GB2351406A - Video data compression with scene change detection - Google Patents

Video data compression with scene change detection Download PDF

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Publication number
GB2351406A
GB2351406A GB9914476A GB9914476A GB2351406A GB 2351406 A GB2351406 A GB 2351406A GB 9914476 A GB9914476 A GB 9914476A GB 9914476 A GB9914476 A GB 9914476A GB 2351406 A GB2351406 A GB 2351406A
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Prior art keywords
image
scene change
quantity
block
operable
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GB9914476A
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GB9914476D0 (en
Inventor
Timothy Stuart Roberts
Nicholas Ian Saunders
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Sony Europe Ltd
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Sony Europe Ltd
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    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04NPICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N5/00Details of television systems
    • H04N5/76Television signal recording
    • H04N5/91Television signal processing therefor
    • H04N5/92Transformation of the television signal for recording, e.g. modulation, frequency changing; Inverse transformation for playback
    • H04N5/926Transformation of the television signal for recording, e.g. modulation, frequency changing; Inverse transformation for playback by pulse code modulation
    • H04N5/9261Transformation of the television signal for recording, e.g. modulation, frequency changing; Inverse transformation for playback by pulse code modulation involving data reduction
    • H04N5/9264Transformation of the television signal for recording, e.g. modulation, frequency changing; Inverse transformation for playback by pulse code modulation involving data reduction using transform coding
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04NPICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/10Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
    • H04N19/134Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the element, parameter or criterion affecting or controlling the adaptive coding
    • H04N19/142Detection of scene cut or scene change
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04NPICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/10Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
    • H04N19/169Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding
    • H04N19/179Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding the unit being a scene or a shot
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04NPICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/50Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using predictive coding
    • H04N19/503Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using predictive coding involving temporal prediction
    • H04N19/51Motion estimation or motion compensation
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04NPICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/85Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using pre-processing or post-processing specially adapted for video compression
    • H04N19/87Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using pre-processing or post-processing specially adapted for video compression involving scene cut or scene change detection in combination with video compression
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04NPICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N5/00Details of television systems
    • H04N5/14Picture signal circuitry for video frequency region
    • H04N5/144Movement detection
    • H04N5/145Movement estimation
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04NPICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N5/00Details of television systems
    • H04N5/14Picture signal circuitry for video frequency region
    • H04N5/147Scene change detection

Abstract

A video data compression apparatus comprises a motion vector generator for performing block matching by detecting block differences between a block of one image and a plurality of blocks of an adjacent image for generating a motion vector from the pair of blocks having the best match. A combining means combines the block differences across blocks of each image and a scene change detector detects scene change in the input video signal by detecting changes in the combined block difference values from image to image. A second invention describes an adaptive encoder that operates to compress an image in accordance with a target data output. Where a scene change is detected the target output is adjusted to take this into account.

Description

2351406 VIDEO DATA COMPRESSION This invention relates to video data

compression.

It has long been recognised that some video data compression systems, such as systems broadly defined by the MPEG-2 standard, use compression techniques in which the number of compressed data bits generated for a picture, or a part of a picture, depends on the nature of the image represented by that picture. Also, the main compression parameter which can be altered from block to block or picture to picture to change the bit rate, namely the degree of quantisation, has a somewhat non linear and difficult to predict effect on the resulting bit rate.

These characteristics are of particular concern in systems such as video tape recorders, where there is generally a fixed allocation of bits for each picture or group of pictures (GOP) and little or no scope for exceeding that fixed allocation, As a result, techniques for bit rate control in video data compression are very important.

The so-called "Test Model 5" of the MPEG 2 system proposes a rate control algorithm that allocates bits between pictures in accordance with a "global complexity estimation" dependent upon the actual number of bits generated in respect of a preceding picture and the quantisation parameters used to achieve this. The actual bit rate achieved during compression of a picture is then monitored and the degree of quantisation varied during compression to try to achieve the desired total bit rate for that picture. This system can, however, be slow to react to changes in image type during the compression of a picture and cannot predict the presence of difficult-to encode image portions (requiring a higher bit rate) towards the end of a particular picture.

However, a problem can occur if a scene change takes place in the input video signal. Here a "scene change" means either a literal cut form one scene or camera to another, or just any type of video effect or scene which gives rise to a rapid change of the image from one image to the next. If such a scene change occurs, the bit allocations previously established for use in compressing the input video signal may be no longer valid, which can mean that problems can occur such as encoders over-running the allowable number of bits for an image.

