Error concealment in digital television signals

Concealing errors in a digital television signal, which has a plurality of component sample values corresponding respectively to sample positions along a horizontal scan line of a television picture made up of a plurality of scan lines, in respect of each sample value which is in error, by calculating a first concealment accuracy by determining the difference between two immediately adjacent sample values both in the scan line immediately preceding or following the scan line which the error sample is in, and one of which sample values is immediately adjacent to the error sample, calculating a second concealment accuracy by determining the difference between two immediately adjacent sample values both in the vertical line immediately preceding or following the vertical line which the error sample is in, and one of which sample values is immediately adjacent to the error sample, selecting from the available concealment accuracies a preferred direction of the television picture for concealing the error sample, calculating a corrected value of the error sample using available sample values disposed along the preferred direction, and substituting the corrected sample value for the error sample so as to conceal the error.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to error concealment in digital television signals. 
2. Description of the Prior Art 
If errors occur in the handling of digital television signals, for example 
due to noise or drop-out occurring in a digital video tape recorder (VTR), 
the digital signals are lost or corrupted, and then the reformed analog 
television signal does not correspond exactly to the original analog 
television signal, and a resulting 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 purpose 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 lost or 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. 
In our UK Pat. No. 2,073,534 and the corresponding European Pat. No. 
0,037,212, we have disclosed a method of error concealment which comprises 
selecting from a plurality of algorithms a preferred algorithm for 
calculating a corrected value for use in concealment of an error sample, 
calculating a corrected value for the error sample using the preferred 
algorithm, and replacing the error sample by the corrected value sample. 
This method works well in most situations, but we have found that there 
are certain error situations and also certain signal frequency conditions 
which result in problems in selecting the preferred algorithm. These 
problems will be discussed in more detail below. 
SUMMARY OF THE INVENTION 
One object of the present invention is to provide a method of concealing 
errors in a digital television signal in which these problems are reduced. 
Another object of the present invention is to provide a method of 
concealing errors in a digital television signal which is effective when 
there is a high error rate. 
Another object of the present invention is to provide a method of 
concealing errors in a digital television signal which takes account of 
the frequency response of the sampled input television signal. 
According to the present invention there is provided a method of concealing 
errors in a digital television signal, which television signal comprises a 
plurality of component sample values corresponding respectively to sample 
positions along a horizontal scan line of a television picture made up of 
a plurality of said lines, the method comprising, in respect of each said 
sample value which is in error: 
calculating a first concealment accuracy by determining the difference 
between two immediately adjacent sample values both in the said line 
immediately preceding or following the said line which said error sample 
is in, and one of which sample values is immediately adjacent to said 
error sample; 
calculating a second concealment accuracy by determining the difference 
between two immediately adjacent sample values both in the vertical line 
immediately preceding or following the vertical line which said error 
sample is in, and one of which sample values is immediately adjacent to 
said error sample; 
selecting from the available said concealment accuracies a preferred 
direction of said television picture for concealing said error sample; 
calculating a corrected value of said error sample using available sample 
values disposed along said preferred direction; and 
substituting said corrected sample value for said error sample so as to 
conceal the error. 
According to the present invention there is also provided apparatus for 
concealing errors in a digital television signal, which television signal 
comprises a plurality of component sample values corresponding 
respectively to sample positions along a horizontal scan line of a 
television picture made up of a plurality of said lines, the apparatus 
comprising: 
means for calculating a first concealment accuracy by determining the 
difference between two immediately adjacent sample values both in the said 
line immediately preceding or following the said line which an error 
sample is in, and one of which sample values is immediately adjacent to 
said error sample; 
means for calculating a second concealment accuracy by determining the 
difference between two immediately adjacent sample values both in the 
vertical line immediately preceding or following the vertical line which 
said error sample is in, and one of which sample values is immediately 
adjacent to said error sample; 
means for selecting from the available said concealment accuracies a 
preferred direction of said television picture for concealing said error 
sample; 
means for calculating a corrected value of said error sample using 
available sample values disposed along said preferred direction; and 
means for substituting said corrected sample value for said error sample so 
as to conceal the error. 
