Signal correction for composite triple beat impairments

A method for reducing the effect of a composite triple beat (CTB) of a visual image is obtained by digitizing the signal for the image frame, dividing the digital signal into a plurality of stripes extending perpendicular to the horizontal lines of the image to form subpictures and determining the average intensity of the signal in each of the horizontal lines of each of the subpictures. An average value for each of the lines in each of the subpictures is then obtained and the component of the signal forming the composite triple beat signal for each line of each subpicture is estimated. Then the estimated triple beat signal is subtracted from its corresponding portion of the digitized signal to provide corrected subpictures. The corrected subpictures are then recombined to form a clean signal providing the cleaned visual image. In a preferred arrangement plurality of consecutive frames are treated as above described to provide average corrections for each line based on the average for the corresponding lines in the subpictures in the plurality of frames and then the subpictures for the frame to be shown next are corrected using the average corrections values to provide an even clearer signal.

FIELD OF THE INVENTION 
The present invention relates to a method of substantially removing the 
visual effect of a composite triple beat (CTB) in a cable television 
signal. 
BACKGROUND OF THE PRESENT INVENTION 
Composite triple beat (CTB) impairments, one of the major impairments in 
cable television, are known to be caused by the non-linearity of 
amplifiers in the cable distribution network. Composite triple beats (CTB) 
manifest themselves as horizontal streaking patterns on the picture and 
are extremely irritating to viewers, to the extent that the threshold of 
perceptibility is found to be significantly higher than other noise levels 
(currently the CTB is noticeable at readings about 57 dB in terms of 
carrier-to-CTB noise ratio (CNR)). 
When a number of different channels are transmitted simultaneously through 
the same cable network, these signals must be amplified at various 
locations along the network and since the amplifiers are not ideally 
linear, third-order distortion inevitably occurs forming what is known as 
third-order intermodulation beat products having frequencies that are 
combinations of the carrier frequencies of any two or three of the 
transmitted channels. A composite triple beat (CTB) is a cluster of such 
spurious signals having similar frequencies. 
There is no known system for reducing or substantially eliminating the 
visual effect of CTB. 
BRIEF DESCRIPTION OF THE PRESENT INVENTION 
It is the object of the present invention to reduce the visual effect of 
CTB so that it will become significantly less noticeable to viewers and in 
some cases to substantially eliminate the effect. 
Broadly the present invention relates to a method of reducing the composite 
triple beat in a visual image of a channel transmitted via cable 
comprising digitizing frames forming images of said channel to provide 
digitized frame signals for each said digitized frames, dividing each said 
digitized frame signal into a plurality of stripes extending perpendicular 
to horizontal lines of said digitized framing signal to form subpictures, 
determining the average intensity of the signal in each horizontal line of 
each of said subpictures to provide an average intensity value for each 
said line in each said subpicture, filtering the said average intensity 
values for each said horizontal of each said subpicture to provide an 
estimated composite tripe beat signal, subtracting said estimated 
composite triple beat signal for each said line of each said subpicture 
from its equivalent portion of said digitized frame signal to provide a 
corrected subpicture for each said frame and combining said corrected 
subpictures to provide a clean signal and creating a clean visual image 
based on said combined corrected subpictures. 
Preferably said process will further comprise processing in sequence a 
selected number of consecutive frames to provide a combined average value 
of said composite triple beat for each line of each subpicture for said 
selected number of frames and correcting the frame of said selected set of 
frames to be shown next using said combined average composite triple beat 
of said selected sequence of frames. 
Preferably said estimate composite triple beat signal for each said line in 
each said subpicture is used to estimate the signal to composite triple 
beat ratio calculated as 
##EQU1## 
where .alpha..sub.i (n)=the estimate composite triple beat intensity value 
for the n.sup.th line of the i.sup.th subimage 
L=the number of subimages 
N=the length of each subimage in the vertical direction.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Before discussing in detail the procedure of the present invention it is 
believed to be in order to discuss the basic concept leading to the 
present invention. 
