WO1994024824A1

WO1994024824A1 - Method and device for correcting a distortion of a video line signal

Info

Publication number: WO1994024824A1
Application number: PCT/EP1994/001199
Authority: WO
Inventors: Joel Hamon; Albert Dorner; Eric Diehl
Original assignee: Thomson Consumer Electronics S.A.
Priority date: 1993-04-21
Filing date: 1994-04-18
Publication date: 1994-10-27
Also published as: GB9308244D0; GB2277420A; AU6679194A

Abstract

This invention presents a system, including a method and an appropriate device, for the correction of distortions of a video signal. First, amplitude values of samples (P1, P2) of an active video line, with a known or the same luminance amplitude values or with known luminance amplitude difference values (DN) prior to transmission and a defined distance from each other are measured and compared. In dependence of the amplitude differences the amplitude values of other samples of the active video line are corrected and by this a possible distortion can be corrected. The advantages of the system are, that differences in the luminance of video pictures can be compensated or if a system with scrambled video signals is used, the streaky noise can be reduced or avoided. On the other side it is also possible to use video transmission systems with a larger tilt value without increasing the inconvenience of a viewer. The inventive system can be used preferably for downloading video systems.

Description

Method and Device for Correcting a Distortion

of a V i d e o Line Signal

The present invention relates to a method for correcting a distortion of a video line signaL and a device able to execute said method.

It is known, that a video Line signal, which has no distortion at the transmitter, may get a distortion on its transmission channel and/or by a receiver or other devices which are connected with the receiver.

By this, changes in luminance from one side of the picture to the other or additional Luminance amplitude differences with- in the lines can occur.

It is also known, that a so called line-tilt, or just ti lt, can occur due to a Large variety of causes in connection with Vestigal Side Band Amplitude Modulation CVSB-AM) transmission systems. If tilt occurs anywhere in the transmission of an Active Line Rotation (ALR) scrambled signal then the descrambled picture has a Low-Level scrambling pattern superimposed on it. This appears as "streaky" noise.

Informal subjective tests on critical pictures indicated, that streaky noise becomes perceptible on the descrambled ALR picture when the Line-ti lt exceeds a given value. If the Line-ti lt is defined as the additional difference in amplitude between the beginning and the end of the active Line expressed as a percentage of the b Lack-to-white excursion of the wanted Luminance signal (nominally 0,7 volts), the given value is usually about 0,5%. But it was found, that a value of 1.5% is a realistic value for so called downloading systems, in which descrambled pictures are viewed after recording and replay by a domestic videorecorder. When using relay stations this value can easily extend to 3%.

The present invention seeks to provide a method and an appropriate device to correct the distortion of a line signal, preferably of the active line.

The present invention also seeks to provide a system which allows an increased value of line-tilt in a video transmitting system.

According to a first aspect of the present invention there is provided a method for correcting a distortion of a video line signal comprising the steps of a pixel acquisition of samples (Pl, P2) of the video line signal with known or the same luminance amplitude values or with known luminance amplitude difference values prior to transmission and with a known distance from each other or from the beginning or the end of the active line, a calculation of luminance amplitude values (A1,A2) of said samples (P1,P2), a calculation of luminance amplitude correction values, and a correction of said luminance amplitude values of said pixels of said video line signal.

According to a second aspect of the present invention there is provided a device for correcting a distortion of a video line signal comprising means for a pixel acquisition of samples (P1,P2) of the video line signal with known or the same luminance amplitude values or with known luminance amplitude difference values prior to transmission and with a known distance from each other, a calculation of luminance amplitude values (A1,A2) of said samples (P1,P2), a calculation of luminance amplitude correction values, and a correction of said luminance amplitude values of said pixels of said video signal.

Briefly, the system according to the invention makes a pixel acquisition of samples (pixels) with known or the same amplitude values or amplitude difference values prior to transmission and a known distance from each other, compares the amplitude values belonging, calculates correction values from the amplitude differences of the received samples and compensates the amplitude values of all the pixels (samples) of the active line.

It is an advantage of the inventive system by correcting distortions of luminance amplitude values of a line signal, to generate video pictures of high quality.

It is another advantage of the present invention, that it is possible to decrease a line tilt of the video-signal. If it is fed to a decoder for descrambling the received video signal it is possible to reduce or to avoid streaky noise.

On the other hand it is also possible to use video transmission systems, which have a relative high value of line tilt.