The above, and other objects, features and advantages of this invention 
will be apparent from the following detailed description of illustrative 
embodiments which is to be read in connection with the accompanying 
drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before describing an embodiment of the invention, and to assist 
understanding of the embodiment, further reference will first be made to 
the problems mentioned above. 
Referring to FIG. 1, this shows part of the television raster of one field, 
and in particular parts of three consecutive horizontal scan lines 
labelled line n-1, line n and line n+1. The luminance sample positions are 
disposed at regular intervals along each of the lines, the intervals 
corresponding to a sampling frequency of say 13.5 MHz, and the sample 
positions being aligned in the vertical direction. In other words, the 
field is orthogonally sampled, although this is not essential to the 
invention, it being sufficient that the sample positions are substantially 
aligned in vertical lines. Reading from the left, consecutive sample 
positions in each line are labelled S-3, S-2, S-1, S0, S1, S2 and S3. 
Using this notation, any sample position in the matrix can be designated 
by the line and the sample number, and for the purpose of this discussion 
it is assumed that the sample position at which there is an error sample 
signal requiring concealment is in line n at position S0, this being 
designed n, S0. 
As disclosed in our above-mentioned patents, a corrected value for the 
sample position n, S0 could be estimated in one of four different ways. 
Firstly, the average could be taken of the two sample values in line n 
adjacent to and on each side of the sample position n, S0. Secondly, the 
average could be taken of the two sample values in line n-1 and line n+1 
adjacent to and vertically above and below the sample position n, S0. 
Thirdly, the average could be taken of the two sample values in line n-1 
and line n+1 and on either side of the sample position n, S0 along the 
positive diagonal direction. Fourthly, the average could be taken of the 
two sample values in line n-1 and line n+1 adjacent to and on either side 
of the sample position n, S0 and along the negative diagonal direction. 
These four directions are indicated by the arrows A, B, C and D 
respectively. 
Each of these possibilities may be thought of as an algorithm for 
calculating a corrected value, and it will be appreciated that it is 
likely that one of these algorithms will give a better result than any of 
the others. The direction to be used is therefore selected by testing each 
algorithm using known sample values to see which gives the best result. A 
corrected value derived using the direction corresponding to that 
preferred algorithm is then used when substituting a corrected value 
sample. 
As a further refinement, the results derived from the respective algorithms 
can be weighted. In other words, a value can be placed on the likely 
accuracy of the results obtained. This is necessary because the distance 
between adjacent sample positions is less in the horizontal direction than 
in the vertical direction, the difference amounting to a factor of 
approximately 1.75. For this reason, the algorithm using the horizontal 
direction is in fact most likely to give the nearest result, with the 
algorithm for the vertical direction being next best, and the two 
algorithms for the diagonal directions being the next best. 
The four algorithms referred to above will now be specified in mathematical 
terms. Thus, the decision as to which concealment direction to use is made 
by investigating the adjacent sample values and obtaining the concealment 
accuracy for each direction. If the concealment accuracy is H for the 
horizontal direction, V for the vertical direction, D.sup.+ for the 
positive diagonal direction and D.sup.- for the negative diagonal 
direction, then these concealment accuracies can be defined as follows: 
EQU H=1/2.vertline.1/2{(n-1), S-1+(n-1), S1}-(n-1), 
S0.vertline.+1/2.vertline.1/2{(n+1), S-1+(n+1) S1}-(n+1), S0.vertline.(1) 
that is to say, the concealment accuracy H equals the average of the 
horizontal concealment accuracy from the horizontal line immediately above 
and the horizontal line immediately below the horizontal line containing 
the error sample. 
