System and method for filtering video signals

• US 5,438,374 A
• Filed: 12/10/1993
• Issued: 08/01/1995
• Est. Priority Date: 12/10/1993
• Status: Expired due to Fees

• Current Assignee: American Telephone & Telegraph Company (AT&T, Inc.)
• Original Assignee: AT&T Corporation (AT&T, Inc.)
• Inventors: Yan, Li
• Primary Examiner(s): Groody, James J.
• Assistant Examiner(s): Murrell, Jeffrey S.
• Application Number:
  US08/165,271
• Time in Patent Office: 599 Days
• Field of Search: 348/619, 348/620, 348/416, 348/413
• US Class Current: 348/620
• CPC Class Codes: H04N 19/80 Details of filtering operations specially adapted for video compression, e.g. for pixel interpolation H04N19/635, H04N19/86 take precedence
• CPC Class Codes: H04N 5/21 Circuitry for suppressing or minimising disturbance, e.g. moiré or halo
• CPC Class Codes: H04N 9/8042 involving data reduction
• CPC Class Codes: H04N 9/877 by assembling picture element blocks in an intermediate memory

Abstract

Motion video is represented by digital signals. The digital signals can be compressed by coding to reduce bitspace. Noise in the signal, however, reduces the efficiency of coding. The present invention is a system and method for reducing noise in video signals by filtering. The signal for the current frame is weightedly averaged with signals for a future and prior frame. The future and prior frames are given less weight as they differ more from the current frame. When motion compensation information is available, the motion compensated future and prior frames can be used for averaging, further improving filtering.

Claims

1. A video filtering system for reducing noise in a motion video sequence comprising:

a source of frame signals;

a computer processor for filtering frame signals in the video sequence;

an input line for sending said frame signals from the source to the computer processor;

an encoder having a frame buffer for storing frame signals corresponding to at least a current frame and a prior frame; and

a frame buffer input line for sending said frame signals from the frame buffer to the computer processor;

wherein the computer processor comprises;

means, operably engaged to the frame buffer, for comparing the frame signals for the current frame with the prior frame to generate a first frame difference;

means, operably engaged to the comparing means, for determining a weighting factor based, at least in part, on the first frame difference; and

means, operably engaged to the determining means, for generating the filtered signal for an intermediate frame based, at least in part, on the weighting factor and the signal for the prior frame.

2. The system of claim 1 further comprising:

means for comparing the intermediate frame signal with a future frame signal to generate a second fame difference;

means for determining a second weighting factor based, at least in part, on the second frame difference; and

means for generating a filtered frame signal based, at least in part, on the second weighting factor and the future frame signal.

3. The system of claim 2 wherein the prior frame is an adjacent prior frame to the current frame.

4. The system of claim 2 wherein the future frame is an adjacent future frame to the current frame.

5. The system of claim 2 wherein the encoder further comprises a motion estimator operably engaged to the frame buffer, which motion estimator can generate a motion compensated frame signal and wherein the computer processor further comprises:

means for separating the current frame signal into current macroblocks;

means for comparing the current macroblocks with macroblocks in the prior frame signal to generate a prior error signal;

means for comparing the current macroblocks with macroblocks in a motion-compensated prior frame signal to generate a motion-compensated prior error signal; and

means for combining the macroblocks with the smallest prior error signals into a prior best frame signal.

6. The system of claim 5 wherein computer processor further comprises:

means for comparing the current macroblocks with macroblocks in the future frame signal to generate a future error signal;

means for comparing the current macroblocks with macroblocks in a future motion-compensated frame signal to generate a motion-compensated future error signal; and

means for combining the macroblocks with the smallest future error signals into a future best frame.

7. The system of claim 1 wherein the means for calculating a weighting factor further comprises:

means for setting the weighting factor to a lower constant if the frame difference is below a lower threshold; and

means for setting the weighting factor to an upper constant if the frame difference is above an upper threshold.

8. The system of claim 7 wherein the means for calculating a weighting factor comprises a means for increasing the weighting factor as the frame difference increases when the frame difference is between the lower threshold and the upper threshold.

9. A method of filtering noise-contaminated video signals comprising the steps of:

storing the signals for a current frame, an adjacent prior frame and an adjacent future frame;

calculating a first frame difference between the current frame and the adjacent prior frame;

calculating a first weighting factor based, at least in part, on the frame difference;

multiplying the first weighting factor by the first frame difference to create a first product;

adding the first product to the adjacent prior frame to create an intermediate frame;

calculating a second frame difference between the intermediate frame and the adjacent future frame;

calculating a second weighting factor based, at least in part, on the second frame difference;

multiplying the second weighting factor by the second frame difference to create a second product; and

adding the second product to the adjacent future frame to create a filtered frame.

10. The method of claim 9 wherein the step of calculating a first frame difference comprises the step of subtracting pixel values of the adjacent prior frame from the corresponding pixel values of the current frame.

11. The method of claim 9 wherein the step of calculating a second frame difference comprises the step of subtracting pixel values of the adjacent future frame from the corresponding pixel values of the intermediate frame.

12. The method of claim 9 wherein the first weighting factor is calculated such that the prior adjacent frame is given less weight as the frame difference increases.

13. The method of claim 12 wherein the prior adjacent frame is given no weight when the frame difference is above an upper threshold.

14. The method of claim 12 wherein the prior adjacent frame and the current frame are given equal weight when the frame difference is below a lower threshold.

