5,469,212
`(11) Patent Number:
`United States Patent
`Lee
`[45] Date of Patent:
`Nov. 21, 1995
`
`
`,19
`
`CTO AUTAT AA
`
`US005469212A
`
`[54] APPARATUS FOR
`COMPRESSION-ENCODING AND
`DECODING VIDEO SIGNALS
`
`[75]
`
`Inventor: Choon Lee, Seoul, Rep. of Korea
`
`[73] Assignee: Goldstar Co., Ltd., Seoul, Rep. of
`Korea
`
`L.
`No.:
`[21] Appl. No.: 43,370
`{22} Filed:
`Apr. 6, 1993
`£30]
`Foreign Application Priority Data
`Apr. 11, 1992
`[KR]
`Rep. of Korea wesc 6061/1992
`[SU]
`Tints Co ooecccccccscsssssssssseuesseessessesessennvann HOAN 7/48
`[52] U.S. Ch. wesc 348/392; 348/409; 348/412;
`348/424
`(58] Field of Search 0... 348/392, 409,
`348/412, 415, 419, 424; HO4N 7/137
`.
`References Cited
`U.S. PATENT DOCUMENTS
`~
`11/1989 Ninomiya 0... eeseseseceseseeeeee 348/392
`
`4,884,136
`
`[56]
`
`[57]
`
`ABSTRACT
`a,
`.
`.
`An apparatus for compression-encoding and decoding digi-
`tal video signals, capable of additionally transmitting a
`differential signal
`indicative of a difference between an
`original video signal and an encoded video signal obtained
`by encoding the original video signal by an existing video
`compression system, so as to efficiently cope with a varied
`bandwidth of a recording medium or a channel. The appa-
`ratus comprises an encoding device for encoding an original
`digital video in a sampling manner, encoding a differential
`signal indicative of a difference between eachoftheoriginal
`video signals and a signal obtained by expanding each
`corresponding one of the encoded original video signals,
`together with edge information, adding the encoded com-
`posite differential signal
`to the encoded original video
`signal, and outputting the resulting signal. The apparatus
`also comprises a decoding device for decoding each of the
`encoded original video signals,
`reading video signals,
`indicative of video portions including no differential signal,
`from video signals resulting from the decoding of the
`encoded original video signals, based on the edge informa-
`tion, adding each of the differential signals to each corre-
`sponding one of the read video signals, and outputting
`recovered video signals.
`
`Primary Examiner—Howard W. Britton
`Attorney, Agent, or Firn—Fiiesler, Dubb, Meyer & Lovejoy
`
`9 Claims, 6 Drawing Sheets
`
`CODED SIGNAL
`
`
`
`ORIGINAL
`
`
`DECIMATION
`UNIT
`ENCODER 1
`ENCODER 27)
`SIGNAL .
`
`
`10
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`EXPAND
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`VLC/
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`VIDEO SIGNAL Hy)
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`40
`
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`

`

`US. Patent
`
`Nov. 21, 1995
`
`Sheet 1 of 6
`
`5,469,212
`
`
`
`PRIOR ART
`
`FIG.—1
`
`COMPRESSION
`ENCODED
`VIDEO DATA
`FROM CHANNEL
`
`
`—) DECODER 1
`
`
`DECODED
`VIDEO
`DATA
`
`PRIOR ART
`
`PIG.—e2
`
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`

`

`U.S. Patent
`
`Novy. 21, 1995
`
`Sheet 2 of 6
`
`3,469,212
`
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`

`

`U.S. Patent
`
`Nov. 21, 1995
`
`Sheet 3 of 6
`
`5,469,212
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`
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`

`

`U.S. Patent
`
`Nov. 21, 1995
`
`Sheet 4 of 6
`
`5,469,212
`
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`U.S. Patent
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`Noy. 21, 1995
`
`Sheet 5 of 6
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`5,469,212
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`

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`US. Patent
`
`Nov. 21, 1995
`
`Sheet 6 of 6
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`5,469,212
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`

