken and won't let you turn on interlacing.
- Your graphics card must be able to support a very low dot-clock frequency (14.76MHz for PAL, something like 13.34MHz for NTSC).
- You must tell your graphics card to use negative vsync and hsync polarities.
- You're not going to be able to use anything remotely similar to this circuit to display PAL or NTSC signals on a VGA monitor. Most VGA monitors won't sync to PAL or NTSC frequencies so you would need to sample the signal into a framebuffer and read it out at about double the frequency. Not to mention all the deinterlacing you'd need to do to make the picture watchable.
- Some TVs need pin 16 of the SCART connector driven high to enable the input. You could pull power from the keyboard connector, etc. Bob van Loosen has provided a circuit which derives the required voltage from the vsync pin. (Note: I have not tried Bob's circuit myself).
Have a look at the circuit. It does no frequency conversion or scaling, etc. so your graphics card must output a signal at PAL or NTSC specification if your TV is going to have any chance of syncing with it. If you see 2 images side by side then your graphics card is outputting a standard VGA signal and your TV just happens to be syncing with every other sync pulse (the horizontal sync in VGA runs at exactly twice the frequency of PAL). You should also connect pins 5 and 11 on the VGA connector together to tell the PC that there's a colour monitor plugged in.
Using this circuit instead of using a TV output of a graphics card has some advantages:
- Cost.
- You get to fiddle with all the timings yourself by programming your VGA card (e.g. setting modelines in X), so you can set exactly how much overscan the picture has.
- One-to-one relationship between the lines - if you run your graphics card in 800x600 and then use the TV output, the card needs to scale the picture down (576 lines for PAL). So either some of the lines get blurred together or some of the lines don't get drawn at all. If you're really unlucky then the graphics card won't pay attention to interlacing and it'll blend some of the odd lines into even lines. If you drive the graphics card at the native PAL specification and feed the signal straight into the TV then you can guarantee that each even line in the framebuffer will match up with exactly one line in the even field on the screen and every odd line will match exactly one line in the odd field.
- VBI sync - your graphics card can tell the software when the vertical blanking interval has started so the software can synchronise video playback, etc with the TV so you don't get tearing.
- RGB output - graphics cards generally give you composite and S-Video outputs. If your TV will accept RGB then use it and get a better quality picture.
The following XFree86 modelines should work. PAL is specified to be 768x576 although 800 pixel wide modes are also listed below. Similarly, NTSC is 720x480 but 800 pixel wide modes are listed below too. Use the extra-wide modes at your own risk - they probably won't look too good since the dot pitch of the TV's CRT is probably too large and there is a small chance you could damage the TV.
Modes ending in "pali" run in the usual interlaced PAL mode, "pal-half" modes provide half the vertical resolution so you don't get an interlaced display. Similarly the "ntsci" modelines give you full interlaced NTSC whilest "ntsc-half" is non-interlaced NTSC whith half the vertical resolution.
Modeline "800x576pali" 15.38 800 823 895 984 576 580 583 625 -hsync -vsync interlace Modeline "800x288pal-half" 15.38 800 823 895 984 288 290 292 313 -hsync -vsync Modeline "768x576pali" 14.76 768 789 858 944 576 580 583 625 -hsync -vsync interlace Modeline "768x288pal-half" 14.76 768 789 858 944 288 290 292 313 -hsync -vsync Modeline "720x576pali" 13.88 720 742 808 888 576 580 583 625 -hsync -vsync interlace Modeline "720x288pal-half" 13.88 720 742 808 888 288 290 292 313 -hsync -vsync Modeline "704x576pali" 13.50 704 722 786 864 576 580 583 625 -hsync -vsync interlace Modeline "704x288pal-half" 13.50 704 722 786 864 288 290 292 313 -hsync -vsync Modeline "650x576pali" 12.50 650 669 728 800 576 580 583 625 -hsync -vsync interlace Modeline "800x480ntsci" 15.10 800 808 879 960 480 484 487 525 -hsync -vsync interlace Modeline "800x240ntsc-half" 15.10 800 808 879 960 240 242 244 263 -hsync -vsync Modeline "720x480ntsci" 13.59 720 727 791 864 480 484 487 525 -hsync -vsync interlace Modeline "720x240ntsc-half" 13.59 720 727 791 864 240 242 244 263 -hsync -vsync Modeline "704x480ntsci" 13.34 704 714 776 848 480 484 487 525 -hsync -vsync interlace Modeline "704x240ntsc-half" 13.34 704 714 776 848 240 242 244 263 -hsync -vsync Modeline "657x480ntsci" 12.50 657 667 725 792 480 484 487 525 -hsync -vsync interlace Modeline "657x240ntsc-half" 12.50 657 667 725 792 240 242 244 263 -hsync -vsync Modeline "640x480ntsci" 11.32 640 646 703 786 480 484 487 525 -hsync -vsync interlace Modeline "640x240ntsc-half" 11.32 640 646 703 786 240 242 244 263 -hsync -vsync