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YUV(YCbCr) to RGB converter


May 6, 2002


Recently, most digital video equipments, such as video recorder, DVD player and TV game, have component video output. The component video signal is like RGB video signal, but it cannot connect to RGB monitor directly. Thus I designed and built YUV(YCrCb) to RGB converter to use old TV monitor or video equipment which does not have component video input.

For old RGB monitor....no, my principal aim is to generate RGB signal from component signal from PlayStation 2 for scan converter because recent shipped PlayStation 2 fixes configuration of video output format to component video when used as DVD player :-( Converting the component signal into RGB signal can enjoy the game and DVD with high quality picture.
However, the PlayStation 2 can be fixed to RGB output configuration with only a jumper wire. If you wish to get only RGB signal from the PlayStation 2, I recommend this way instead :-)

The theory of RGB to YUV conversion

The component video signals are generated by separating a luminance component and two chrominance components from RGB signals with following formulae:

Y   = 0.299R + 0.587G + 0.114B        Luminance component

R-Y = R - (0.299R + 0.587G + 0.114B)
    = 0.701R - 0.587G - 0.114B        Chrominance component (Red)

B-Y = B - (0.299R + 0.587G + 0.114B)
    = -0.299R - 0.587G + 0.886B       Chrominance component (Blue)

Note: These parameters are for SDTV(525/625), not for HDTV(750/1120).

G-Y crominaice component can also be generated. However, to restore the RGB signal, Y and two chrominance components will do. The G-Y component contains least chrominance in the three chrominance components so that this term is omitted to minimize conversion error.

When restore RGB signals from component signals, following formulae are applied.

R = Y + (R-Y)
G = Y - 0.51(R-Y) - 0.186(B-Y)
B = Y + (B-Y)

The contents of component video signal are these three signals, Y, R-Y and B-Y. You will able to understand that these are loss-less conversion. However, in order to reduce overall video signal band-width to be recorded or transmitted, the band-width of chrominance components are reduced and some compression processes are applied. Human eyes are sensitive to luminance but not sensitive to chrominance. The quality of chrominance components, such as band-width, SNR and digitizing resolution, can be reduced compared with luminance component.

The chrominance signal levels at the transfer line are normalized to luminance amplitude because it is easy to A-D, D-A conversion and transmittion processes. Following are attenuation ratio for the chrominance signals:

Cr = 0.713(R-Y)
Cb = 0.564(B-Y)

And sync pulse is added to luminance signal as -0.3V pulses. These are the component video signals that is appering at input/output point. Typical signal levels at the interface connector are from -0.3V to +0.7V for luminance signal, 0.35V for chrominance signals.

Hardware

To convert component video signals into RGB video signals, the alithmetic circuit composed with some OPAMPs is used. However, only it is not sufficient to build complete set, some glue logics are also required.

Most video signal outputs except some high end equipments are AC coupled, DC restore circuit is needed to video input part. To remove sync pulses added to Y signal, blanking circuit is also required. These function should apply for completed process. The timing generator for the functions is realized with only a CPLD because to realize it with discrete components will be complex. Following images show the generated wave form of blanking pulse and clamping pulse. Top one is Y input, below two are /BLANK and /CLAMP. Left image is near horizontal blanking area, right image is near vertical blanking area.




In this project, not only RGB output, NTSC outputs (YC and CVBS) are also available. To encode RGB signals into NTSC signals, line locked clock is required to the NTSC encoder so that base clock is generated with a PLL locked to incoming video signal. If you don't require NTSC outputs, the Circuit Diagram 2 with fixed clock source can be used instead, PLL and NTSC encoder is not required.
No special part is used in this project, most parts will able to be obtained with ease. OPAMPs can be replaced with equivalents which is high speed voltage feed-back OPAMPs avobe 50 MHz. Negative voltage generator can be replaced with any DC-DC converter.

Relationship of picture quality between component video and its source

Difference between component video signal and YC video signal is the transfer method of chrominance components.

At YC video signal, a color sub-carrier is modulated by two chrominance signals with quadrature balanced modulation, and transferred as a chrominance signal. Therefore, the band-width of chrominance signals are reduced to half of the sub-carrier frequency. At NTSC video format, I,Q signal which is shifted 33 degree from R-Y,B-Y axis is used instead of R-Y,B-Y signal, the band-width for I signal is 1.5 MHz, for Q signal is 0.5 MHz. However, most video decoders seem decode in R-Y,B-Y axis and both signals are limited to 0.5 MHz.

At component video siglal, two chrominance signals are transferred with two separated lines directly. This is full band-width transmittion. When video signal is from TV game whose source is RGB video buffer, difference between component signal and YC signal appers conspiculusly as some effects shown in following images. In this case, component signal is better than YC signal.


Left image shows the image from component signal and right image shows the image from YC signal. You will able to recognize difference between two images, the color of green letters in the right image is diffused.


At boundary between hair tips and skin, when chrominance band-width is limitted, the colors are mixed at complex area.


When video source is DVD, the data on the DVD media is recorded as 4:2:0 sampled data. The sampling method reduces horizontal and vertical resolution of chrominance signals by half of luminance signal. Therefore, the band-width of the chrominance signals are alredy reduced, we cannot recognize any diffference between the video signals at most natural picrures except for CG pictures.

Thus YC signal is sufficient for DVD. However as for HDTV signals, they cannot transfer as YC signal. In this case, there is not that no merit of the component signal.

Technical Data

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