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Cable components

TV-Out pin header connections


The S-Video cabled version has been tested and works!
The RGB cabled version has been tested and works!
Regardless of this, the information presented here is by no means guaranteed - modify your hardware at your own risk!

Different video formats

First a little introduction into the different video formats available to us.
  • Composite
  • S-Video
  • RGB
Composite is where the colour and the black and white signals are merged into one, and then split out at the other end. A standard PAL aerial uses this type of format. The quality is degraded because the black and white, and colour components are all transmitted along the same line.

S-Video is better quality. It transmits the black and white signal (Y) separately from the combined colour signal (C). The black and white signal is send at full bandwidth.

RGB is where the colours are transmitted separately, in full bandwidth, in a similar way to a computer monitor. As an aside, an intermediate format derived from RGB is YPbPr where (like S-Video) the black and white signal is sent at full bandwidth. However the colour components are sent as difference signals which are mathematically obtained.

Generally, RGB components give a better picture quality than S-Video, although it has been argued that with good cabling this relationship can be reversed. Composite video is of a poorer quality than both RGB and S-Video.

For our purposes, making a connector to facilitate RGB and S-Video connections will be more than adequate. Most TV's now have SCART plugs, and SCART to S-Video adaptors are readily available, so this would be the easiest option with good results to get the box up and running. However, later we might want to compare the quality of RGB and S-Video to get the best from the board, and so adding a socket for RGB will allow us this freedom.

The SCART standard

The SCART standard can carry both RGB component video and S-Video. Here is a run down of the pin outs used for the different formats:

Pin RGB S-Video Composite
1 Audio Output Right Audio Output Right Audio Output Right
2 Audio Input Right Audio Input Right Audio Input Right
3 Audio Output Left Audio Output Left Audio Output Left
4 Audio Ground Audio Ground Audio Ground
5 Blue Ground    
6 Audio Input Left Audio Input Left Audio Input Left
7 Blue    
8   Aspect Ratio Switching Aspect Ratio Switching
9 Green Ground    
10 Vertical sync Digital Data Bus Input  
11 Green    
12 Horizontal sync Digital Data Bus Output  
13 Red Ground Chrominance (C) Input/Output Ground  
14 RGB Status / Fast Blinking Ground Digital Data Bus Ground  
15 Red Chrominance (C) Input/Output  
16 RGB Status / Fast Blanking Output    
17 CVBS (Sync) Output Ground Luminance (Y) Ground CVBS Output Ground
18 CVBS (Sync) Input Ground Luminance (Y) Input Ground CVBS Input Ground
19 CVBS (Sync) Output Luminance (Y) Output CVBS Output
20 CVBS (Sync) Input Luminance (Y) Input CVBS Input
21 Common Ground (casing) Common Ground (casing) Common Ground (casing)
Key: CVBS = Composite video, blanking, sync

The signal on pin 8 to signify the aspect ratio can be in one of the following ranges:
  • 0V - 2V, no aspect ratio specified
  • 5V - 8V, 16:9 aspect ratio
  • 9.5V - 12V, 4:3 aspect ratio

Pin 16 specifies whether RGB is on or off:
  • 0V - 0.4V, RGB off
  • 1V - 3V, RGB on


Here is a list of what each of the pins does, and which need to be connected under the different formats:

Pin Name RGB S-Video Composite
1 +2.5V RGB Status Output    
2 -      
3 B_Pb Blue Output    
4 Yh_G Green Output    
5 GND CVBS (Sync) Ground Luminance (Y) Ground Composite Ground
6 GND Red, Green, Blue Ground Chrominance (C) Ground  
7 CVBS CVBS (Sync) Output Luminance (Y) Output Composite Output
8 C_R_Pr Red Output Chrominance (C) Output  
9 CSO      
10 -      
Key: CSO = Colour Separation Overlay - Chroma key

Wiring the TV-out socket

Since we will only be using either RGB out or S-Video out, it makes sense (if possible) to re-use the socket functionality as much as possible. We need to connect 3 extra pins for RGB output, and so a second S-Video socket will be used for this purpose. Hence, in S-Video mode only one socket will be connected, whereas in RGB mode we will utilise the S-Video socket and an extra custom wired socket.

The 4-pin S-Video sockets will look like this:

4-pin S-Video socket 4-pin custom RGB socket

Each pin's function for S-Video, and the function we will assign for RGB output, is specified below. The connections from socket pin to header pin are also listed below.

Socket Pin S-Video function RGB function Connect to header pin
1 1 Luminance Ground CVBS (Sync), RGB Status Ground 5
1 2 Chrominance Ground Red, Green, Blue Ground 6
1 3 Luminance (Brightness) + Sync CVBS (Sync) Output 7
1 4 Chrominance (Colour) Red Output 8
2 1   Blue Output 3
2 2   Green Output 4
2 3   RGB Status Output 1

Creating the custom RGB lead

The custom RGB lead should give a better quality picture compared to the S-Video solution. Two S-Video plugs will be required for this, along with the SCART plug and the audio jack plug. Remember that the pinout images given above relate to either looking into a socket, or looking from the wire end into a plug - not looking at the end of a plug. The plugs need to be wired as follows:

SCART Pin Function Plug Pin
2 Audio Right Jack plug Ring
4 Audio Ground Jack plug Sleeve
5 Blue Ground 1 2
6 Audio Left Jack plug Tip
7 Blue 2 1
9 Green Ground 1 2
11 Green 2 2
13 Red Ground 1 2
14 RGB Status / Fast Blinking Ground 1 1
15 Red 1 4
16 RGB Status / Fast Blanking 2 3
18 CVBS (Sync) Ground 1 1
20 CVBS (Sync) 1 3
21 Common Ground (casing) 1 1

Comparison of picture quality

There is a huge difference in quality that the two leads offer. When using the S-Video lead, there is a netting effect that can be seen on all of the picture, and this is particularly obvious where blocks of the same colour are being shown. That said however, the colours displayed are rather natural, and as you would expect to find in the "real world".

In comparison the RGB cable results in a much brighter picture, with more vivid colours. The netting effect that is visible in the S-Video solution doesn't appear when using the RGB lead. The picture is also shown throughout the boot process (which isn't the case with the S-Video lead - only when the system has loaded the gnome login will a picture be shown).