Multimedia Systems

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  • Topic: Video, NTSC, PAL
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MULTIMEDIA SYSTEMS

Video
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion. Raw video can be regarded as being a series of single images. There are typically 25, 30 or 50 frames per second. There are two types of video recording systems/formats. Also, the term video ("video" meaning "I see", from the Latin verb "videre") commonly refers to several storage formats for moving pictures. These are Analog Video and Digital Video. The world we sense is full of analog signal; electrical sensors such as transducers, thermocouples, microphones convert the medium they sense into electrical signals. These are usually continuous and still analog. These analog signals must be converted or digitised into discrete digital signals that computer can readily deal with. Special hardware devices called Analog-to-Digital converters perform this task. For playback Digital-to-Analog must perform a converse operation.

Analog Video
Analog Video is usually captured by a video camera and then digitised. Digital Video
Digital video is a type of video recording system that works by using a digital rather than an analog video signal. Digital video comprises a series of orthogonal bitmap digital images displayed in rapid succession at a constant rate. In the context of video these images are called frames. We measure the rate at which frames are displayed in frames per second (FPS). Since every frame is an orthogonal bitmap digital image it comprises a raster of pixels. If it has a width of W pixels and a height of H pixels we say that the frame size is WxH. Pixels have only one property, their color. The color of a pixel is represented by a fixed amount of bits. The more bits the more subtle variations of colors we can reproduce. This is called the color depth (CD) of the video. An example video can have a duration (T) of 1 hour (3600sec), a frame size of 640x480 (WxH) at a color depth of 24bits and a frame rate of 25fps. This example video has the following properties: * pixels per frame = 640 * 480 = 307,200

* bits per frame = 307,200 * 24 = 7,372,800 = 7.37Mbits
* bit rate (BR) = 7.37 * 25 = 184.25Mbits/sec
* video size (VS) = 184Mbits/sec * 3600sec = 662,400Mbits = 82,800Mbytes = 82.8Gbytes

The most important properties are bit rate and video size. The formulas relating those two with all other properties are: BR = W * H * CD * FPS
VS = BR * T = W * H * CD * FPS * T
(Units are: BR in bits/sec, W and H in pixels, CD in bits, VS in bits, T in seconds) While some secondary formulas are:
Pixels per frame = W * H
Pixels per second = W * H * FPS
Bits per frame = W * H * CD
Regarding Interlacing
In interlaced video each frame is composed of two halves of an image. The first half contains only the odd-numbered lines of a full frame. The second half contains only the even-numbered lines. Those halves are referred to individually as fields. Two consecutive fields compose a full frame. If an interlaced video has a frame rate of 15 frames per second the field rate is 30 fields per second. All the properties and formulas discussed here apply equally to interlaced video but one should be careful not to confuse the fields per second with the frames per second. Properties of compressed video

The above are accurate for uncompressed video. Because of the relatively high bit rate of uncompressed video, video compression is extensively used. In the case of compressed video each frame requires a small percentage of the original bits. Assuming a compression algorithm that shrinks the input data by a factor of CF, the bit rate and video size would equal to: BR = W * H * CD * FPS / CF

VS = BR * T / CF
Please note that it is not necessary that all frames are equally compressed by a factor of CF. In practice they are not so CF is the average factor of compression for all the frames taken together. The...
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