As above described a composite triple beat (CTB) is a cluster or sum of 
many third-order intermodulation beat products. Generally the central 
frequency of this cluster will be at the video carrier frequency of the 
victim channel and each third order intermodulation beat product of this 
cluster is a linear combination of the carrier frequencies of any two or 
three channels transmitted by the cable. Because of the random shifting of 
the carrier frequencies, the frequency of each resulting third-order 
intermodulation beat product randomly shifts within about 30 kHz centred 
at the carrier frequency of the victim channel. 
After a CTB signal is scanned onto the TV screen, its intensity along an 
arbitrary line is the sum of the intensities of many (r) third-order 
intermodulation beat products and is represented by the formula 
##EQU2## 
where A.sub.i =amplitude of the i.sup.th third order intermodulation beat 
product 
.omega..sub.i =angular frequency of the i.sup.th third order 
intermodulation beat product 
.phi..sub.i =phase angle of the i.sup.th third order intermodulation beat 
product 
t.sub.d =time the scanner takes to scan to the beginning of this line, and 
T=the time to scan the whole line. 
f.sub.ctb is the sum of many sinusoidal terms each of which has a frequency 
lying in the range 0 to 15 Khz. These frequencies are very small compared 
to that of the picture which has a range of 0-4250 Khz. Consider only one 
sinusoid of (1) whose .omega.=2.pi., i.e., its frequency is of low value 
equal to 1 hz. It can be shown that when this signal is mapped on the TV 
screen and only one frame is grabbed, this frame looks like a constant DC 
luminance value everywhere, specifically one line of the frame will have a 
constant DC luminance value everywhere. Now consider the sinusoid whose 
frequency is 9.5 Khz. When this signal is mapped on the TV screen, it can 
be shown that the luminance values across one line of a frame takes the 
shape of half a period of a sinusoid. If the frequency of the sinusoid 
signal is 15 Khz (the largest frequency a component of the CTB can assume) 
the luminance values across one line of a frame will appear as part of a 
sinusoid period (see FIG. 5). More specifically this sinusoid signal S is 
EQU S=A.sub.i cos (2.pi..times.15000t+.phi..sub.i +.omega..sub.i t.sub.d) 
Assume .phi..sub.i +.omega..sub.i t.sub.d is proportional to 2.pi. then 
across one line of a frame the luminance will be proportional to cos (x) 
where 
0.ltoreq..times..ltoreq.1.57.pi. (as shown in FIG. 5) 
Thus, over a small region of x any sinusoidal component of a CTB may be 
approximated as a constant luminance. Thus, by dividing a line into small 
segments a sinusoid signal having a low frequency (between 0-15 Khz) can 
be represented as a stairway signal as shown in FIG. 5 (for the frequency 
15 Khz). Thus, the effect of the CTB over a small segment of a line in the 
image can be modelled as a change in the DC luminance by a constant over 
that segment. 
The impairment process can be expressed as 
EQU g(n,k)=f(n,k)+f.sub.ctb (n,k) (2) 
where 
f.sub.ctb (n,k)=the digitized CTB signal. 
g(n,k)=the impaired digitized signal. 
f(n,k)=the unimpaired or original picture, and 
where 
1.ltoreq.n.ltoreq.N; 1.ltoreq.k.ltoreq.K and N and K are the length and 
width of the frame respectively. 
By dividing the picture into L vertical stripes or subpictures then each 
stripe has width M.times.K/L. 
When L is large then the equation for each stripe can be approximated as 
EQU g.sub.i (n,m)=f.sub.i (n,m)+.alpha..sub.i (n) (3) 
where 
g.sub.i (n,m) is the i.sup.th stripe of the impaired picture 
f.sub.i (n,m) is the i.sup.th stripe of the original picture and 
.alpha..sub.i (n) is the constant by which the intensity of the CTB signal 
is approximated by over the n.sup.th line of the i.sup.th stripe. 
Taking the average of intensities of each line along each of the stripes, 
equation (3) is simplified to 
EQU g.sub.i (n)=f.sub.i (n)+.alpha..sub.i (n) (4) 
where 
##EQU3## 
From (4) it is clear that f.sub.i (n) the average DC luminance of the 
original uncorrupted picture over a line n of subimage i is changed by the 
addition of a constant term .alpha..sub.i (n). To clean the image from the 
effects of the CTB, it is necessary to restore the DC luminance of each 
line of each subimage, that is to subtract .alpha..sub.i (n) from g.sub.i 
(n) for every line n in very subimage i, since .alpha..sub.i (n)=g.sub.i 
(n)-f.sub.i (n) to obtain .alpha..sub.i (n) one must first find f.sub.i 
(n) for every line n in every subimage i. But since the values of the 
different f.sub.i (n)'s are unknown, the problem becomes that of 
estimating every f.sub.i (n). 