This invention provides video data compression apparatus in which motion 2 vectors are generated to represent image motion between pairs of successive images of an input video signal, the apparatus comprising:

a motion vector generator for performing block matching by detecting block differences between a block of one image and a plurality of blocks of an adjacent image, and for generating a motion vector from the pair of blocks having the best match; means for combining the block differences across blocks of each image; and a scene change detector operable to detect scene change in the input video signal by detecting changes in the combined block difference values from image to image.

The invention recognises the need to detect scene changes in a video data compression system, and provides a particularly convenient way to do this as a straightforward by-product of the motion vector estimation process carried out in such systems.

The block difference - used during motion vector estimation to derive the best match between a block of a one image and a search area in an adjacent image - is combined (e.g. summed) over the blocks of an image. This then gives a measure which can be compared from image to image, although if a few previous values are averaged it can give a more stable response. If there is a change of at least a certain amount in this variable, a scene change is indicated.

The invention also provides video data compression apparatus comprising:

an adaptive compression encoder operable to compress each image of an input video signal in accordance with a target output data quantity for that image, the encoder being operable to vary data compression parameters during compression of an image in order to substantially achieve the target output data quantity; means for detecting scene changes in the input video signal; and means for setting the target output data quantity to a first quantity when a scene change has not been detected and to a second, lower, quantity for an image in which a scene change is detected.

This aspect of the invention recognises that an adaptive encoder may find it very difficult to achieve its target data amount after a scene change, and so the target is reduced (e.g. for one or two images) in those circumstances. The temporarily 3 lower target means that if the adaptive encoder exceeds its target as a result of the scene change, the overall data limits of the system need not be exceeded.

While a preferred embodiment of the invention relates to an 11-VTR using intra (1) pictures only, it will be appreciated that the invention is equally applicable to a system employing GOPS also including B pictures, P pictures or both.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, throughout which like parts are described by like references, and in which:

Figure I is a schematic diagram of a video tape recorder (VTR) using data compression; Figure 2 is a schematic diagram of a data compression apparatus; Figure 3 is a schematic diagram of a scene change detector; and Figure 4 is a schematic diagram of a bit allocator.

Figure 1 is a schematic diagram of a video tape recorder (VTR) using data compression. Video data received by the VTR is supplied first to a data compression apparatus 10 in which the data quantity of the video data is reduced by compression techniques to be described below. The compressed video data is then passed to an error correcting code (ECC) processor and formatter which formats the data into an appropriate form for storage on tape and adds various error correcting codes in accordance with conventional techniques. The fon-natted data is then stored on a tape medium 30.

At replay, data is read from the tape medium 30 and processed by an ECC processor and formatter 40. This uses the ECC to detect any errors resulting from the data storage process and, hopefully, to correct them. It also re- formats the data into an appropriate form for decompression. Decompression is then carried out by a data decompression apparatus 50 which is arranged to provide a decompression process complimentary to the compression process applied by the data compression apparatus 10.

The key features of the embodiment which will be described below are found in the data compression apparatus 10. The remaining parts of Figure I may be implemented using known techniques.

Figure 2 is a schematic diagram of the data compression apparatus 10.

4 The VTR described in connection with this embodiment uses so-called "I" (intra) pictures only. So, unlike some implementations of systems such as MPEG-2, each picture (generally a frame) is compressed without reference to adjacent or nearby pictures. While this means that some of the compression efficiency which is possible with a long-GOP system using P frames and B frames cannot be achieved, it does mean that editing can easily take place at any desired frame boundary in the video signal. So, an I-frame VTR is particularly suited for studio use.

The fact that only I-frames are used means that the diagram shown in Figure 2 is much simpler than a conventional long-GOP encoder.

So, Figure 2 illustrates a DCT encoder 60, a bit allocator 70, a motion estimator 75, a quantizer 80 and an entropy encoder 90. The DCT encoder 60 operates to decompose the picture into blocks of 8x8 pixels and to apply a discrete cosine transform to generate a corresponding matrix of 8x8 DCT coefficients representing increasing spatial frequency components.

In parallel with the DCT process, the bit allocator 70 examines the input images and allocates a proportion of the available number of bits for encoding each image (which is generally fixed quantity because of storage constraints imposed by the tape medium 30) to different areas of the image. In the present example, the allocation is carried out on a macroblock (MB) by macroblock basis. Here, the term macroblock refers to an array of 16x16 pixels, i.e. four DCT blocks. The specific operation of the bit allocator will be described in much more detail below, but as its output it supplies target data quantities for each macro to the quantizer 80.

