5. Coding the video signals:

DBS video compression uses an algorithm called ISO-MPEG2. This and many other forms of audio and video coding provide what is known as lossy compression. When the signals are decompressed, a subjectively pleasing signal is recovered, but much of the original is lost, having been discarded during the encoding process. Examples of lossless compression include data compression algorithms like zip, arj, lzh and many others.

The following is just a quick once-over of the process but it is still pretty complicated. So take a deep breath, and read on!

Video compression involves the removal of redundancies from the video signal. Predictable things like the synchronization signals and blanking can be defined in a lookup table, video can be digitally low-pass filtered by varying the number of samples per line, the frame rate can be dropped from about 30 fps, and the color information can be subsampled at half the number of samples used for the luminance (the brightness information) information.

The real compression power is done by forming the video data stream into 8 x 8 picture element (pixel) blocks, scanning each block diagonally, and performing a discrete two-dimensional cosine transform on each block's data. This converts the temporal and spatial domains into the frequency domain. The less significant coefficients are then truncated (discarded).

Another part of compression is quantization, the reduction of the number of shades of gray available for each pixel from 256 shades to some lower value. What happens is that each two or three gray scale values are combined into a new single value.

Another important part of video compression is called motion prediction, motion compensation or loop filtering, depending upon context or standard. The transmit codec buffers, or temporarily stores, two frames of video, and compares the video blocks between both frames. Blocks or groups of blocks can then be assigned a vector amplitude, direction and rate of change, allowing the receive decoder to predict where certain parts of the picture are moving to. Sometimes, if the codec is incorrectly set up and with high-contrast lighting, this can produce interesting artifacts like a nose periodically detaching from a person.

The final coding technique used is called Huffman coding, where the statistically most common digital patterns are replaced with terse codes from a lookup table. This is a bit of lossless coding that is useful for video compression.

There are lots of other techniques in the video compression cookbook with names like conditional replenishment, intra-frame coding, various forms of differential coding, field repeat, frame repeat, buffer overflow and so forth. For this paper, the above should whet your interest.

The various television signals making up each transponder's 23 Mbps capacity are managed by a device known as a statistical multiplexer. Each statistical multiplexer accommodates digital signal inputs from a number of MPEG2 video encoders. For DBS, these transmit-only codecs take the uncompressed analog audio and video signals, convert them to a digital signal of about 140 Mbps and then compress the signal to between 1.5 Mbps and 15 Mbps.

The better encoders are fully programmable, and can have their algorithm adjusted dynamically, as conditions change.

The set top box essentially undoes the encoding process in reverse order.

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