The invention is further described by way of i llustrative examples by reference to the drawings in which

Fig. 1 shows in principle the transmission way of a video signal;

Fig. 2 shows in principle timing diagrams of lines of

video signals, which are present at different points during the transmission according to the system of fig. 1;

Fig. 3 shows a block diagram of a first embodiment of the

invention;

Fig. 4 shows a flow chart of the method used in the

device of Fig. 3;

Fig. 5 shows a block diagram of a second embodiment of the

invention

Fig. 6 shows a flow chart of the method used in the device of Fig. 5.

Before the detailed description of the preferred embodiments it should be mentioned, that the blocks are separately shown for a better explanation. In practice at least some of these blocks can be integrated using an Integrated Circuit technology, a hybrid technology or a microcomputer. Some of these blocks can also be part of a program for such a microcomputer.

Fig. 1 shows in principle the known transmission channel of a video signal. A video source 1, like a video generator, a video recorder, etc., is connected to a transmitter 2. The transmitter 2 transmits its output signals via a not shown antenna or a cable to a receiver 3. The received signals are given to a monitor 4.

The function of the transmission line of fig. 1 will be explained with the aid of timing signals of fig. 2a and fig 2b.

In fig. 2a, 2b several timing diagrams of a video line are in principle shown. They start with tπe so called "sync" for the horizontal synchronization and it follows the so called "burst". Between the beginning point PB and the end point PE follows the active part of the line.

The video source 1 delivers video signals with no distortion. A timing diagram of such a video line is shown in fig. 2a. It can be seen, that the samples PB, PE and all according samples between them have the same luminance value in this example.

This signal is taken for modulation for getting a signal, which is sent by the transmitter 2. This transmission can be done via a satellite, terrestrial or via cable (electrical, optical) .

The transmitted signal is received and demodulated by the receiver 3. If some distortions occur on the transmission channel, the receiver 3, as is state of the art, demodulates the video signal, as depicted in fig. 2b.

The receive/ 3 according to the invention includes the inventive device, corrects the distortion and has an output signal nearly as the one shown in fig. 2a.

The demodulated video signal is monitored by the monitor 4.

Though the active line should have only a constant luminance value in this example, it can be seen (fig. 2b), that its amplitude changes. The amplitude difference between the one of the samples PB and PE and the one of samples P1, P2 with the maximum change of luminance amplitude is the distortion to be corrected.

Fig. 3 shows a block diagram of a first preferred embodiment. A video input terminal 10 is connected to a first input of a correction device 17, which is connected with its output to an output terminal 11, which is generally connected to other devices of a video receiver, which are not shown, because they are not part of the invention. The correction device 17 and the terminal 11 are connected to an acquisition device 12. This gives signals derived from samples P1, P2 of the active line to an amplitude measurement device 13 and another signal to a mean value block 15. The resulting signaL of the device 13 is given to a difference calculation device 14, which gives a second signal to the mean value block 15. Signals from this block 15 are Led to the device 12 and to a correction value calculator 16, which gives its result to a second input of the correction device 17.

The function of the device according to fig. 3 is now explained with the aid of the flow chart of fig. 4.

After switching on the device, the method starts with step 100, where parameters are set to zero or another specified value. In step 101 the variable N, which has been zero before, is increased and in step 102 the actual value N is compared with a maximum value MM. If N is smaller than NM, the method continues with step 103, where the first sample P1, which is part of the active Line, is sampled. In step 104 the second sample P2 of the active line is sampled. The steps 100 to 104 are executed by the acquisition device 12.

According to fig. 2b the samples P1 and P2 can be the samples with the maximum distortion of the luminance amplitude.

These samples P1, P2 can be found by the acquisition device 12, e.g. by an adaptive method, when a line signal with the same or known amplitude signals is transmitted.

Adaptive methods are known and a further description of them doesn't seem necessary.

The amplitude measuring device 13 determines the amplitude values A1, A2 of the samples P1, P2 respectively (step 105, 106). The difference calculation device 14 calculates difference values DN1 = A1 - A0 and DN2 = A2 - A 0 (step 107). Thereby A0 is the amplitude value of P1 and P2 before the distortion occurs. This value A0 may be known, e.g. due to the use of a test line, or may be calculated, e.g. from the amplitude values of other known samples, e.g. PB, PE.

The mean value block 15 calculates mean values D1 = 1/N * DN1 and D2 = 1/N * DN2 in step 108. After this calculation, the block 15 gives a signal to the acquisition device 12, which restarts with step 101.

If the result of step 102 is "no", that means, that N is Larger than or equal NM, the method continues with step 109, where correction values Korr are determined by the correction value calculator 16. The correction of the videosignal is done by the correction device 17 (step 110). At last the variable N is reset to zero in step 111 by the acquisition device 12 after receiving an appropriate signal from the block 15.