To find an estimate of every f.sub.i (n) we consider the i.sup.th subimage 
or stripe the values of f.sub.i (n) change very slowly as n is varied but 
those of .alpha..sub.i (n) change very rapidly as n is varied. These 
variations are depicted in FIG. 4. Furthermore, it has been found that 
.alpha..sub.i (n) has characteristics similar to those of white noise. 
Thus, to estimate the different values of f.sub.i (n) for the i.sup.th 
stripe, the (moving) average or the (moving) median filter may be used. 
For the average filter of size 3 the estimate of f.sub.i (n) is 
##EQU4## 
and the average filter of size 5 yields the estimate 
##EQU5## 
For the median filter of size 3, the estimate of f.sub.i (n) is the median 
of g.sub.i (n-1), g.sub.i (n), g.sub.i (n+1). 
For the median filter of size 5, the estimate of f.sub.i (n) is the median 
of g.sub.i (n-1), g.sub.i (n-1), g.sub.i (n+1), g.sub.i (n+2). 
The estimate f.sub.i (n) for every stripe i is found using the average or 
median filter for every stripe in the image. Then, an estimate 
.alpha..sub.i (n) for every .alpha..sub.i (n) is found using 
EQU .alpha..sub.i (n)=g.sub.i (n)-f.sub.1 (n) (9) 
Subtracting .alpha..sub.i (n) from g.sub.i (n) for every n and every stripe 
i results in a picture which is relatively clean from the effects of the 
CTB. 
Referring back to FIG. 1 the process of the present invention has been 
represented schematically as shown visual frame signal 10 is digitized as 
indicated at 12 and then divided into subpictures by forming stripes 
extending substantially perpendicular to the horizontal signal line as 
indicated at 14. 
Next the average intensity of each horizontal line of the received frame is 
determined for each stripe or subpicture as indicated at 16 and the each 
line segment defined in each stripe is filtered as indicated at 18 to 
eliminate a substantially constant value. The filter 18 may be a median 
filter, an average filter or any suitable low pass filter and provides an 
estimate of the strength of the CTB signal as indicated at 20. This CTB 
signal is then subtracted from the corresponding portion of the original 
signal as indicated at 24 to produce a corrected subpicture for each 
stripe as also indicated at 24. These corrected subpictures are then 
recombined as indicated at 26 to form the processed (corrected) picture 
that is then displayed as indicated at 28. 
The triple beat signal illustrated in FIG. 5 via the line 100 is a 
substantially sinusoidal wave and as can be seen by the steps 102, 104, 
106, 108, 110, 112, 114, and 118 where the horizontal arrow represents a 
line along a picture digitized frame that each of the steps 102, 104, 106, 
etc. provides a reasonably accurate indication of the intensity of the 
composite triple beat signal over a short length of the scan line. 
As shown in FIG. 6 the picture 200 is divided into vertical scans 201, 202, 
204, 206, 207, 208, 210, 212 which correspond with their similarity 
numbered in 100 series intensities shown in FIG. 5 for a given scan line 
perpendicular to the stripes. 
The concept is depicted in simplified form in FIG. 6 which shows to the 
left of the picture 200 a typical plot of average intensity of a stripe 
204 along a vertical line as indicated at 300 then this same plot of 
average intensity is rotated 90.degree. clockwise as indicated at 310 and 
a signal as indicated at 320 based on smoothing the signal 310 (filtering 
signal 310). Subtracting 320 from 310 provides a representation of the CTB 
signal for every line along stripe 204. The difference of the signal 310 
and the signal 320 is used to define the CTB signal in a stripe. 
The above description has dealt with primarily single frame CTB removal 
using the signal from a single frame only. 
