RGB Scart

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Warning

Warning: As these types of cables are directly driving the television, there exists potential for damage through mistakes in cabling or even sending an out-of-spec signal to the television. Consider yourself warned and proceed at your own risk.

Forword

This guide is dedicated to all UK GAME employees. Me: "Have you any proper scart cables?" Employee: "There's one included." Me: "That's a composite over scart converter." Employee: "They're all the same." Me: "No they're not, you see RGBs is not composite sync, and a scart can cary s-video, etc..." Employee: "They're all the same." Me: "Err... ok then."

I'm ( pepsi_max2k ) about to edit and update the whole of this page after completing a cable based on Steve Hill's guide at http://www.nexusuk.org/projects/vga2scart/ . Therefore if you're looking for info on a VGA to Scart (RGBs) cable then bear with me for the next few weeks while I update the page.

If you happen to live in the UK, I'll also have a spare one made up within a couple weeks which I'll send you for cost price + postage (only a few £ max), so don't worry if you can't solder owt, although mine aren't exactly works of art...

--Pepsi max2k 20:55, 11 January 2008 (UTC)

Introduction

SCART is a European standard for connecting audio-visual equipment together. Different methods of transmitting video such as composite, Y/C (S-video) and RGB are supported on the same cable. The RGB method of transmission is of particular interest as it uses Red, Green, Blue and Composite Sync pins, which is very close to the way signal is carried over a VGA cable, Red, Green, Blue, Horizontal Sync and Vertical Sync. This makes is possible to adapt a VGA output to drive a RGB-capable SCART television directly.

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. You need to convert the component sync signals into a composite sync compatable with SCART. The red, green and blue signals can be connected through as-is.

Advantages

Driving a TV directly has some advantages. The quality is superior to composite or S-video, and as the video card's TV encoder is completely bypassed, custom resolutions and refresh rates can be programmed. On the down side, this method can require some experimentation to get right.

1. 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.
2. 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 very odd line will match exactly one line in the odd field.
3. 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.
4. 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.

Requirements

• A television with an RGB capable SCART socket. Just the presence of a SCART socket does not mean the television accepts RGB, as it will also accept a composite over SCART image. Check the manual, or try changing between Composite and RGB output on a SCART connected device to see if there is any change in image quality (the yellows will be noticeably more vibrant in RGB mode).
• A video card and drivers capable of supporting interlaced resolutions - some don't, notably early versions of the official nVidia drivers for Linux are broken and won't let you turn on interlacing.
• A video card capable of supporting a very low dot-clock frequency (14.76 MHz for PAL, 13.34 MHz for NTSC.)
• With Nvidia cards, you must tell your graphics card to use negative vsync and hsync polarities.
• You need to be able to tell your graphics card to use your own resolutions and timings - if you're running Windows you should try using Powerstrip. See below for suitable modelines.
• Some TVs need pin 16 of the SCART connector driven high (powered with 1 to 3V) to enable the input. You can check this by blocking pin 16 (with some electrical tape) on a seperate RGB device; if the output changes to composite or no image with pin 16 blocked, your TV needs 16 driven high. You could pull power from the keyboard connector, connect up a 1.5V battery, etc. Bob van Loosen has provided a circuit which derives the required voltage from the vsync pin.

Cable types

These fall into two groups. Some Radeon video cards are capable of generating composite sync directly and therefore need only a simple cable connecting the pins together. All other cards require a small circuit to convert the horizontal/vertical sync into composite sync. You can tell the circuits apart by the because the more complex version will have at least one transistor.

Nvidia Cable

This circuit 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).

Video cards

Cards known to support interlaced resolutions and TV frequency dot clocks
Name	Interlacing?	<15 MHz dot clock?
nVidia 7800 GS	YES	?
nVidia FX 5200 series	YES	YES
nVidia GeForce4 MX	YES	YES
ATI Radeon 8000 series
ATI Radeon 7500	YES	YES
ATI Radeon X300SE (RV370 - Dell Optiplex GX280)	YES	YES
Via Unichrome IGP
Intel GMA950

Xorg Configuration

X Windows needs to be configured to generate the signals at the right frequencies for your TV.

Example xorg.conf

This is a working Interlaced PAL TV setup. The relevant sections of xorg.conf are below. Note the "composite" keyword which causes the Radeon to generate composite sync directly.

xorg.conf

...

  Section "Monitor"
        Identifier   "Generic Monitor"
        HorizSync    14.0 - 18.0
        VertRefresh  45.0  - 65.0
        Option      "DPMS"
        Modeline     "720x576" 15.125 720 778 834 968 576 579 607 625 composite interlace +hsync +vsync
  EndSection
  Section "Device"
        Identifier  "ATI Technologies, Inc. RV370 5B60 [Radeon X300 (PCIE)]"
        Driver      "ati"
        Option      "DynamicClocks" "true"
        Option      "ForceMinDotClock" "14MHz"
        Option      "MergedFB" "false"
        Option      "IgnoreEDID" "true"
        Option      "VGAAccess" "false"
        BusID       "PCI:1:0:0"
  EndSection

...

Links

Here are some links to information on how to convert the VGA out signal to a suitable signal for a RGB Scart input.

• VGA to RGB SCART Direct Drive with ATi Radeon 7500 / 8500 / 9x00
• VGA-TV
• AV Forums : VGA - Scart RGB?
• VGA to SCART Converter
• VGATV Homepage
• VGA to TV converter
• X on TV