The correction values calculated in step 109 may be of different kinds. One possibility is, that they form ramps for additional corrections according to D1, D2 respectively. But it is also possible, that multiplicative correction factors are formed. Then the difference values DN1, DN2 calculated in step 107 must be calculated by

DN1 = A1/A0,

DN2 = A2/A0.

The correction executed in step 110 is dependent on the form of the correction factor Korr and on the distance of the samples P1 and P2 from the beginning or the end of the active line. If Korr is to have a multiplicative effect, then in step 110 a multiplicative correction is executed like

A_x = K * t_x * Korr, where K is a corrective factor including the distance of P1 and P2 from the beginning of the active line, A is the amplitude of a sample of the active Line and t_x is the corresponding time value starting with t =0 at the beginning of the active Line.

Fig. 5 shows a second embodiment of the invention including additional digital means.

Means and blocks with the same function as the embodiment of FIG.3 have the same reference numbers and they are only explained as it is necessary for the understanding of the invention.

The input terminal 10 is connected to the first input of the correction device 17, which gives its signals to an analog- to-digital (A/D) converter 21. This converter 21 is connected to the input of digital video means 22, like a decoder, and to the acquisition device 12. The output of the digital video means is connected to a digital-to-analog (D/A) converter 23, which is connected with its output to the output terminal 11.

The acquisition device 12 gives data to a microcontroller 24 and receives signals on another line from this microcontroller 24. The microcontroller 24 sends data to a first counter 25 and to a second counter 26. The second counter 26 also receives signals from the first counter 25. Both counters 25, 26 are also connected to a not shown trigger modul, which sends trigger signals corresponding to the line signal. The result of the second counter 26 is sent to a D/A converter 27, which gives its output signals to the correction device 17.

The function of the device according to fig. 5 is now explained with the aid of the flow charts of fig. 4 and fig. 6, including fig. 6a and fig. 6b.

The method that is executed by the device according to fig. 5 is the same , as the one shown in fig.4. But the step 109 is substituted by the steps shown in the flow chart of fig.6.

The steps 100 to 108 are executed by the microcontroller 24.

If the decision in step 102 is "no", that means that N is larger than or equal NM, the method according to this embodiment continues with step 201. In this step a test is made, if a flag a1, which has been set to zero at step 100, has still the value zero.

If a1=0, then it is checked in step 202, if the mean difference D1, calculated in step 108, is lower than zero.

If the result of the step 202 is "yes", follows a step 203 where the microcontroller 24 gives an UP-signal to the second counter 26.

During an UP-signal the value of the second counter 26 increases in dependence on the signal of the first counter 25.

If the result of the step 202 is "no", follows a step 204, where it is checked, if the mean difference D1 is larger than zero. If "yes", follows a step 205, where the microcontroller 24 gives a DOWN-signal to the second counter 26.

During a DOWN-signal the value of the second counter 26 decreases in dependence on the signal of the first counter 25. If the result of step 204 is "no", that means that D1=0, follows a step 206 where the flag a1 is set to one. This step follows also after the execution of the steps 203 and 205.

If the result of step 201 is "no", that means that the flag a1 has been already set to one, follows the same step as after step 206: step 207. Here it is checked, whether the mean difference is larger than zero. If so (yes), follows step 208, where it is checked, if the microcontroller 24 is giving an UP-signal to the second counter 26. If so (yes), the value which is sent from the microcontroller 24 to the first counter 25 is increased (step 209), otherwise (no) this value is decreased (step 210).

The function of the first counter 25 is as follows:

It devides the clock frequency of the system by the actual value C1. That means, that the result of the first counter 25 increases, if the value of C1 decreases and vice versa.

If the result of step 207 is "no", which means that the mean difference D1 is smaller than or equal zero, it is checked whether the second counter 26 receives an UP-signal (step 211). If the result is "yes", C1 is decreased (step 212), otherwise C1 is increased (step 213).

After the execution of the steps 209, 210, 212, 213 the steps 301 to 313 of fig. 6b follow.

The steps 201 to 213 of fig. 6a determine correction values for the luminance amplitude values of the active line around the sample P1 and the steps 301 to 313 of fig. 6b which execute adequate functions as the steps 201 to 213, determine correction values for the luminance amplitudes of the active line around the sample P2. All these steps are executed by the microcontroller 24 in the preferred embodiment.

After the steps 309 to 313 follow the steps 110 and 111 of fig. 4.

The device according to fig. 5 works in that way, that if the method is executed for the first time after start, the microcontroller sends an UP-signal or a DOWN-signal to the second counter 26 in dependence of the fact, whether the distortion is positive (as at P2 in fig. 2b) or negative (as at P1 in fig. 2b).

