Temporal filtering of video signals

• US 8,493,313 B2
• Filed: 02/13/2008
• Issued: 07/23/2013
• Est. Priority Date: 02/13/2008
• Status: Active Grant

• Current Assignee: Dolby Laboratories Licensing Corporation (Dolby Laboratories Incorporated)
• Original Assignee: Dolby Laboratories Licensing Corporation (Dolby Laboratories Incorporated)
• Inventors: Damberg, Gerwin, Seetzen, Helge
• Primary Examiner(s): Mengistu, Amare
• Assistant Examiner(s): Nadkarni, Sarvesh J
• Application Number:
  US12/030,448
• Publication Number:
  US 20090201320A1
• Time in Patent Office: 1,987 Days
• Field of Search: 345/102, 345/589, 345/694, 345/691, 348/844
• US Class Current: 345/102
• CPC Class Codes: G09G 2320/0261 in the context of movement of objects on the screen or movement of the observer relative to the screen
• CPC Class Codes: G09G 2320/0646 Modulation of illumination source brightness and image signal correlated to each other
• CPC Class Codes: G09G 2320/0653 Controlling or limiting the speed of brightness adjustment of the illumination source
• CPC Class Codes: G09G 2320/066 for control of contrast
• CPC Class Codes: G09G 2360/16 Calculation or use of calculated indices related to luminance levels in display data
• CPC Class Codes: G09G 3/3426 the different display panel areas being distributed in two dimensions, e.g. matrix

Abstract

A process for reducing noise and temporal artifacts (e.g. walking LEDs) on a dual modulation display system by applying temporal filtering to rear modulation signals of a sequence of video frames. Flare and dimming rates are calculated for a current frame in the video. If a flare rate threshold is exceeded, an intensity of the backlight is limited to a predetermined flare rate. If a dimming rate threshold is exceeded, the backlight intensity is limited to a predetermined dimming rate. The limitations are applied, for example, on an element-by-element basis. In the event of a scene change, the limitations do not need to be applied. A forward modulation signal is calculated by taking into account any applied backlight limitations.

Claims

1. A method comprising the steps of:

receiving a segment of a video;

calculating a rear modulation signal for the received segment wherein the rear modulation signal comprises a plurality of control signals, the control signals respectively corresponding to separately-controllable elements of a backlight;

calculating a difference in intensity between the rear modulation signal of the received segment and a rear modulation signal of a previous frame corresponding to the received segment; and

modifying the rear modulation signal for the received segment with a temporal filtering limit R to obtain an actual rear modulation signal for the received segment;

wherein the temporal filtering limit R limits at least a rate of increase or a rate of decrease in intensity of elements of the backlight; and

wherein calculating the rear modulation signal comprises;

performing first downsampling on the received segment;

applying a spatial filter to the downsampled received segment; and

performing a second downsampling on the spatially-filtered, downsampled, received segment.

2. The method according to claim 1, wherein the received segment comprises one of a full frame of the video, a fixed partial frame of the video, a variable partial frame of the video, and a scanned portion of the video.

3. The method according to claim 1, further comprising the step of determining the filtering limit R.

4. The method of claim 1 wherein the temporal filtering limit comprises a flare rate wherein modifying the rear modulation signal comprises limiting increases in intensities of the backlight elements to less than the flare rate.

5. The method of claim 4 wherein the temporal filtering limit comprises a dimming rate wherein modifying the rear modulation signal comprises limiting decreases in intensities of the backlight elements to less than the dimming rate.

6. The method of claim 5 wherein the dimming rate is variable and is a function of intensity of the backlight elements.

7. The method of claim 4 wherein the flare rate is variable and is a function of intensity of the backlight elements.

8. The method of claim 1 comprising applying the temporal filtering limit on an element-by-element basis to a plurality of backlight elements.

9. The method of claim 8 comprising calculating a forward modulation signal for driving a front modulator unit based on the actual rear modulation signal.

10. The method of claim 4 wherein applying the filtering limit comprises averaging light intensities across multiple previous frames.

11. The method of claim 1 wherein the first downsampling comprises taking maximum values within regions of an image in the received segment.

12. The method of claim 11 wherein the second downsampling comprises applying a mean-downsample.

14. The method according to claim 1, wherein the filtering limit R is based on at least one of performance characteristics of a display on which the video is to be displayed and characteristics of the video signal.

13. A method comprising the steps of:

receiving a segment of a video;

calculating a rear modulation signal for the received segment wherein the rear modulation signal comprises a plurality of control signals, the control signals respectively corresponding to separately-controllable elements of a backlight;

calculating a difference in intensity between the rear modulation signal of the received segment and a rear modulation signal of a previous frame corresponding to the received segment; and

modifying the rear modulation signal for the received segment with a temporal filtering limit R to obtain an actual rear modulation signal for the received segment;

wherein the temporal filtering limit R limits at least a rate of increase or a rate of decrease in intensity of elements of the backlight; and

the method further comprising forming a three-dimensional data structure of backlight element drive values, the data structure having two spatial dimensions and a temporal dimension wherein applying the filtering limit comprises applying a three-dimensional filter to the data structure.

15. The method according to claim 13, wherein:

said method comprises executing by a computer a set of computer instructions stored on a computer readable media.

16. The method according to claim 15, wherein said computer instructions are compiled computer instructions stored as an executable program on said computer readable media.

17. The method of claim 13 wherein applying the temporal filtering limit comprises multiplying one or more values for a previous frame by a value corresponding to a maximum flare rate.

18. The method of claim 13 comprising applying the filtering limit on an area-by-area basis to the received segment such that some of the control signals are changed by applying the filtering limit to yield the actual rear modulation signal and others of the control signals are not changed by applying the filtering limit.

19. The method of claim 13 wherein applying the filtering limit comprises comparing a change in commanded light output for a backlight element to a threshold.

20. The method of claim 19 comprising adjusting the threshold dynamically based on a luminance level for the backlight element.

21. The method of claim 19 wherein applying the filtering limit comprises comparing a change in commanded light output for a backlight element to a flaring threshold and a dimming threshold wherein the flaring threshold is different from the dimming threshold.

22. The method of claim 13 wherein the three-dimensional filter has a kernel of dimension 3×

3×

2.

23. The method of claim 13 wherein the data structure comprises a matrix of backlight element drive values for a current frame stacked with a matrix of backlight element drive values for a previous frame.

24. The method of claim 13 wherein applying the filtering limits comprises applying a single-stage filter that operates simultaneously on spatial and temporal information.