`

`2
`Onthe other hand, when a signal obtained by reproducing
`the signal transmitted from the video signal compression
`encoding device or recorded on the recording medium is
`received in the decoding device,it is decoded in the decoder
`4 and then expandedtothe size of the original videosignal,
`in the expand unit 5, so as to be recovered.
`The first encoding unit 1 utilizes high technical video
`The present invention relates to an apparatus for com-
`compression techniques, so as to transmit video data with a
`pression-encoding and decoding digital video signals, and
`proper size and a proper amount within a certain limited
`more particularly to an apparatus for compression-encoding
`bandwidth. For example, video compression techniques
`and decoding digital video signals, which intends to recover
`which are regarded as optimum onesfor respective channels
`fine information disappearing upon interpolating a digital
`are selected in various fields such as HDTV fields, digital
`VCR fields, and multimedia fields.
`video compressed by a decimation and to enableacircuit at
`the receiving side to be efficiently designed.
`Where a higher compressionratio is desired, in spite of a
`2. Description of the Prior Art
`degradation in picture quality, a device such as the decima-
`tion unit 1 is used. In this case, there is used a method for
`Generally, compression of digital video signals means
`reducing a video in size in the sub-sampling manner and
`that a great quantity of video data is efficiently converted
`then compressing it. When this method is used, fine infor-
`into a small quantity of video data to meet the recording
`mation of the video disappear during when the size of video
`bandwidth of a recording medium or a channel, although
`information is optionally reduced.
`there may be a degradation in picture quality of an original
`video.
`In cases of video data compression methods used in
`HDTV digital VCRs and digital broadcasting satellites
`(DBSs), the use of the decimation unit 1 is eliminated so that
`an Original video subjected to no sub-sampling is compres-
`sion encoded, in order to generate a video signal with a
`certain limited bandwidth. In this case, there are various
`problemsrelating to the data amount.
`Each of the above-mentioned video data compression
`techniques are designed for one selected channel or record-
`ing medium. If a new application requiring a larger band-
`width is made, an optimum video data compression tech-
`nique will be newly developed. However, such a video data
`compression technique newly developed may havenorela-
`tion with the known video data compression techniques. In
`this case, there is a problem about a compatibility to systems
`using the known compression techniques. For example,it is
`impossible to utilize the new compression technique in
`systems using the known compression techniques, since a
`new decoding technique should be used, in order to decode
`a video signal encoded according to the new compression
`technique. Furthermore, a new system to which a new
`decoding technique is applied can not decode video data
`encoded according to the known compression techniques.
`Due to such a problem about compatibility, the manufac-
`turers need a long time for developing new systems. On the
`other hand, the consumers bear expenses for purchasing the
`systems. As a result, there is also a problem of requiring a
`long system replacementtime for enabling a commercial use
`of more efficient video processing systems meeting new and
`larger bandwidths.
`Until an optimum video compression technique is devel-
`oped,
`it
`is necessary to provide compression encoding/
`decoding devices capable of being applied to new band-
`widths and yet minimizing an alternation of existing
`systems, even though they exhibit a slight degradation in
`picture quality.
`
`BACKGROUNDOF THE INVENTION
`
`1. Field of the Invention
`
`As conventional video compression techniques, various
`coding methods have been known, which include a trans-
`form coding method, a predictive coding method, a hybrid
`coding method, a vector quantization method, and a sub-
`band coding method. Recently, such video compression
`techniques have been making fast progress. In particular,as
`they have been applicable to technical fields such as ISDNs
`and multimedia, international standardsrelating to the video
`compression have been continuously reported.
`For example, the international standard so called a mov-
`ing picture experts group (MPEG)for recording a com-
`pressed video on a digital recording medium such as a
`CD-ROMis currently extending.
`In accordance with this MPEG standard,it is possible to
`achieve an encoding at a high compression ratio, using
`methods such as a discrete cosine transform (DCT), a
`motion estimation (ME) and a variable length coding
`(VLC).
