Component video
From Free net encyclopedia
Image:Component video RCA.jpg Component video is a type of analog video information that is transmitted or stored as two or more separate signals. Component video can be contrasted with composite video (such as NTSC or PAL) in which all the video information is combined into a single signal such as a TV broadcast. However, component video cables are gradually being replaced by the higher quality DVI and HDMI cables.
Analog component video
Analog video signals (also called components) must provide red, green and blue signals to create a television image. The simplest type, RGB, consists of the three discrete red, green and blue signals sent down three coaxial cables. There are a number of variant schemes which vary according to how synchronization is handled. If a synchronisation signal is sent on the green channel, it is called sync-on-green. Some schemes use a separate sync channel, for instance the European SCART connection scheme in which the video signal occupies four (R,G,B + sync) of the 21 pins in the interface. SVGA, another RGB scheme, is used worldwide for computer monitors (this is somethimes known as RGBHV, as the horizontal and vertical synchronisation pulses are sent on separate lines).
An alternative type of componentization does not use R,G,B components but rather a colorless component, termed luminance combined with one or more color-carrying components, termed chrominance, that give only color information. Mulitiple chrominance channels allow for more precision and speed in mapping the RGB colour space. This componentization scheme is a linear transformation of the sRGB color space. This type of signal is usually what is intended when people talk of component video today. Variants of this format include YUV, YCbCr, YPbPr and YIQ commonly used in video systems.
In component video systems, additional synchronization signals may need to be sent along with the images. The synchronization signals are commonly transmitted on one or two separate wires, or embedded in the blanking period of one or all of the components. In computing, the common standard is for two extra wires to carry the horizontal and vertical components ('separate syncs'), whereas in video applications it is more usual to embed the sync signal in the Y component ('sync on luminance').
S-Video is another type of component video signal, because the luminance and chrominance signals are transmitted on separate wires. This connection type, however, cannot produce high definition or digital-quality pictures (pictures with more than 480 interlaced lines of video for NTSC or more than 576 lines of interlaced video for PAL). Component video is capable of producing signals such as 480p, 720p, 1080i and 1080p, but digital connections such as DVI (video only) and HDMI (which can also include up to 8 channels of audio) give better results at the higher resolutions (up to 1080p). Template:Analogvideo
Examples of international component video standards are:
- RS-170 RGB (525 lines, based on NTSC timings, now EIA/TIA-343)
- RS-343 RGB (525, 625 or 875 lines)
- STANAG 3350 Analogue Video Standard (NATO military version of RS-343 RGB)
Digital component video
The digital component video is sometimes referred to as 4:2:2. This means that for every 4 pixels of luminance (Y) information, only 2 pixels of Pb (Blue Difference), and 2 pixels of Pr (Red difference) are encoded. This is the scheme used for the DVD format. The numbers also represent the relative number of bits (but not the actual number) used to carry the three pieces of information at each pixel. The colour information is spread across the pixels it represents.
Another scheme encountered will be 4:1:1. This is the scheme used on the NTSC version of the DV (and hence miniDV) tape formats for camcorder use. In this case for every 4 pixels of luminance, only 1 pixel of the Pb and Pr difference information is encoded for each line.
To provide apparent confusion, the PAL version of the DV and miniDV tape format use 4:2:0 which would appear to suggest the absence of the Pr signal altogether. In fact, the system takes advantage of features of the PAL colour system, and encodes 2 pixels of Pb colour difference for every 4 luminence pixels on odd lines only. On even lines, 2 pixels of Pr colour difference are encoded for every 4 luminance pixels. In each case the missing information is recovered from the previous line (and is thus closer to the French SECAM system than PAL). This doubles the horizontal resolution but halves the vertical resolution. This is acceptable, because the PAL analogue colour system does in fact have half the vertical resolution of the NTSC system.