Likewise: 
EQU V=1/2.vertline.1/2{(n-1), S-1+(n+1), S-1}-n, S-1}+1/2.vertline.1/2{(n-1), 
S1+(n+1), S1}-n, S1} (2) 
EQU D.sup.+ =1/2.vertline.1/2{(n-1), S0+(n+1), S-2}-n, 
S-1}+1/2.vertline.1/2{(n-1), S2+(n+1), S0}-n, S1.vertline.(4) 
EQU D.sup.- =1/2.vertline.1/2{(n-1), S-2+(n+1), S0}-n, 
S-1.vertline.+1/2.vertline.1/2{(n-1), S0+(n+1), S2}-n, S1.vertline.(3) 
These four values H, V, D.sup.+ and D.sup.- represent the accuracy of 
concealment for the sample values most closely connected with the error 
sample. Preferably these concealment accuracies H, V, D.sup.+ and D.sup.- 
are each assigned a weighting coefficient to take account of the unequal 
spacings of the horizontal, vertical and diagonal samples. The smallest 
value is then used to select the direction of concealment. 
One of the problems that can arise with this method, and which was referred 
to briefly above, will now be further described with reference to FIG. 2. 
This again shows part of the raster of one field, and in this case each 
sample position is marked by a cross. It is possible during severe error 
conditions for a high proportion of the sample values to be in error. A 
25% error rate is not unusual, and may arise, for example, where due to 
drop-out there is no output from one of the heads of a four-head digital 
VTR. It will be usual for the error samples in such a case to be 
symetrically distributed throughout the field, so that every fourth sample 
value is in error, as indicated by the crosses enclosed in boxes in FIG. 
2. 
If now FIG. 2 is compared with FIG. 1, it will be seen that due to the 
error samples it is not possible to determine the concealment accuracies 
D.sup.+ and D.sup.- corresponding to the diagonal directions C and D, 
because each of the two terms bounded by the magnitude signs in the 
relevant algorithm (3) or (4) involves use of an error sample. It is to be 
noted that it is merely the calculation of the concealment accuracies 
which has failed, and it is clear from FIG. 2 that there are non-error 
samples available for actually effecting concealment in the positive and 
negative diagonal directions C and D. 
The other problem referred to briefly above will now be further described 
with reference to FIG. 3. The specifications of the abovementioned patents 
refer to the frequency response of this form of error concealment, and 
this second problem arises in this area of frequency response. It is to be 
remembered that when a television signal is orthogonally sampled, the 
sample positions along each horizontal scan line have a predetermined 
phase. The frequency response is the average of all the possible phases of 
the signals at the sample positions. FIG. 3 shows the case where 
successive sample positions differ in phase by .pi./2, and using the 
designation plus, minus and zero for the phases at respective sample 
positions as indicated by the waveform in FIG. 3, it will be seen that it 
may not be possible to determine the concealment accuracies H, V, D.sup.+ 
and PG,9 D.sup.- because terms in the relevant algorithm disappear when 
the phases are considered. Thus a particular phase of the signal gives 
rise to nulls at most or all of the sample positions which are to be used 
in calculating the concealment accuracies H, V, D.sup.+ and D.sup.-. In 
these circumstances the method fails to select a preferred concealment 
direction correctly, even although there are no other error samples in the 
region of the error sample to be corrected. 
This problem can be demonstrated, for example, by using a zone plate. A 
zone plate is a television picture comprising concentric circles which 
move outwards from the centre of the screen, increasing in diameter and 
speed, and hence in signal frequency, as they approach the periphery of 
the screen. With such a zone plate, it is found that the problem arises on 
each of the two diagonals of the screen, about midway between the centre 
and the corner of the screen. 
In the method according to the present invention, both these problems are 
overcome by modifying the way in which the concealment accuracies H, V, 
D.sup.+ and D.sup.- are calculated. 
FIG. 4A indicates the modified calculation for the horizontal direction A. 
For the calculation the same sample values as in the algorithm (1) are 
used, but the sample values are subtracted from one another in pairs as 
indicated by the dotted lines in FIG. 4A. 
FIG. 4B indicates the similarly modified calculation for the concealment 
accuracy in the vertical direction V. 
FIG. 4C indicates the modified calculation for the positive diagonal 
direction C. For the calculation, each of the four sample values nearest 
to the error sample in the same horizontal line are used. The first two 
are subtracted from the two sample values in the preceding horizontal line 
and located in the respective positive diagonal direction, and the two 
succeeding the error sample have subtracted from them the two sample 
values in the succeeding horizontal line located in the respective 
positive diagonal direction. 