In order to improve the operation even further one can apply a multi-frame 
CTB removal scheme wherein X consecutive frames of TV pictures are 
processed. Preferably X the number of consecutive frames to be processed 
to produce one clean or corrected frame will generally be an odd number, 
preferably 3 or 5 frames. The number of consecutive frames remains 
constant but one is added as another one is dropped off. Each frame is 
processed as above described but the average used to determine the CTB in 
a given line in a given stripe is based on the averages for the given line 
in the given stripe in all frames of the sequence being processed and this 
combined average is applied to correct the frame about to be shown which 
will normally be the middle frame in the sequence. A delay of 1 frame is 
preferably imposed if three consecutive frames are used or preferably 
(X-1)/2 frame delay if X consecutive frames are used. This process is 
repeated in order to present corrected fully combined picture signals to 
the TV. 
This multi-frame CTB removal scheme provides a much cleaner signal but 
obviously is a more complex system to implement and requires extra 
hardware. This system requires both intra-frame and inter-frame filters. 
The above procedure which corrects for the CTB can also be used for 
measuring the strength of the CTB. 
Based on the above it will be apparent that the estimate of composite 
triple beat signal for each line in each subpicture may be used to measure 
the signal-to-composite triple beat (CTB) ratio, this is calculated as 
##EQU6## 
where .alpha..sub.i (n)=the estimate composite triple beat value for the 
n.sup.th line of the i.sup.th subimage 
L=the number of subimages 
N=the length of each subimage in the vertical direction. 
If X consecutive frames are used in the processing then each .alpha..sub.i 
(n) in equation (11) is the average of the .alpha..sub.i (n)'s of the X 
consecutive frames. 
Experimental results show that measuring the signal to composite triple 
beat noise ratio by (10) and (11) provides correct estimates to values 
within .+-.2 dBs for S/N ratios up to 47 dBs. 
EXAMPLES 
In each of the examples two types of pictures were chosen. Type 1 are 
pictures consisting of people and background scenarios and Type 2 are 
pictures consisting of text. A random composite triple beat impairment was 
added to these pictures by computer simulation. 
The results obtained were evaluated both objectively and subjectively. The 
objective evaluation was done by measuring the signal-to-noise ratio and 
the subjective evaluation by visual inspection of the processed pictures. 
EXAMPLE 1 
For intra-frame processing (single frame processing) a number of different 
filter sizes 3, 5, 7 and 9 were applied and the filter size number 5 was 
found to give the best result and was used in subsequent experiments. 
In this example both the average and the median filters were used; the 
results showed that the average filter reduces the visual effect of CTB 
more than the median filter does but at the same time filters out more of 
the fine pictorial details and is therefore better for processing Type 1 
pictures and the median filter is better for processing Type 2 pictures. 
Experiments were conducted testing different subpicture (stripe widths) 
using stripe widths of 256, 128, 64, 32, 16, 8 and 4 pixels. Obviously the 
smaller the width of the stripe, the more the visual effect of the CTB was 
reduced but at the expense of finer detail of the picture. Based on both 
an objective and a subjective evaluation 64 pixel width was deemed to 
produce the best results. 
EXAMPLE 2 
The present invention was tested on pictures with different levels of 
impairments ranging from normal pictures having extremely high 
signal-to-noise ratio (SNR) to impaired signals with SNR at around 26 dBs. 
The results show that as the noise level increased more noise was removed 
but the noise level in the restored picture also increases. 
The overall results indicate that the present invention can reduce the 
visual effect of CTB. CTB's are normally noticed when the SNR is less or 
equal to 53 dbs. However, after processing by the method of the invention 
the level at which CTB's are perceived is lowered to about 42 dbs. for 
single frame processing scheme and about 38 dbs. for a multi-frame 
processing scheme. 
Besides reducing the visual effect on the picture of a CTB, the present 
invention can also reduce the visual effect of any impairment which can be 
modelled as a special case of a CTB such as co-channel interference 
impairments. A CTB is a sum of beats or sinusoids with relatively low 
frequencies interfering with the intended or original signal. The 
co-channel interference being composed of a single beat or sinusoid whose 
frequency lies around 10 Khz or 20 Khz is thus a special case of a CTB. 
The procedure of the present invention will also reduce the visual effect 
of the co-channel interference impairment. 
Having described the invention modifications will be evident to those 
skilled in the art without departing from the scope of the invention as 
defined in the appended claims.