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7. Artifacts:

In the days of analog television broadcasting, viewers put up with a variety of picture impairments, generally caused by the analog transmission chain from TV camera to home TV receiver. Things like snow, ringing, differential gain, criminate-luminance delay inequality, and co-channel interference were part of normal television reception. The introduction of modern digital production and distribution using MPEG2 and MPEG4 has totally eliminated these classes of impairments, and the old terminology. But the digital age has introduced its own unique set of impairments.

The term artifact is commonly applied to digital video impairments, essentially any unintended aspect not present in the original image, and usually as a result of the image coding, decoding, storage or transmission processes. The most significant are described below:

Blocking

Blocking is the most common artifact identified by DBS viewers. It only happens when the transmission system is over-stressed. Blocking is the preferred term as it implicitly refers to the part of the coding process causing the problem. When severe, the visible tiling is known as mosaic effect. Occasionally, the term pixelization is used in the news groups to refer to blocking, but pixelization is really an incorrect term — each block has 64 pixels.

An important part MPEG2 compression, the two-dimensional discrete cosine transform (DCT), operates on parts of the image known as blocks, each 8 by 8 pixels (picture elements) in size. Part of the coding algorithm may include block-edge filtering or use similar techniques to ensure that there is no visible difference in values in adjacent pixels in adjacent block edges. Normally, this works well and there is no visible blocking or mosaic pattern visible in an image.

There are, however, two MPEG2 situations where one or more individual blocks may momentarily become visible:

Blocking caused by high bit error rate transmission

During transmission from a direct broadcast satellite (DBS), the signal path introduces errors. About 40% of the bandwidth of each satellite transponder is used for forward error correction (FEC), described on page 3 of this paper. FEC allows the receiver to restore or correct a certain number of signal bits in error, a normal, ongoing process.

In a lower-quality DBS mode of about 1.5 Mbps per television channel, there are only about 0.2 bits transmitted per picture element per video frame. This paucity of data per pixel is why video compression is often referred to as entropy coding and why it is important that each and every bit transmitted can be perfectly decoded by the receiver.

As heavy rainfall approaches, digital transmission errors gradually build up until a threshold is reached where the forward error correction can no longer correct the errors, and induced errors start to show up on the video screen. The most usual impairment under these conditions is blocking, where the individual image blocks become visible, as the MPEG2 decoder starts to decode data in error rather than the wholly corrected image data. Just a single bit in error can be part of the bit stream used to recreate perhaps several thousand bits of the original, uncompressed video signal. This can result in one or more 8 x 8 pixel blocks (64 pixels, about 1,500 decoded bits) being incorrectly decoded. All or part of a block in error might be the wrong color, or artifacts like phantom lines or shapes could appear within the blocks.

As soon as the bit error rate falls to the point where the FEC can work as intended, the blocks which were in error will once again appear normal.

Blocking from insufficient bandwidth

During fair weather conditions, detectable to objectionable blocking is sometimes seen on lower priority channels, often visible when there is rapid movement in a close-up image. This is caused by insufficient bandwidth being allocated to the channel in question or a mistuned MPEG2 video compression algorithm. (In a modern encoder, most coding parameters are readily adjustable, often in real-time, as part of a dynamic bandwidth control protocol.)

A second order cause is that with a contemporary DBS system, the service providers are continuously faced with the need to add "just one more channel". Eventually, there are just too few system resources available to faithfully reproduce all the video signals. However, often there are options which can be applied which can reduce the degree of blocking visible.

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