FIG. 4D indicates the similarly modified calculation for the concealment 
accuracy D.sup.- in the negative diagonal direction D. 
Written in the form of algorithms the concealment accuracies H, V, D.sup.+ 
and D.sup.- at (3) and (4) above are modified as follows: 
EQU H=1/4{.vertline.(n-1), S-1-(n-1), S0.vertline.+.vertline.(n-1), S0-(n-1), 
S1.vertline.+.vertline.(n+1), S-1-(n+1), S0.vertline.+.vertline.(n+1), 
S0-(n+1), S1.vertline.} (5) 
EQU V=1/4{.vertline.(n-1), S-1-n, S-1.vertline.+.vertline.n, S-1-(n+1), 
S-1.vertline.+.vertline.(n-1), S1-n, S1.vertline.+.vertline.n, S1-(n+1), 
S1.vertline.} (6) 
EQU D.sup.+ =1/4{.vertline.(n-1), S-1-n, S-2.vertline.+.vertline.(n-1), S0-n, 
S-1.vertline.+.vertline.n, S1-(n+1), S0.vertline.+.vertline.n, S2-(n+1), 
S1.vertline.} (7) 
EQU D.sup.- =1/4{.vertline.(n-1), S0-n, S1.vertline.+.vertline.(n-1), S1-n, 
S2.vertline.+.vertline.n, S-2-(n+1), S-1.vertline.+.vertline.n, S-1-(n+1), 
S0.vertline.} (8) 
The method has been described as applied to the luminance channel, that is 
to say concealment of errors occuring in luminance sample values. It is 
also necessary to consider the colour difference channels, and here two 
possibilities arise. 
Firstly, each colour difference channel can be provided with a separate 
concealment selection arrangement independent of the arrangement for the 
luminance channel. 
Secondly, because the first solution referred to above increases the amount 
of hardware required by approximately three, an alternative method which 
economizes on the amount of hardware required makes use of the fact that 
the chrominance information is related to the luminance information. That 
is, where a chrominance edge exists, so usually does a luminance edge. 
Based on this assumption it is possible to select the direction of colour 
difference concealment to be the same as that selected for luminance 
concealment. However, because the chrominance samples occur at only one 
half the frequency of the luminance samples along each horizontal line, a 
different set of weighting coefficients has to be used, these being 
optimized to the chrominance bandwidths. 
Referring to FIG. 5, this shows apparatus in accordance with the present 
invention for concealing errors in a digital television signal. The 
apparatus comprises a luminance sample storage means 1 to which the 
luminance input samples are supplied by way of an input terminal 2. The 
luminance sample storage means 1 supplies outputs of a luminance sample 
matrix storage means 3 which stores a moving matrix of sample values 
corresponding to the sample positions: 
EQU (n-1), S-1; (n-1), S0; (n-1), S1; n, S-2; n, S-1; n, S0; n, S1; n, S2; 
(n+1), S-1; (n+1), S0; (n+1), S1. 
Four concealment accuracy detectors are provided, these being a horizontal 
concealment accuracy detector 4, a vertical concealment accuracy detector 
5, a positive diagonal concealment accuracy detector 6 and a negative 
diagonal concealment accuracy detector 7. Each of the concealment accuracy 
detectors 4 to 7 is continuously supplied with the appropriate part of the 
sample matrix from the luminance sample matrix storage means 3. Thus the 
horizontal concealment accuracy detector 4, for example, receives or 
selects the sample values necessary to calculate the concealment accuracy 
H using the algorithm (5) above, and supplies a signal representing the 
concealment accuracy H by way of a weighting multiplier 8 to a luminance 
direction processor 12. Likewise the concealment accuracy detectors 5 to 7 
supply a respective signal representing the vertical concealment accuracy 
V using the algorithm (6) above, the positive diagonal concealment 
accuracy D.sup.+ using the algorithm (7) above, and the negative diagonal 
concealment accuracy D.sup.- using the algorithm (8) above, by way of 
weighting multipliers 9, 10 and 11 respectively to the luminance direction 
processor 12. The weighting multipliers 8 to 11 effect the weighting 
referred to above to compensate for the different distances between 
adjacent sample positions in the various directions. The weighting may be 
done simply on the basis of distance between adjacent sample positions, in 
which case each weighting multiplier multiplies by the distance between 
adjacent sample positions in the relevant direction. Other weightings can, 
however, be used. 
