Difference between RGB (VGA) and YPrPb (YUV)
YUV or YPrPb
Analog component video contains three feeds. The luminance signal (Y) contains the black & white (brightness) information from the original RGB signal. The color difference signal B-Y (PB) contains the blue information, minus the luminance information. The color difference signal R-Y (PR) contains the red information minus the luminance information. In the display device the three channels are recombined to produce a full color picture, recreating the green information within this process.
RGB
The analog RGB signal is carried over three, four or five lines (depending on the sync characteristics). Three lines carry the color information for the red, green and blue components: R,G,B. Each of the three channels R, G and B include the color signal and the luminance information.
The sync signal can be provided as sync on green RGsB, one combined sync signal RGBS, separate sync signal for horizontal and vertical RGBHV or sync on all three RsGsBs. For fully separated sync transmission five individual channels (cables) are used and the H and V channels carry the horizontal and vertical synchronization.
RGB video signals and RGB computer grphics signals have some significant differences and are not compatible without a scaling unit!
Difference between RGB (VGA) and YPrPb (YUV)
Viewing any of the three channels of a RGB transmission separately, a black and white image is always visible, because the Y information is included in all three channels. This is a large wast of bandwidth, because actually the same signal is carried three times.
By using just one channel for the luminance information and two channels for the color information, bandwidth can be saved drastically by carrying the same information.
Component video YPrPb (YUV) consists of three signals. The first is the luminance signal, which indicates brightness or black and white information that is contained in the original RGB signal. Monochrome signals contain only intensity luminance information, also called luma. It is referred to as the Y component. The apostrophe indicates that the component is gamma corrected.
The second and third signals are called 'color difference' signals which indicate how much blue and red there is relative to luminance. The blue component is B-Y and the red component is R-Y. The color difference signals are mathematical derivatives of the RGB signal.
When color information is added to the video signal, the luma signal is left intact
for compatibility with existing equipment, and two color information components are added. They are called U'
and V'.
Y'Pr'Pb' (Y'U'V') signals are matrixed components. The luminance Y signal is derived from the Red, Green, and Blue components by the formula 30% Red, 59% Green, and 11% Blue. The R-Y signal (Pr) is derived by subtracting this luminance signal from the Red signal. The resulting signal contains only color difference component for red.
The B-Y or Pb signal is derived by subtracting the Y signal from the blue signal to get the color component of blue. A G-Y component for green is not needed because all information is carried with the Y signal and the red and blue color component.
The color components are also called color difference signals. The U' component is the difference between blue and luma and the V' component is the difference between red and luma.
The reason to use YPrPb instead of RGB is the large amount of saved bandwidth. In the RGB signal the luminance component is carried three times. Typically, the color components are about half the bandwidth of the luminance component. Therefore about 4 Mhz of the necessary RGB bandwidth of about 12 Mhz can be saved.
The luminance image (Y) will usually be transmitted 25 or 30 times per second, while the two difference signals will alternate at half that rate.
In the receiving unit the signal is matrixed and the original information is available again.
Y is luminance, U and V are the color differences: U = R-Y, V = B-Y
Sometimes there is confusion about connecting a YUV source (DVD player) to a tv-/plasma-/whatsoever-monitor or a projector.
Both can be handled on the DB-15 connector, but the signal formats are different.
A 'conversion cable' can be used to connect the YUV source on the DVD player to the DB-15 input on the display device, but this display device must have the input format switching between RGB and YUV. This cannot be switched automatically, connecting YUV to RGB will result in a green picture.
RGB is expecting and displaying red, green and blue, component video is transmitting red minus Y, Y and blue minus Y. Thus from the red cable it gets zero red, from the green cable it gets the green signal from the full spectrum of the Y signal and from the blue cable it gets zero blue signal, the only data it has to display is green.
YPrPb Pinout on 15-pin D-Sub
| |
| 1 | Pr |
| 2 | Y |
| 3 | Pb |
| 4 | n.c. |
| 5 | n.c. |
| 6 | Pr Ground |
| 7 | Y Ground |
| 8 | Pb Ground |
| 9 | n.c. |
| 10 | Ground |
| 11 | n.c. |
| 12 | n.c. |
| 13 | n.c. |
| 14 | n.c. |
| 15 | n.c. |
Digital Components Video Y' Cb'Cr'
Digital component video uses a modified form of the Y' U' V' color format. This format is called Y' Cb'Cr'. It is a scaled and offset version of Y' U' V'. The Cb' component is the blue color difference component, similar to the U' component in Y' U' V'. Likewise, Cr' is the red color difference, similar to V'.
In 10-bit digital video, the Y' component has a range of 64 to 940. The Cr' and Cb' components have ranges of 64 to 960. Values above and below the specified component ranges are reserved.
4:2:2 Sampling
In 4:2:2 digital component video, the Y' component is sampled at twice the rate as each of the
chroma components Cb' and Cr'. If the basic sample rate is 74.25 MHz, the
Y' component is sampled at a rate of 74.25 MHz and the Cb' and Cr' components are sampled at
half the rate: 37.125 MHz.
In the 4:2:2 sampling per sample we have 50% luma and 50% chroma information (the chrome word is again 50% Cb' and 50% Cr').
In the HDTV digital component video the luma and the chroma information is treated separately. For 10-bit digital video, there is a 10-bit luma channel and a 10-bit chroma channel. The chrome samples alternate between a Cb' word and
a Cr' word each time.
MPEG compression, as used in DVDs, digital TV Receivers and digital camcorders (MiniDV, DV), is coded in YCbCr. This signal is transmitted over a digital link such as FireWire or SDI. The international standard ITU-R BT.601 for digital video defines both YCbCr and RGB color spaces.