After that or during another run according to the method, the first counter 25 gives a value to the second counter 26. This value will be increased, if the second counter 26 receives an UP-signal and the distortion is negative (D1, D2 is smaller than zero) or if the second counter 26 receives a DOWN-signal and the distortion is positive.

Otherwise the second counter 26 will receive a decreasing signal.

It may be said again, that the value sent from the first counter 25 to the second counter 26 increases, if the value of the signal, which the first counter 25 receives from the microcontroller, decreases, and vice versa.

The preferred embodiments of the invention work as a closed-loop control device.

A device, which works as open-loop control could be another embodiment of the invention.

In this case it is also necessary, to measure the amplitude values of samples of an active line, which have a known or the same amplitude values and a defined distance from each other, to calculate the (mean) amplitude difference and to correct the amplitudes of the samples of the active line.

But in this case the corrected video signal is not fed back to the input terminal 10, but led to the output terminal 11.

Versions of the preferred embodiments may contain at least one of the following variations:

- More than two samples inside the active line may be taken for the calculation of correction values;

only the samples PB and PE may be taken for calculation - of correction values of the samples of the active line.

This is preferred, if it can be assumed that the distortion of the active line is constant or with a known curve, e.g. a linear line ti lt;

- samples of an overlay area, which can be used for active line rotation (ALR) systems, may be taken as samples with the same amplitude value and a known distance from each other or with a known distance from the beginning or the end of the active line. In this case the acquisition device 12 may find the samples P1, P2 by a proper timing of sampling the active line in dependence on the content and the form of the overlay areas;

- the amplitude values AO, which are used in step 107 may be different for the sample P1 and the sample P2;

- samples P1, P2 with known amplitude difference values

before transmitting may be taken;

- samples P1, P2 may be taken with the same amplitude

value, but the other samples of the active line may have different amplitude values. These values might be known and corrected appropriately.

By this invention a system, including a method and an appropriate device, is presented, which measures and compares amplitude values of samples P1, P2 of an active video line, with known or the same amplitude values or with known amplitude difference values prior to transmission and a defined distance from each other. In dependence on the (mean) amplitude differences of the received samples P1, P2 the amplitude values of the other pixels (samples) of the active video line are corrected and by this a possible distortion, e.g. a line ti lt is corrected.

The advantages of the system are, that error differences in the luminance of video pictures can be compensated or if a system with scrambled video signals is used, the streaky noise can be, reduced or avoided.

On the other side it is also possible to use video transmission systems with a larger tilt value without increasing the inconvenience of a viewer.

Claims

C L A I M S

1. Method for correcting a distortion of a video line signal characterized by the steps of

- a pixel acquisition of samples (P1, P2) of the video line signal with known or the same luminance amplitude values or with known luminance amplitude difference values prior to transmission and with a known distance from each other or from the beginning or the end of the active line,

- a calculation of luminance amplitude values (A1, A2) of said samples (P1, P2),

- a calculation of luminance amplitude correction va lues,

- a correction of said luminance amplitude values of said pixels of said video line signal.

2. Method according to claim 1, characterized in that said calculation of luminance amplitude correction values comprises the calculation of mean values of said luminance amplitude values (A1, A2) and/or the calculation of mean values .of one or more difference values (D1, D2) of said luminance amplitude values (A1, A2).

3. Method according to claim 1 or 2, characterized in that said correction of said luminance amplitude values of pixels of said video signal includes a close loop control.

4. Method according to one of the claims 1 to 3, characterized in that said distortion to be corrected is a line ti lt.

5. Method according to one of the claims 1 to 4, characterized in that said samples (P1, P2) with known or the same amplitude values or with known amplitude difference values and a known distance from each other or from the beginning or the end of the active line are samples of an overlay area.

6. Device for correcting a distortion of a video line signal characterized by means for

a pixel acquisition (12) of samples (P1, P2) of the video line signal with known or the same luminance amplitude values or with known luminance amplitude difference values prior to transmission and with a known distance from each other,

a calculation (13) of luminance amplitude values (A1, A2) of said samples (P1, P2),

a calculation (14, 15, 16, 24) of luminance amplitude correction values,

a correction (17, 25, 26, 27) of said luminance amplitude values of said pixels of said video signal.

7. Device according to claim 6, characterized in that said means for said calculation of luminance amplitude correction values include means (15, 24) for the calculation of mean values of said luminance amplitude values (A1, A2) and/or of difference values (D1, D2) of said luminance amplitude values (A1, A2).

8. Device according to claim 6 or 7, characterized in that it works as a closed loop control.