`Referring to FIG. 1, there is illustrated a conventional
`device for compression-encoding video signals. As shown in
`FIG. 1, the compression encoding device comprises a deci-
`mation unit 1 for reducing the amountof data for an original
`video in a sub-sampling manner,a first encoding unit 2 for
`encoding output data from the decimation unit 1 to compress
`it, and a VLC/formatter 3 for compression-formatting an
`encoded signal from the first encoding unit 2 in a VLC
`manner.
`
`Ontheother hand, FIG.2 illustrates a conventional device
`for decoding compressed video signals. This decoding
`device comprises a decoder 4 for receiving compression-
`encoded video data and decodingit, and an expand unit 5 for
`expanding video data decoded in the decoder 1.
`Operations of these devices for compression-encoding
`video signals and decoding them will now be described.
`As an original digital video signal
`is received in the
`compression encoding device, the amount of data thereofis
`reduced in the decimation unit 1, in a sub-banding coding
`manner. Thereafter, the video data reduced in data amountis
`encoded in the first encoding unit 2 to be compressed. The
`compression-encoded signal is then formatted in the VLC/
`formatter 3, in a variable length coding manner, so as to be
`compressed at a higher rate. This compressed video datais
`recorded on a recording medium or transmitted via a trans-
`mitting medium.
`
`5,469,212
`
`1
`APPARATUS FOR
`COMPRESSION-ENCODING AND
`DECODING VIDEO SIGNALS
`
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`SUMMARY OF THE INVENTION
`
`Therefore, an object of the invention is to provide an
`apparatus for compression-encoding and decoding digital
`video signals, which is capable of additionally transmitting
`a differential signal indicative of a difference between an
`original video signal and an encoded video signal obtained
`by encoding the original video signal by an existing video
`compression system, so as to efficiently cope with a varied
`
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`5,469,212
`
`3
`bandwidth of a recording medium or a channel, and of
`having a compatibility with existing systems and yet mini-
`mizing alternations in existing devices for compression-
`encoding and decoding video signals, so as to enable a
`commercial use thereof in a short time.
`
`In accordance with the present invention, this object can
`be accomplished by providing an apparatus for compres-
`sion-encoding and decoding digital video signals compris-
`ing: encoding meansfor encoding an original digital video
`in a sampling manner to produce encoded original video
`signals, encoding a differential signal indicative of a differ-
`ence between each ofsaid original video signals and a signal
`obtained by expanding each corresponding one of said
`encodedoriginal video signals, together with edge informa-
`tion, to produce an encoded composite differential signal,
`adding said encoded composite differential signal to the
`encodedoriginal vidco signal, and outputting the resulting
`signal; and decoding means for decoding each of the
`encodedoriginal video signals and each corresponding one
`of the differential signals encoded in said encoding means
`and then received via a transmitting medium or played back
`from a recording medium, reading video signals, indicative
`of video portions including nodifferential signal, from video
`signals resulting from said decoding of the encodedoriginal
`video signals, based on the edge information, adding each of
`the differential signals to each corresponding one ofsaid
`read video signals, and outputting recovered video signals.
`In accordance with the present invention, the encoding
`means comprises a decimation unit for reducing the size of
`each ofsaid original digital video signals in a sub-sampling
`manner, a first encoder unit for encoding each of signals
`subsampled by said decimation unit to video information
`properto a limited bandwidth of said recording medium or
`said transmitting medium, an expand unit for expanding a
`signal from said first encoder unit up to the size of each
`original video signal, in reversely proportional to the reduc-
`tion achieved by the decimation unit, a delay unit for
`delaying each original video signal for a predetermined time
`during which the original video signal is processed through
`the decimation unit, the first encoder unit and the expand
`unit, an adderunit for calculating a difference between each
`original video signal delayed by said delay unit and each
`corresponding expanded signal from the expand unit, a
`second encoder unit for encoding a differential signal from
`said adder unit, and a VLC/formatter unit for encoding a
`signal from said second encoder unit in a variable length
`coding and formatting it to send the resulting signal to a
`channel of the transmitting medium or
