1. Field of the Invention
This invention relates to error concealment in digital television signals.
2. Description of the Prior Art
Recently there has been an increasing interest in the use of digital techniques for television signals. Such techniques are, for example, used in some video tape recording arrangements where an incoming television signal to be recorded is sampled, the samples are coded into digital form, the digital data signals are recorded and subsequently reproduced by a video tape recorder (VTR), the reproduced digital data signals are decoded, and the decoded signals are used to form an analog signal corresponding to the original television signal.
If errors occur in the handling of the digital signals, for example due to noise or tape drop-out occurring in the VTR, the digital signals are corrupted and then the reformed television signal does not correspond exactly to the original television signal, and a resulting television picture is degraded.
There are two main approaches to dealing with errors in digital television signals. The first approach is correction, which involves the production and use of additional data signals purely for the purposes of error detection and correction, these additional data signals otherwise being redundant. While correction provides good results, it cannot generally be used as the sole means of dealing with errors, because a comprehensive correction capability would require an excessive amount of additional data which might overload the data handling paths or raise the data rate to an unacceptable level. The second approach, with which the present invention is more particularly concerned, is concealment. This comprises the replacement of corrupted data signals by data signals generated using available uncorrupted data signals. This method relies largely for accuracy on the strong correlation that exists in a television signal.
Had a frequency of four times the frequency of the color sub-carrier been adopted as the base sampling frequency for digital television systems, then there would have been a substantial margin between the highest video frequency (5.5 MHz) and the Nyquist frequency (8.8 MHz). This would have allowed a very useful headroom for correcting errors using a fairly straight-forward concealment technique. In general, a concealment technique using standard digital filtering in one dimension gives the kind of response indicated in FIG. 1 of the accompanying drawings in which the concealment error is plotted as ordinates against the usable concealment frequencies as abscissae. The Nyquist fraction is the ratio of the frequency at which the concealment error exceeds a certain limit to the value of the Nyquist frequency. It will be seen from FIG. 1 that the Nyquist frequency represents the point on the graph at which concealment error rises rapidly to unacceptably poor levels.
As it is practically impossible to achieve a Nyquist fraction of unity, some lower value must be chosen and it has been shown that a value of up to 0.85 can be achieved. The current recommendation for the sampling frequency for digital television systems is for a component system using 12 MHz for the luminance signal and 4 MHz for each of the color difference signals, the lower frequency being acceptable for the color difference signals because the eye is less sensitive to differences in color than to differences in luminance. The luminance Nyquist frequency is therefore 6 MHz. If a Nyquist fraction of 0.85 is chosen, then the highest frequency which can satisfactory be concealed is 5.1 MHz. This means that if the video signal is not filtered correctly, and in particular if there is not a very rapid roll-off from 5 MHz onwards, then any errors occurring in signals having these higher frequencies will be concealed very poorly, to the extent of being more visible than the original error.
There are two problems which can arise:
Firstly, the concealment will only work correctly if all the samples which are to be used for concealment and which surround the sample in error are themselves error-free. Statistically, this situation is highly improbable. For example, if the off-tape error is one in 10.sup.-5 and this error is taken as p, then two or more errors are likely to occur within a limit of .+-. eight samples on either side of another error with a frequency which is approximately equal to 8.5 p.sup.2, which is 0.85.times.10.sup.-9. At present data rates this is approximately once every 6 seconds. However, two considerations are known to increase the error rate substantially. The first of these is that when an error is detected in a data word it is common to assume that five data words on either side of the error data word are also in error, and this of course increases the probability of an error or assumed error by a factor of about ten. Secondly, in order to reduce the rate of statistical errors the bandwidth of the off-tape signal is reduced to the minimum possible. This, however, increases the data-dependent errors such as pattern sensitivity, and these error types do not obey the normal probability equations. It is believed that this will increase the probability of multiple errors, although the level of increase is unknown.
Secondly, at the start and end of a horizontal scan line, there are no picture samples in existence since most of the blanking period is removed prior to recording. The vertical edge of a television picture may well therefore be prone to poor concealment, since the criterion for developing the concealment coefficients assumes useful samples on either side of the error sample. In practice, this may be overcome reasonably well by generating a suitable number of artifical samples at the beginning and end of each horizontal line scan.