Error Rates

Practical communications systems, are not completely reliable in delivering information intact. There are errors and data losses, which affect more or less the quality at the receiver. Uncompressed video information is considered one of the least sensitive targets of transmission errors, as occasional leakages are usually not noticeable in the bulk of information received. Compressed video, on the other hand, is extremely vulnerable to errors or packet loss, as described in the following paragraphs.

The entropy encoding stage of popular video compression schemes uses Variable Length Codes (VLC) to take advantage of long runs of bit patterns. However, this makes the stream asynchronous, and more prone to packet loss or bit errors, if resynchronising markers are not used. Even with their use the error is spatially propagated until the reception of the next marker.

The motion compensation used to reconstruct images at the decoder can provide low quality results if erroneous reference images are used, or if motion vector information is not correctly received. This makes the errors to propagate temporally as well as spatially until the next intra-coded frame appears in the sequence, and, if this is not done in regular intervals, the results could be very frustrating for the user.

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