The luminance direction processor 12 supplies an output signal representing 
the selected direction of concealment to a sample value calculator 13 
which operates to select the appropriate samples from the luminance sample 
matrix storage means 3 and calculate therefrom the required concealment 
value to be used to conceal the error sample. For example, if the 
horizontal direction is selected, the sample value calculator 13 uses the 
sample values for the sample positions n, S-1 and n, S1 to calculate the 
value to be used to conceal the error sample at the sample position n, S0. 
The concealment value is supplied to a selector 14 to which a switching 
signal is supplied by way of a terminal 15. The selector 14 is also 
supplied with the sample value from the sample position n, S0 by way of a 
terminal 16. 
Preferably the apparatus as so far described operates continuously, that is 
to say concealment values are determined as described for every sample 
position and supplied to the selector 14. Only, however, when it has been 
determined that there is an error at a given sample position n, S0, is a 
signal supplied to the selector 14 by way of the terminal 15, whereupon 
the concealment value supplied from the calculator 13 is supplied to a 
luminance output terminal 17 in place of the sample value supplied by way 
of the terminal 16. At all other times, the sample value supplied by way 
of the terminal 16 is supplied to the luminance output terminal 17. 
The fact that there is an error at a given position n, S0 can be determined 
in any suitable manner. For example, it may be determined that the data 
word representing the sample value is not valid, or the data words may be 
grouped into error-detecting blocks for recording and an error-detecting 
code such as a cyclic redundancy check code associated with each block. 
The apparatus may also include arrangements for calculating concealment 
values for the colour difference channels U and V. For simplicity, only 
that part of the apparatus necessary to calculate concealment values for 
the difference channel U is shown and will be described. For this purpose 
the apparatus comprises a chrominance sample storage means 21 to which 
chrominance input samples are supplied by way of an input terminal 22. The 
chrominance sample storage means 21 supplies outputs to a chrominance 
signal matrix storage means 23 which stores a moving matrix of sample 
values corresponding to those listed above in connection with the 
luminance sample matrix storage means 3, but adjusted to take account of 
the different spacing between adjacent chrominance samples. 
Operating in time division multiplex for the luminance and chrominance 
samples respectively, the concealment accuracy detectors 4 to 7 derive 
signals representing the horizontal, vertical, positive diagonal and 
negative diagonal concealment accuracies H, V, D.sup.+ and D.sup.- using 
the algorithms (5) to (8) respectively for the chrominance difference 
channel U and supply the signals by way of respective chrominance 
weighting multipliers 24, 25, 26 and 27 to a chrominance direction 
processor 28 which supplies an output signal representing the selected 
direction of concealment to a sample value calculator 29 which operates to 
select the appropriate samples from the chrominance sample matrix storage 
means 23 and calculate therefrom the required concealment value to be used 
to conceal the error sample. The concealment error is supplied to a 
selector 30 to which a switching signal is supplied by way of a terminal 
31. The selector 30 is also supplied with the sample value from the sample 
position n, S0 by way of a terminal 32. 
As with the luminance part of the apparatus, the chorminance part of the 
apparatus preferably operates continuously. Only, however, when it has 
been determined there is an error at a given sample position n, S0, is a 
signal supplied to the selector 30 by way of the terminal 31, whereupon 
the concealment value supplied from the calculator 29 is supplied to a 
chrominance output terminal 33 in place of the sample value supplied by 
way of the terminal 32. 
The chrominance part of apparatus may be duplicated for the colour 
difference channel V or alternatively hardware can be saved by also using 
the direction selected for the colour difference channel U for the colour 
difference channel V. 
