In this article, we look at component video signal: what it is, where it is used, and what cables are used for component video. This is the forth in this series of articles outlining the different video signals used in home entertainment for connecting VCRs, DVDs, set top boxes, cameras and laptops to TVs and projectors. In the previous articles we looked at Composite Video, S-video and RGB video.
“Component video” could well be referred to as “confused video”. Component video is often confused with RGB video in terms of what it is. Component video can be confused with S-video as the luma (picture) and chroma (colour) components of S-video are kept as separate components. Even the phrase “component video” is easily confused with “composite video” during discussions about either. So, lets try to reduce the confusion.
What is Component video?
In the last article we saw how RGB is three coloured picture signals, and the problem is that this is a lot of information to process and store. So when DVDs were invented, they needed a way of preserving the separation of the three coloured signals, and yet reduce the overall amount of information. So instead of having the complete picture for each colour, component video uses one channel/cable to carry the basic black & white picture information (luma). As we saw in the article on S-video, the letter “Y” is used to designate the luma channel. The horizontal and vertical sync (timing information) is also combined in this channel. So the “Y” component is the basic black and white picture bit of component video. To try to confuse you, the Y (luma + sync) component is connected to the green plug and socket of connectors (even though it doesn’t carry any colour information at all – certainly no green).
So all the black & white picture information (luma) is on one channel/cable. Now what about the colour information? This is where it gets tricky. The other channels/cables in component video are called the “colour difference” signals. That is, how much blue and red there is relative to the black & white picture (luma). Mathematically, the blue component is said to be blue minis luma (B-Y), and the red component is red minus luma (R-Y).
So that is component video, just the three channels (cables) are required: Y, B-Y and R-Y. The more astute among you will want to know about the green signal. This is the clever bit: the display (TV or projector) knows how bright the image is (from the Y component). From the B-Y and R-Y components it knows how much of the image is blue and red, so it simply says the rest must be green – simple eh?
To add even more to the confusion, different terms are used to indicate each component. This table tries to make sense of it all.
|Description||Symbol||Cable Colour||Label 1||Label 2|
|Luma + Sync||Y||Green||Y||Y|
|Blue – Luma||B-Y||Blue||Pb||Cb|
|Red – Luma||R-Y||Red||Pr||Cr|
Although there are technical differences between the labels often used for component video, they are generally used to mean the same thing. So don’t be confused when you see the green, blue and red sockets labelled Y B-Y R-Y or Y Pb Pr or Y Cb Cr. These labels are mostly used interchangeably these days. As in the photo of the back of a DVD player, either type of label may be used, and sometimes both are used.
Where Component Video is Used
Component video is the highest quality analogue video signal commonly used on domestic equipment. It is used extensively on DVD players – indeed the three components of component video are separately stored on a DVD in digital form. Component video is also available from set top boxes, satellite receivers and digital recorders. It can be used for standard definition and high definition images.
Most recent TVs, screens and projectors also have the three component video connectors. Connecting the “component video out” of a device to the “component video in” of a display will give you the highest quality analogue picture available.
Cables to use for Component Video
Red, green and blue cables are most commonly used for component video. The green connector normally connects to the green socket on the sending device (DVD player or STB) and to the green socket of the display device (screen or projector). The “green” channel is normally labelled “Y”, as it is carrying the luma (black and white picture information). The red and blue plugs connect to their corresponding coloured sockets also. The cables use the same RCA (phono) plugs we discussed in the article for composite video.
There is nothing magical about the colour of the plugs and sockets. The colours are there simply to make it easy for you to connect each component of the component video from the sending device to the correct input of the display device. If you have other coloured leads, you can also use them – just make sure you connect the “Y” signal out to the “Y” signal in, and the same for the red and blue difference signals. If need be, you can even use the yellow, red and white composite video (and audio) leads for component video over short distances – just make sure the right output component video signals are connected the their respective inputs of the display device.
If the cables are not connected correctly, there is normally no damage done, but the colour of the image will be wrong. Obviously, if the “green” (Y) only is connected, you only get a black & white image. If the “red” and “blue” cables are swapped, the colours will simply be wrong (as the red and blue colour information is swapped). If the “green” is not connected correctly most displays will indicate “no signal” as it can’t detect the basic picture.
Good quality cables are required for component video, especially over longer distances. As mentioned in the article on composite video, good quality RG59 video cable can be used for lengths up to 100 metres (300 ft).