`the recording
`medium.
`
`The decoding means comprises a demultiplexor unit for
`receiving each of said encoded video signals from said
`encoding means via said transmitting medium or said
`recording medium, demultiplexing it to separate each cor-
`responding composite differential signal therefrom and out-
`putting them,a first decoderunit for decoding said separated
`encoded video signal from said demultiplexor unit and
`outputting it, an expand unit for expanding a decodedsignal
`from said first decoder unit and outputting it, a frame
`memory unit for storing an output signal from said expand
`unit, a delay unit for delaying said separated encoded
`composite differential signal from the demultiplexor unit for
`a predetermined delay time, a second decoder unit for
`decoding the differential signal delayed by said delay unit
`and generating an address for reading a video signal indica-
`tive of each of portions of the video stored i n said frame
`memory unit, other than white video portions, and an adder
`unit for adding said differential signal decoded by said
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`second decoderunit to said read video signal outputted from
`the frame memory unit according to said address of the
`second decoder unit and outputting a decoded video signal.
`The second encoder unit comprises an edge detector for
`detecting information about edges of each of said original
`video signals, an edge information memory for storing said
`edge information detected by said edge detector, a Quadtree
`encoder for generating a control signal for controlling an
`encoding for each of said differential signals from said adder
`unit and encoding the edge information in a Quadtree
`manner, a differential signal encoder for receiving the dif-
`ferential signal from the adder unit via a switch and encod-
`ing it, a formatter for multiplexing the edge information
`encoded by said Quadtree encoderand the differential signal
`encodedby said differential signal encoder and outputting an
`encoded composite differential signal, and a threshold cal-
`culator for receiving a bit count value obtained by counting
`the numberof bits of said encoded differential signal from
`the differential signal encoder, varying a threshold for an
`edge detection, in reversely proportional to said bit count
`value and sending said varied threshold to the edge detector.
`The second decoder unit comprises a demultiplexor for
`separating said edge information and said differential signal
`from each of said composite differential signals including
`the edge information, a Quadtree decoder for decoding said
`Quadtree-coded edge information from said demultiplexor,
`an address generator for generating an address for reading a
`video signal indicative of each of portions of the video
`stored in said frame memory unit, other than white video
`portions, based on said edge information decoded in said
`Quadtree decoder, a switch for permitting outputting of said
`DPCM-codeddifferential signal from demultiplexor under a
`control of the Quadtree decoder, and a differential signal
`decoder
`for
`recovering the differential signal
`received
`through the switch.
`according to the
`The encoding/decoding apparatus
`present
`invention can be applied to all
`types of video
`encoding/decoding system, differently from conventional
`devices. The encoding/decoding apparatus according to the
`present invention require only a simple hardware and enable
`its use with existing encoding/decoding devicesin a parallel
`manner. Accordingly,it is applicable to most technical fields
`utilizing the video compression. For example, the apparatus
`can be applied to the MPEG standard having a bit rate of
`1.15 Mbps, at a bandwidth increase of about 5 nbps. The
`present
`invention also makes it possible to apply video
`encoding/decoding devices of the NTSC grade to the band-
`width of the HDTV grade. This provides an effect of
`maintaining the compatibility with the lower-grade format.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`Other objects and aspects of the invention will become
`apparent from the following description of embodiments
`with reference to the accompanying drawings in which:
`FIG. 1 is a block diagram of a conventional device for
`compression-encoding video signals;
`FIG. 2 is a block diagram of a conventional device for
`decoding compressed video signals;
`FIG. 3 is a block diagram of a device for compression-
`encoding video signals according to the present invention;
`FIG. 4 is a block diagram of a device for decoding
`compressed video signals in accordance with the present
`invention;
`FIG. 5 is a block diagram of a second encoder unit
`included in the device shown in FIG.3;
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`5,469,212
`
`5
`FIG. 6 is a block diagram of a second decoder unit
