3.12 Concealment by Interpolation

3.12 Concealment by Interpolation

There are some practical differences between data representing audio or video and generic data. Video and audio must be reproduced in real time, but have the advantage that there is a certain amount of redundancy in the information conveyed. Thus if an error cannot be corrected, it can be concealed. If a sample is lost, it is possible to obtain an approximation to it by interpolating between samples in the vicinity of the missing one. Clearly, concealment of any kind cannot be used with computer instructions or compressed data, although concealment can be applied after compressed signals have been decoded.

If there is too much corruption for concealment, the only course in video is repeat the previous field or frame in a freeze as it is unlikely that the corrupt picture is viewable. In audio the equivalent is muting.

In general, if use is to be made of concealment on receipt, the data must generally be reordered or shuffled prior to transmission. To take a simple example, odd-numbered samples are sent with a delay with respect to even-numbered samples. If a gross error occurs, depending on its timing, the result will be either corrupted odd samples or corrupted even samples, but it is most unlikely that both will be lost. Interpolation is then possible if the power of the correction system is exceeded. NICAM 728 uses such a system.

It should be stressed that corrected data are indistinguishable from the original and thus there can be no visible or audible artefacts . In contrast, concealment is only an approximation to the original information and could be detectable. In practical equipment, concealment occurs infrequently unless there is a defect requiring attention, and its presence is difficult to detect.



Digital Interface Handbook
Digital Interface Handbook, Third Edition
ISBN: 0240519094
EAN: 2147483647
Year: 2004
Pages: 120

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net