Again, in parallel with the DCT process, the motion estimator 75 performs block matching to establish motion vectors representing image motion between adjacent images of the video signal. Although these motion vectors are not employed in the present system for compression purposes (as the images are all encoded as I pictures), they can be added (e.g. under user control) to the bitstream to assist later in error concealment, as defined by MPEG.

During the block match process, a block in one image is compared with an array of possible block positions in a preceding image. For each of these possible block positions, the sum of absolute pixel differences between pixels at corresponding positions in the pair of blocks under test is calculated. This gives an indication of the quality of the match between the pair of blocks.

The vector output by the motion estimator 75 is that corresponding to the block position giving the best match. However, the block match results (the sums of absolute difference for the best block match for each block, i.e. that block match giving rise to the output motion vector) are also summed across the image to provide a measure, referred to as ME - dist, of the quality of block matching across the image.

In other words, ME-dist is a simple by-product of the motion estimation process.

Sudden changes in ME - dist are detected by the bit allocator 70 and influence the allocation of target bits to each image. This operation will be described in more detail with reference to Figures 3 and 4.

The quantizer 80 caff ies out a conventional thresholding and quantization process which involves zeroing coefficients below a predetermined threshold and quantizing the remaining ones, with the degree of quantization being selected in order to control the resulting output data quantity and also to account (in a conventional way) for image attributes such as so-called image activity which can vary from area to area within the image.

The quantiser 80 operates as an adaptive compression encoder operable to compress each image of an input video signal in accordance with a target output data quantity for that image, the encoder being operable to vary data compression parameters during compression of an image in order to substantially achieve the target output data quantity. This uses known techniques such as those defined by the MPEG Test Model 5 specification.

Finally, the entropy encoder 90 carries out run length coding and variable length (Huffmann) coding so that more frequently occurring bit patterns within the run length encoded sequence are encoded to form shorter output data words.

Figure 3 is a schematic diagram of a scene change detector comprising a shift register 100, an average calculator 110, a multiplier 120 and a comparator 130.

The vector residual values ME-dist, one per picture (frame), are received and stored into the shift register. So, the shift register may contain up to four consecutive frames' values of ME-dist. Stored values are clocked along the shift register 100 at each frame.

The average calculator 110 calculates an average value of the ME-dist values 6 stored in the shift register 100 according to rules outlined below.

The average value ME - dist - ave calculated by the average calculator is multiplied by a threshold value - e.g. 1.3 or 1.6 - at the multiplier 120. The weighted average value is then compared with the current ME - dist value. If the current value is greater than the product of ME-dist ave and the threshold value, then a scene change is detected and a scene change output signal (s.c.) is set. The actions taken in response to the s.c. signal will be described below with reference to Figure 4.

In other embodiments, a detection can also (or instead) be made as to whether the current ME-dist value is less than the rolling average by a certain threshold amount - perhaps 30 or 60% less, This too could be used to detect a scene change.

In further embodiments, the difference necessary to indicate a scene change could be absolute, rather than being a proportion of the average value as in Figure 3.

The rules for the calculation of the rolling average of ME-dist will now be described. It will be seen that the current ME-dist value is compared with an average of up to four preceding values. To achieve this, the average calculator latches its output value at each calculation so that the comparator 130 makes a comparison with the previous ME-dist-ave value. A simple alternative would be for the shift register to have an extra input cell, so that an ME-dist value does not take part in the calculation by the average calculator until the next frame after it has been received.

7 The following nomenclature will be used:

Frame ME-dist n-4 N-4 n-3 N-3 n-2 N-2 n-1 N-1 n N n+1 N+1 n+2 N+2 n+3 N+3 n+4 N+4 So, the ME-dist value corresponding to frame number (n+2) is represented by the symbol "N+2".

In a first embodiment, the average calculator calculates a simple arithmetic mean of the previous four values of ME-dist:

Frame ME-dist-ave n-4 (N-5 + N-6 + N-7 + N-8)/4 n-3 (N-4 + N-5 + N-6 + N-7)/4 n-2 (N-3 + N-4 + N-5 + N-6)/4 n- 1 (N-2 + N-3 + N-4 + N-5)14 n (NI + N-2 + N-3 + N-4)/4 n+1 (N + N-1 + N-2 + N-3)/4 n+2 (N+1 + N + N-1 + N-2)/4 n+3 (N+2 + N+1 + N + N-1)/4 n+4 (N+3 + N+2 + N+1 + N)/4 So, for example, at frame n-3, the ME-dist value N-3 is compared by the comparator 130 to the ME-dist-ave value (N-4 + N-5 + N-6 + N-7)/4, as weighted by the multiplier 120.