`included in the device shown in FIG. 4; and
`FIG. 7 is a schematic view explaining a Quadtree encod-
`ing in accordance with the present invention.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`FIG. 3 is a block diagram of a device for compression-
`encoding video signals according to the present invention.
`As shown in FIG. 3, the compression encoding device
`comprises a decimation unit 10 for reducing the size of an
`original digital video signal in a sub-sampling manner,a first
`encoder unit 20 for encoding a signal sub-sampled by the
`decimation unit 10 to video information proper to a limited
`bandwidth of a recording medium or a transmitting medium,
`and an expand unit 30 for expanding a signal from thefirst
`encoder unit 20 up to the size of the original video signal, in
`reversely proportional
`to the reduction achieved by the
`decimation unit 10, A delay unit 40 is also provided, which
`is adapted to delay the original digital video signal for a
`predetermined time during which the original video signal is
`processed through the decimation unit 10, the first encoder
`unit 20.and the expand unit 30. The compression encoding
`device also comprises an adder unit 50 for calculating a
`difference between an expanded signal from the expand unit
`30 and the original video signal delayed by the delay unit 40,
`a second encoder unit 60 for encoding a signal from the
`adder unit 50 indicative of the difference, namely, a differ-
`ential signal, and a VLC/formatter unit 70 for encoding a
`signal from the second encoder unit 60 in a variable length
`coding and formatting it to send the resulting signal to a
`channel of a transmitting medium or a recording medium.
`As shownin FIG.5, the second encoder unit 60 comprises
`an edge detector 61 for detecting information about edges of
`the original video signal and an edge information memory
`62 for storing the edge information,indicative of the number
`of edges and each edge position, detected by the edge
`detector 61. A Quadtree encoder 63 is also provided, which
`is adapted to generate a control signal for controlling an
`encoding for the differential signal from the adder unit 50
`and encoding the edge information in a Quadtree manner.
`The second encoder unit 60 also comprises a differential
`signal encoder 66 for receiving the differential signal from
`the adder unit 50 via a switch 65 according to the control
`signal when the numberof detected edgesis not less than a
`predetermined reference value and encoding the received
`differential signal, a formatter 67 for multiplexing the edge
`information encoded by the Quadtree encoder 63 and the
`differential signal encoded by the differential signal encoder
`66 and outputting an encoded composite differential signal,
`and a threshold calculator 64 for receiving a bit count value
`obtained by counting the number of bits of the encoded
`differential signal from the differential signal encoder 66,
`varying a threshold for an edge detection,
`in reversely
`proportional to the bit count value and sending the varied
`thresholdto the edge detector 61. In the illustrated case, the
`differential signal encoder 66 is a differential pulse coded
`modulation (DPCM) encoder and so will be referred to as
`the DPCM encoder.
`
`On the other hand, FIG. 4 is a block diagram of a device
`for decoding compressed video signals in accordance with
`the present invention.
`As shown in FIG. 4, the decoding device comprises a
`demultiplexor unit 110 for receiving encoded video signal
`from the channel of the encoding device of FIG. 3 and
`
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`demultiplexing it to output the video signal and the com-
`posite differential signal separated from each other,a first
`decoder unit 120 for decoding the encoded video signal from
`the demultiplexor unit 110 and outputting it, an expand unit
`130 for expanding a decoded signal from the first decoder
`unit 120 and outputting it, and a frame memory unit 160 for
`storing an output signal from the expand unit 130 therein. A
`delay unit 140 is also provided, which is adapted to delay the
`encoded composite differential signal from the demulti-
`plexor unit 110 for a predetermined delay time. The decod-
`ing device also comprises a second decoder unit 150 for
`decodingthe differential signal delayed by the delay unit 140
`and generating an address for reading a video signal indica-
`tive of a portion of the video stored in the frame memory unit
`160, other than white video portions, and an adder unit 170
`for adding the differential signal decoded by the second
`decoder unit 150 to the video signal outputted from the
`frame memory unit 160 according to the address of the
`second decoder unit 150 and outputting a decoded video
`signal.
`FIG.6 is a biock diagram of the second decoder unit 150
`shown in FIG. 4. As shown in FIG. 6, the second decoder