15. A method of filtering noise-contaminated video signals for intercoded pictures based on the signals for a current frame, a prior reference frame and an adjacent future frame, comprising the steps of:

calculating a motion compensated prior reference frame;

comparing the current frame to the prior reference frame to create an error signal;

comparing the current frame to the motion compensated prior reference frame to create a motion compensated error signal;

selecting either the prior reference frame or the motion compensated reference frame which has the smaller error signal as a best frame;

calculating a first frame difference between the current frame and the best frame;

calculating a first weighting factor based, at least in part, on the first frame difference;

calculating a first weighted average of the best frame and the current frame based, at least in part, on the first weighting factor wherein the weighted average is an intermediate frame;

calculating a second frame difference between the intermediate frame and the adjacent future frame;

calculating a second weighting factor based, at least in part, on the second frame difference; and

calculating a second weighted average of the intermediate frame and the adjacent future frame based, at least in part, on the second weighting factor, wherein the second weighted average is a filtered frame.

16. The method of claim 15 wherein the step of selecting the best frame comprises the steps of:

separating the current frame into current macroblocks;

comparing the current macroblocks with spatially co-located macroblocks in the prior reference frame and the motion compensated reference frame; and

combining the selected macroblocks into the best frame.

17. The method of claim 16 wherein the step of comparing the current macroblocks comprises the step of summing the squares of the differences of corresponding pixel values of the macroblocks being compared.

18. A method of filtering noise-contaminated video signals for intercoded pictures based on the signals for a current frame, a prior reference frame and a future reference frame, comprising the steps of:

calculating a motion compensated prior reference frame;

comparing the current frame to the prior reference frame to create a prior error signal;

comparing the current frame to the motion compensated prior reference frame to create a motion compensated prior error signal;

selecting the prior reference frame or the motion compensated reference frame which has the smaller prior error signal as a prior best frame;

calculating a first frame difference between the current frame and the prior best frame;

calculating a first weighting factor based, at least in part, on the first frame difference;

calculating a first weighted average of the prior best frame and the current frame based, at least in part, on the first weighting factor wherein the weighted average is an intermediate frame;

calculating a motion compensated future reference frame;

comparing the current frame to the future reference frame to create a future error signal;

comparing the current frame to the motion compensated future reference frame to create a motion compensated future error signal;

selecting the future reference frame or the motion compensated reference frame which has the smaller future error signal as a future best frame;

calculating a second frame difference between the future best frame and the intermediate frame;

calculating a second weighting factor based, at least in part, on the second frame difference; and

calculating a second weighted average of the intermediate frame and the future best frame based, at least in part, on the second weighting factor, wherein the second weighted average is a filtered frame.

19. The method of claim 18 wherein the step of selecting the prior best frame comprises the steps of:

separating the current frame into current macroblocks;

comparing the current macroblocks with spatially co-located macroblocks in the reference frame and the motion compensated reference frame;

selecting the macroblock closest to the current macroblocks; and

combining the selected macroblocks into the prior best frame.

20. The method of claim 19 wherein the step of comparing the current macroblocks comprises the step of summing the squares of the differences of corresponding pixel values of the macroblocks being compared.

21. The method of claim 18 wherein the step of selecting the future frame comprises the steps of:

separating the current frame into current macroblocks;

comparing the current macroblocks with spatially co-located macroblocks in the future reference frame and the motion compensated future reference frame;

selecting the macroblock closest to the current macroblocks; and

combining the selected macroblocks into the prior best frame.

22. The method of claim 21 wherein the step of comparing the current macroblocks comprises summing the squares of the differences of corresponding pixel values of the macroblocks being compared.

23. A system for filtering noise contaminated signals for motion video comprising:

a means for storing signals for a current frame, a prior frame and a future frame; and

a computer processor operably engaged to said signal storing means having;

means for calculating a first frame difference between the current frame and the prior frame;

means for calculating an intermediate frame signal as a weighted average of the current frame signal and the prior frame signal such that the prior frame is given less weight as the frame difference increases;

means for calculating a second frame difference between the intermediate frame signal and the future frame signal; and

means for calculating a filtered frame as a weighted average of the intermediate frame signal and the future frame signal such that the future frame is given less weight as the second frame difference increases.

24. The system of claim 23 wherein the means for calculating an intermediate frame signal comprises:

a means for calculating a weighting factor based on the frame difference;

means for multiplying the weighting factor and the frame difference to obtain a product; and

means for adding the product to the prior frame.

25. The system of claim 23 wherein the means for calculating a filtered frame comprises:

means for calculating a weighting factor based on the second frame difference;

means for multiplying the weighting factor and the second frame difference to obtain a product; and

means for adding the product to the future frame.

26. The system of claim 23 wherein the prior frame is a prior adjacent frame.

27. The system of claim 23 wherein the future frame is a future adjacent frame.

28. A video filtering system for reducing noise in frame signals in a bitstream comprising:

a computer processor for filtering the frame signals;

a decoder having a frame buffer and a motion estimator;

an input line for sending the frame signals from the decoder to the computer processor;

an output line for sending filtered signals from the computer processor to a display;

a frame buffer input line for sending signals from the frame buffer to the computer processor;

means for comparing a current frame with a prior frame to generate a first frame difference; and

means for calculating a weighting factor based, at least in part, on the first frame difference; and

means for calculating a weighted average of the prior frame and the current frame based, at least in part, on the weighting factor.

29. The system of claim 28 wherein the weighted average is an intermediate frame and further comprising:

means for comparing the intermediate frame with a future frame to generate a second frame difference; and

means for calculating a second weighted average of the intermediate frame and the future frame based, at least in part, on the second weighting factor.