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Chapter: Satellite Communication : Satellite Access

Compression - Encryption

1. Encryption and Transmission 2. Video and Audio Compression 3. MPEG Standards

Compression - Encryption:


At the broadcast center, the high-quality digital stream of video goes through an MPEG encoder, which converts the programming to MPEG-4 video of the correct size and format for the satellite receiver in your house.


Encoding works in conjunction with compression to analyze each video frame and eliminate redundant or irrelevant data and extrapolate information from other frames. This process reduces the overall size of the file. Each frame can be encoded in one of three ways:


 As an intraframe, which contains the complete image data for that frame.

This method provides the least compression.

 As a predicted frame, which contains just enough information to tell the satellite receiver how to display the frame based on the most recently displayed intraframe or predicted frame.

 As a bidirectional frame, which displays information from the surrounding intraframe or predicted frames. Using data from the closest surrounding frames, the receiver interpolates the position and color of each pixel.



This process occasionally produces artifacts -- glitches in the video image. One artifact is macroblocking, in which the fluid picture temporarily dissolves into blocks. Macroblocking is often mistakenly called pixilating, a technically incorrect term which has been accepted as slang for this annoying artifact.


There really are pixels on your TV screen, but they're too small for your human eye to perceive them individually -- they're tiny squares of video data that make up the image you see.


The rate of compression depends on the nature of the programming. If the encoder is converting a newscast, it can use a lot more predicted frames because most of the scene stays the same from one frame to the next.


In more fast-paced programming, things change very quickly from one frame to the next, so the encoder has to create more intraframes. As a result, a newscast generally compresses to a smaller size than something like a car race.


1. Encryption and Transmission:

After the video is compressed, the provider encrypts it to keep people from accessing it for free. Encryption scrambles the digital data in such a way that it can only be decrypted (converted back into usable data) if the receiver has the correct decryption algorithm and security keys.


Once the signal is compressed and encrypted, the broadcast center beams it directly to one of its satellites. The satellite picks up the signal with an onboard dish, amplifies the signal and uses another dish to beam the signal back to Earth, where viewers can pick it up.


In the next section, we'll see what happens when the signal reaches a viewer's house.


2. Video and Audio Compression:


Video and Audio files are very large beasts. Unless we develop and maintain very high bandwidth networks (Gigabytes per second or more) we have to compress to data.


Relying on higher bandwidths is not a good option -- M25 Syndrome: Traffic needs ever increases and will adapt to swamp current limit whatever this is.


As we will compression becomes part of the representation or coding scheme which have become popular audio, image and video formats.


We will first study basic compression algorithms and then go on to study some actual coding formats.



What is Compression?


Compression basically employs redundancy in the data:


 Temporal -- in 1D data, 1D signals, Audio etc.

 Spatial -- correlation between neighbouring pixels or data items

 Spectral -- correlation between colour or luminescence components. This uses the frequency domain to exploit relationships between frequency of change in data.

 psycho-visual -- exploit perceptual properties of the human visual system.


Compression can be categorised in two broad ways:


Lossless Compression :

-- where data is compressed and can be reconstituted (uncompressed) without loss of detail or information. These are referred to as bit-preserving or reversible compression systems also.


Lossy Compression :

-- where the aim is to obtain the best possible fidelity for a given bit-rate or minimizing the bit-rate to achieve a given fidelity measure. Video and audio compression techniques are most suited to this form of compression.


If an image is compressed it clearly needs to uncompressed (decoded) before it can viewed/listened to. Some processing of data may be possible in encoded form however. Lossless compression frequently involves some form of entropy encoding and are based in information theoretic techniques.


Lossy compression use source encoding techniques that may involve transform encoding, differential encoding or vector quantization.


3. MPEG Standards :


All MPEG standards exist to promote system interoperability among your computer, television and handheld video and audio devices. They are:


 MPEG-1: the original standard for encoding and decoding streaming video and audio files.

 MPEG-2: the standard for digital television, this compresses files for transmission of high-quality video.

 MPEG-4: the standard for compressing high-definition video into smaller- scale files that stream to computers, cell phones and PDAs (personal digital assistants).

 MPEG-21: also referred to as the Multimedia Framework. The standard that interprets what digital content to provide to which individual user so that media plays flawlessly under any language, machine or user conditions.

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