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Compression, Encoding & Encryption

At the broadcast center the signal once combined is compressed using MPEG-2 encoding, which is exactly the same format as Digital Versatile Discs (DVD’s). This format is very efficient and can compress a single 270 Mbps stream to anything between 5 & 10 Mbps depending on the content. This increases the satellites capacity from around 30 channels uncompressed to over 200 channels with compression. As with all such technologies there is a trade off between quality and size.

This compression is achieved by a method of encoding which reduces the amount of data required by eliminating redundant or useless information and by extrapolating information from surrounding information or frames.

The encoding takes place frame by frame and each frame will be encoded in one of the following 3 ways:

intraframe – An intraframe is the complete image data for that frame. This method of encoding provides the least compression, but can still compress colors and other information.

Predicted - predicted frames contains only the information for the decoder to work out whats different from the previous frame. Consider a newsreader talking, most of the image will remain constant frame by frame except for the area around his mouth. Predicted frames would be generated just describing the area of the frame that is changing and what the changes are.

bidirectional – this method relies on the decoder using surrounding frames to interpolate the position and color of each pixel. There is in fact no frame information required because the decoder by interpolating a result will produce a suitable image.

Due to the large compression required for satellite transmission the compression will have losses and is not free of mistakes and glitches. The process can occasionally create spots on the screen where the color or the image does not match, these are called “artefacts”. These are called artefacts as they are the visible leftovers from the compression & encoding process or system.

The amount of compression overall depends on the programming. As before consider a entire newscast, which will be full of predicted frames where nothing much changes from one frame to the next. In more active programs like action movies, music videos or sports events where there is a lot of color and movement your will find many more intraframes.

Once compressed, the signal is then encrypted to stop just anyone viewing the programs. As we are not trying to encourage people to steal the programming we will not discuss the details of the encryption here, only to simply state that the encoder knows how to encrypt the signal in precisely the same way the decoder knows how to decrypt it buy the similar use of encryption algorithms and keys.