040 - How Video Works on PC

Video is a visual images, usually used to create a movie. Video frequency (VF) of a signal conveying the image and synchronizing pulses in a television broadcasting system. An video frequency is an electrical alternating current within the 50 to 60,000 hertz range that can be used to produce visual images.

Video are generally referred to as visuals that are viewable to humans. Moving images stored as a sequence of static images called frames representing snapshots of the scene, taken at regularly spaced time intervals, e.g 50 frames per second. Apart from the frame rate, other important properties of a video are the resolution and colour depth of the individual images. Video data is typically stored and transmitted in a format that includes synchoronised sound.

How Video Captured Work

A video camera is similar to a 35mm film camera in that it has a body, lens and light sensitive recording ability. The major difference is of course that instead of silver halide film there is an electronic semiconductor sensor. This needs rather comples control electronics, and differently sized optics compared to a film camera. The advantages include compactness and the abilites to preview the images and store them on computer.

The amount of light falling on the sensor is controlled by camera shutter speed, aperture size and lens, just as in film photography. The simple block diagram below shows the basic structure of a digicam:

A conventional CCD or CMOS sensor can see only in shades of grey. In other words it senses the brightness of the light falling on it. Colour is given by putting a filter in front of the sensor, with a particular pattern of red, green and blue filters, one for each sensor pixel. This is known as a Bayer Filter and is represented diagrammatically in the image below.

Charge-Coupled Device (CCD) • is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by shifting the signals between stages within the device one at a time. CCDs move charge between capacitive bins in the device, with the shift allowing for the transfer of charge between bins.

Complementary Metal-Oxide-Semiconductor (CMOS) • is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits. CMOS technology is also used for several analog circuits such as image sensors, data converters, and highly integrated transceivers for many types of communication.

Analogue to Digital Converter (ADC) • is a device that converts analog signals into digital signals.

Demosaicing • For every group of 3x3 photosites the color of each photosite is surrounded by four of the other two colors. This enables the algorithm to work out the true color of each photosite.

There are more green filters because the human eye is most sensitive to changes in green.

The filter and sensor produces a mosaic of red, green and blue light levels. Powerful computer routines in the camera circuitry then convert this into an image that can be interpreted by the human eye.

Before the image is stored further in-camera processing deals with white balance, contrast and edge sharpness. The final computer readable file is then placed on the camera storage card.

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