The method described works well so long as the average number of error 
samples over the field is not more than approximately 25%. If the error 
rate goes higher than this, as it may well do for example when using 
special reproduction modes such as slow or fast motion, the resulting 
increase in the density of error samples increases the probability of 
there being more than one error sample within the sample space from which 
sample values are used for calculating the concealment accuracies. 
This problem can be alleviated in a modified method in which no direction 
of concealment is used if the calculation of the corrected value involves 
the use of an error sample. 
However, it is not necessary to exclude a direction of concealment merely 
because the concealment accuracy H, V, D.sup.+ or D.sup.- as set out in 
the algorithms (5), (6), (7) or (8) involves the use of one or more error 
samples. Thus if, for example, the algorithm (5) used for calculating the 
horizontal concealment accuracy H is considered, it is seen that the 
algorithm can be viewed as the sum of four component algorithms 
respectively bounded by the magnitude signs, and it is possible for up to 
three of these four component algorithms to be invalidated by error 
samples, while the other component algorithm remains valid. Thus up to 
three of the component algorithms may be dropped in favour of the other 
component algorithm or algorithms in appropriate cases. 
In other words, the algorithm (5) for calculating the horizontal 
concealment accuracy H can be split into four algorithms which can be 
separately used for calculating component concealment accuracies H1 to H4, 
as follows: 
EQU H1=.vertline.(n-1), S-1-(n-1), S0.vertline. (9) 
EQU H2=.vertline.(n-1)S0-(n-1)S1.vertline. (10) 
EQU H3=.vertline.(n+1)S-1-(n+1), S0.vertline. (11) 
EQU H4=.vertline.(n+1), S0-(n+1), S1.vertline. (12) 
Likewise the algorithms (6), (7) and (8) can each be split into four 
component algorithms V1, V2, V3 and V4; D.sup.+ 1, D.sup.+ 2, D.sup.+ 3 
and D.sup.+ 4; and D.sup.- 1, D.sup.- 2, D.sup.- 3 and D.sup.- 4, 
respectively, as follows 
EQU V1=.vertline.(n-1), S-1-n, S-1.vertline. (13) 
EQU V2=.vertline.n, S-1-(n+1), S-1.vertline. (14) 
EQU V3=.vertline.(n-1), S1-n, S1.vertline. (15) 
EQU V4=.vertline.n, S1-(n+1), S1.vertline. (16) 
EQU D.sup.+ 1=.vertline.(n-1), S-1-n, S-2.vertline. (17) 
EQU D.sup.+ 2=.vertline.(n-1), S0-n, S-1.vertline. (18) 
EQU D.sup.+ 3=.vertline.n, S1-(n+1), S0.vertline. (19) 
EQU D.sup.+ 4=.vertline.n, S2-(n+1), S1.vertline. (20) 
EQU D.sup.- 1=.vertline.(n-1), S0-n, S1.vertline. (21) 
EQU D.sup.- 2=.vertline.(n-1), S1-n, S2.vertline. (22) 
EQU D.sup.- 3=.vertline.n, S-2-(n+1), S-1.vertline. (23) 
EQU D.sup.- 4=.vertline.n, S-1-(n+1), S0.vertline. (24) 
This modified method is implemented in practice by calculating from the 
sample values available each of the component concealment accuracies. For 
example, for the horizontal direction A the component concealment 
accuracies H1 to H4 are calculated, but if any one of the four 
calculations involves the use of an error sample then that component 
concealment accuracy H1, H2, H3 or H4 is rejected, and the averge taken of 
the remainder, with the resulting average then scaled in dependence on the 
number of component algorithms used. If none is rejected, that is if no 
error sample is used for either of the calculations, then the full 
horizontal concealment accuracy H is calculated from: 
EQU H=1/4(H1+H2+H3+H4) (25) 
These calculations are performed continuously and are most conveniently 
performed in the concealment accuracy detectors 4 to 7 of the apparatus of 
FIG. 5. 
Although illustrative embodiments of the invention have been described in 
detail herein with reference to the accompanying drawings, it is to be 
understood that the invention is not limited to those precise embodiments, 
and that various changes and modifications can be effected therein by one 
skilled in the art without departing from the scope and spirit of the 
invention as defined by the appended claims.