In an enhancement of this embodiment, when a scene change is detected, the 8 scene change output signal also forms a reset signal supplied to the shift register 100, the average calculator 110 or both. The effect of the reset signal is to cause a temporary change in the set of rules used to calculate ME-dist-ave. In particular, values of ME-dist before the scene change are no longer employed in the average calculations, as their values are not likely to be comparable to ME-dist values after the scene change. Accordingly, a replacement set of rules for the average calculation is as follows, where it is assumed that a scene change is detected when ME-dist for frame N is compared to ME-dist-ave:

Frame NW-dist-ave n-4 (N-5 + N-6 + N-7 + N-8)/4 n-3 (N-4 + N-5 + N-6 + N-7)/4 n-2 (N-3 + N-4 + N-5 + N-6)/4 n- I (N-2 + N-3 + N-4 + N-5)/4 n (N-1 + N-2 + N-3 + N-4)/4 n+1 no valid ME-dist values available n+2 (N+l) n+3 (N+2 + N+1)/2 n+4 (N+3 + N+2 + N+1)/3 n+5 (N+4 + N+3 + N+2 + N+1)14 n+6 etc So, in this enhancement, as frame n + 1 is the first frame of a new scene, there are no valid values of ME-dist relating to the current scene from which an average can be calculated. Thereafter, ME-dist values for frames since the scene change may be validly used, until at frame n+5 the calculation is back to the normal average of four preceding values.

In a further enhancement, it is recognised that a division by 3 is not straightforward to implement in digital hardware, so the value ME-distave for the fourth frame after a scene change (in the above example, frame n+4) could instead be calculated as follows:

ME-dist-ave - (N+3 + 2(N+2) + N+1)/4 9 Figure 4 is a schematic diagram of a bit allocator. The bit allocator receives the scene change (s.c.) signal generated by the detector of Figure 3. The s.c. signal is supplied in parallel to a one-frarne delay 230 and to an OR-gate 240. The output of the OR-gate 240 controls a switch 220 which switches between two predetermined data quantity values 200,21, these being one frame's portion of 49.5 Mbits/second and 47.5 Mbits/second respectively (i.e. 1/30 or 1/25 of these two data rates are allocated to one frame, depending on whether the frame rate is 30 or 25 fps). The data quantity value selected by the switch 220 is supplied as a target data quantity to the quantiser 80 for use in adaptively encoding the current frame.

If a scene change is indicated by the s.c. signal in either the current frame or - by virtue of the frame delay 230 - the preceding frame, the output of the OR-gate is high and the switch is controlled to select the lower of the two data quantity amounts 210, i.e. the data quantity derived from 47.5 Mbits/second. On the other hand, if a scene change was not detected for either the current or the preceding frame, then the higher amount 200 (the data quantity derived from 49.5 Mbits/second) is selected by the switch 220.

The figures of 47.5 Wits and 49.5 Wits are chosen here in the context of a 50 Wits per second system.

So, when a scene change has been detected, it is considered more likely that the adaptive encoder 80 will overrun its target data quantity as the encoding parameters can no longer be approximated from those used in preceding frames. A larger margin between the target supplied to the quantiser and the actual available data rate is therefore employed.

The lower data rate is preferably maintained for two frames to allow the adaptive encoder to assess appropriate parameters for the new scene and also because in the frame after a scene change there is no longer a valid average of previous ME-dist figures with which the current ME - dist value can be compared. So, as scene changes cannot be detected reliably in consecutive frames by the apparatus of Figure 3, it is better to maintain the lower data rate for two frames after a scene change detection.

