Description of Composite Video Signal construction

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Composite Video Signal:
The most elementary job of a video decoder is to take apart the color from the black and white information for the video composite signals. Task has been achieved many ways ever since introduction of color television over fifty years ago. Many different separation methods have been used through years. With accessibility of the new cost effective technologies, consumers have been seeing gradual improvement in the picture quality and the detail. The advances in display tube technology and semiconductor processes have pushed the technological envelope providing sharper, more the robust video but it separating chrominance from luminance information is chiefly challenging due to the fact that the signals overlap each other in frequency spectrum. How do we separate them, while minimizing display artifacts .

Composite video Signal Construction
Composite video signal is constructed with three basic elements:
– Luminance Information from DC to 5.5MHz (B&W Detail)
– Chrominance Information modulated onto carrier (at 3.58MHz or 4.43MHz)
– Synchronization Information (Horizontal and Vertical Sync)
The 3 analog elements of composite video signal carry all information necessary to display 2 dimensional picture on a cathode ray tube (CRT) television.

Horizontal blanking interval refers to a part of the method of displaying images on a computer monitor or television screen via the raster scanning. The CRT screens display images by the moving beams of electrons very quickly across screen. Once beam of monitor has reached the edge of the screen, A beam is off switched , and deflection circuit voltages (or currents) are returned to values they had for the other edge of screen; this would have effect of retracing screen in opposite direction, so the beam is turned off during this time. This part of line display process is Horizontal Blank.
Vertical Interval
After CRT is filled with lines of video, scanning beam must return to top of screen so it can start all over again. This repositioning time, called vertical blanking interval, is composed of 21 horizontal lines which are not displayed .This portion of the composite video waveform is extremely important since it contains timing pulses, FCC regulated test signals, source identification codes, reference signals, and information regarding caption availability for the deaf during a broadcast, Most TV stations transmit special reference signals during the vertical blanking interval. The two most common signals are the VITS (vertical interval test signal) and VIRS (vertical interval reference signal).
The vertical blanking interval also known as the vertical interval or VBLANK, which is the time difference between the last line of 1 frame or field of the raster display, and beginning of first line of next frame. It is present in the analog TV VGA, DVI and the other signals. During the VBI the incoming data stream is not displayed on screen. In the raster cathode ray tube displays  beam is blanked to evade the displaying the retrace line  see raster scan for the details. Signal source, such as a television broadcast , does not supply image information during the blanking period.

Composite Video Signal  graph:

composite vidio

Question answers:
Q1: How to separate the Low Pass or High Pass Filter?
Ans: One of the primary separation techniques used is the Low Pass or High Pass Filter technique to separate the luminance and chrominance signals from the composite video signal. The was the simplest and least costly to implement when color TV was first available.

Q2: Give details the process of putting the scan-lines together to an image?
Ans: An image is built from the 625 of scan lines, but TV doesn’t show 625 numbers of lines. Some of lines are used for synchronization pulses, and some lines are invisible  because old TVs needed some time to move the electron beam from the bottom of screen.

Q3: Note down the basics for color generation?

Ans: As we know video signals, the software needs to create modulated sinus and cosines waveforms for color information and sum them together with the intensity waveform. To get good result the sample rate needs to be much higher than the color carrier frequency, and the software  also be able to do needed calculations for the waveform which in total would need a very powerful processor if there is no hardware to help. The SX processor performing 50MIPS would not be good enough using this technique.

Q4: What Are the Emulators available for color composite video signal?

Ans: There are unfortunately no emulators available for color composite video signal generation with the SX chips. However, there are some interesting open source stuff that might could be used as a good base for developing SX color video game emulator.

Q5: What is NCST video?
Ans: NTSC stands for National Television System Committee is a  TV standard which  is mostly used in North America and Japan. It uses the familiar 4:3 aspect ratio i.e. the ratio of picture width to its height and uses 525 scan lines per frame at 30 frames per second (fps).


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