Claims (15)

  1. I Video data compression apparatus in which motion vectors are generated to represent image motion between pairs of successive images of an input video signal, the apparatus comprising:
    a motion vector generator for performing block matching by detecting block differences between a block of one image and a plurality of blocks of an adjacent image, and for generating a motion vector from the pair of blocks having the best match; means for combining the block differences across blocks of each image; and a scene change detector operable to detect scene change in the input video signal by detecting changes in the combined block difference values from image to image.
  2. 2. Apparatus according to claim 1, in which the scene change detector is operable to compare the combined block difference value for a current image with an average block difference value generated from a group of preceding images.
  3. 3. Apparatus according to claim 2, in which, after a detected scene change, the scene change detector is operable to generate the average block difference value without using block differences derived from images before scene change.
  4. 4. Apparatus according to any one of claims I to 3, comprising an adaptive compression encoder operable to compress each image in accordance with a target output data quantity for that image, the encoder being operable to vary data compression parameters during compression of an image in order to substantially achieve the target output data quantity.
  5. 5. Apparatus according to claim 4, comprising means for setting the target output data quantity to a first quantity when a scene change has not been detected and to a second, lower, quantity for an image in which a scene change is detected.
  6. 6. Apparatus according to claim 5, in which the setting means is operable to maintain the target output data quantity at the second value for one or more images after an image containing a scene change.
  7. 7. Video data compression apparatus comprising:
    an adaptive compression encoder operable to compress each image of an input video signal in accordance with a target output data quantity for that image, the encoder being operable to vary data compression parameters during compression of an image in order to substantially achieve the target output data quantity; means for detecting scene changes in the input video signal; and means for setting the target output data quantity to a first quantity when a scene change has not been detected and to a second, lower, quantity for an image in which a scene change is detected.
  8. 8. A video data storage apparatus comprising apparatus according to any one of the preceding claims.
  9. 9. A method of video data compression apparatus in which motion vectors are generated to represent image motion between pairs of successive images of an input video signal, the method comprising the steps of:
    performing block matching by detecting block differences between a block of one image and a plurality of blocks of an adjacent image; generating a motion vector from the pair of blocks having the best match; combining the block differences across blocks of each image; and detecting scene changes in the input video signal by detecting changes in the combined block difference values from image to image.
  10. 10. A method of video data compression using an adaptive compression encoder operable to compress each image of an input video signal in accordance with a target output data quantity for that image, the encoder being operable to vary data compression parameters during compression of an image in order to substantially achieve the target output data quantity, the method comprising the steps of:
    12 detecting scene changes in the input video signal; and setting the target output data quantity to a first quantity when a scene change has not been detected and to a second, lower, quantity for an image in which a scene change is detected. 5
  11. 11. A computer program having program code means for per-forming a method according to claim 9 or claim 10.
  12. 12. A data carrier on which is stored a computer program according to claim 11. 10
  13. 13. Video data compression apparatus substantially as hereinbefore described with reference to the accompanying drawings.
  14. 14. A method of video data compression, the method being substantially as hereinbefore described with reference to the accompanying drawings.
  15. 15. A video data storage apparatus substantially as hereinbefore described with reference to the accompanying drawings.
GB9914476A 1999-06-21 1999-06-21 Video data compression with scene change detection Withdrawn GB2351406A (en)

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US9955191B2 (en) 2015-07-01 2018-04-24 At&T Intellectual Property I, L.P. Method and apparatus for managing bandwidth in providing communication services

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Publication number Priority date Publication date Assignee Title
GB2263602A (en) * 1992-01-24 1993-07-28 Sony Broadcast & Communication Motion compensated video signal processing
EP0624032A2 (en) * 1993-05-07 1994-11-09 Goldstar Co. Ltd. Video format conversion apparatus and method
EP0649256A2 (en) * 1993-10-19 1995-04-19 Canon Kabushiki Kaisha Motion compensation of a reproduced image signal
WO1997031485A1 (en) * 1996-02-26 1997-08-28 Sarnoff Corporation Method and apparatus for detecting scene cuts in a block-based video coding system
JPH1023329A (en) * 1996-07-01 1998-01-23 Nippon Hoso Kyokai <Nhk> Scene change and/or flash detector

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2263602A (en) * 1992-01-24 1993-07-28 Sony Broadcast & Communication Motion compensated video signal processing
EP0624032A2 (en) * 1993-05-07 1994-11-09 Goldstar Co. Ltd. Video format conversion apparatus and method
EP0649256A2 (en) * 1993-10-19 1995-04-19 Canon Kabushiki Kaisha Motion compensation of a reproduced image signal
WO1997031485A1 (en) * 1996-02-26 1997-08-28 Sarnoff Corporation Method and apparatus for detecting scene cuts in a block-based video coding system
JPH1023329A (en) * 1996-07-01 1998-01-23 Nippon Hoso Kyokai <Nhk> Scene change and/or flash detector

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US9955191B2 (en) 2015-07-01 2018-04-24 At&T Intellectual Property I, L.P. Method and apparatus for managing bandwidth in providing communication services

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