`unit 150 comprises a demultiplexor 151 for separating the
`edge information and the differential signal from the com-
`posite differential signal including the edge information, a
`Quadtree decoder 152 for decoding the Quadtree-coded
`edge information from the demultiplexor 151, and an
`address generator 153 for generating an address for reading
`a video signal indicative of a portion of the video stored in
`the frame memory unit 160, other than white video portions.
`A switch 154 is also provided, which permits outputting of
`the DPCH-codeddifferential signal from demultiplexor 151
`under a control of the Quadtree decoder 152. The second
`decoder unit 150 also comprisesa differential signal decoder
`155 for recovering the differential signal received through
`the switch 154.In the illustrated case, the differential signal
`decoder 155 is a DPCM decoder and so will be referred to
`as the DPCM decoder.
`
`Now, operations of the devices according to the present
`invention will be described, in conjunction with FIGS. 3 to
`7.
`
`When an original digital video signal is received in the
`encoding device shown in FIG, 3,
`it is subjected to a
`sub-sampling process by the decimation unit 10 so that its
`size is reduced. The size-reduced signal is compression-
`encodedin the first encoder unit 20 and then expanded in the
`expand unit 30. The resulting expanded signal is applied to
`the adder unit 50 which also receives the original video
`signal delayed one frame by the delay unit 40. In the adder
`unit 50, a differential signal
`indicative of the difference
`between the expanded signal and the delayed original video
`signal. The differential signal
`is encoded in the second
`encoder unit 60. The differential signal encoded by the
`second encoder unit 60 and the signal encoded by thefirst
`encoder unit 20 are compression-formatted in a variable
`length coding manner, in the VLC/formatter unit 70 from
`which the resulting signal is sent to a channel of a trans-
`mitting medium or a recording medium.
`The operation of the second encoder unit 60 will now be
`described in detail, in conjunction with FIG. 5. The edge
`detector 61 finds edge information aboutthe original video,
`so as to encodefine information ofthe original video. As the
`edge detector 61, a sobel edge detector is used, which has an
`advantageous robustness for noise. The edge detector 61
`detects edges of the original video, based on a threshold
`received from the threshold calculator 64 and outputs posi-
`tion information therefor. This position information about
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`5,469,212
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`7
`the detected edges outputted from the edge detector 61 is
`stored in the edge information memory 62. The edge posi-
`tion information stored in the edge information memory 62
`is sent to the Quadtree encoder 63 in which it is encoded by
`video portions.
`That is, the Quadtree encoder 63 partitions continuously
`the video into four blocksuntil the videois partitioned into
`blocks of the minimum size. The partition operation of the
`Quadiree encoder 63 is repeated according to the edge
`information about the video. The condition for partitioning
`the video into four blocks will now be described.
`
`When the number of edge information in an optional
`block from the same-sized blocks obtained by partitioning
`the videois less than a first predetermined threshold t-black,
`it is determined that the blockhasnofine information.In this
`case, the switch 65 is switched to its OFF state and prevents
`the differential signal from passing therethrough. When the
`number of edge information in the block is more than a
`second predetermined threshold t-white,it is determined that
`the block has sufficient fine information. In this case, the
`switch 65 is switched to its ON state so that all pixels for the
`block is encoded in a DPCM manner, by the DPCM encoder
`66.
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`8
`the resulting histogram with Laplacian
`lated. Thereafter,
`distribution is subjected to a quantization, so as to obtain
`quantized levels. These quantized levels are stored to form
`a table which includes the quantized levels classified into a
`plurality of groups. The t, able is used upon the DPCN
`encoding operation.
`signal
`information
`position
`edge
`Thereafter,
`the
`Quadtree-encoded in the Quadtree encoder 63 and the
`differential signal DPCM-encoded in the DPCM encoder 66
`are multiplexed by the formatter 67 in the following manner.
`For example, assuming that
`‘00’,
`‘01’ and ‘10’ are
`Quadtree codes for a black block B, a white block W and a
`gray block G, respectively, a video shown in FIG. 7 is
`scanned from its left uppermost end, in clockwise. By the
`scanning, the black portion B, and the gray portion G arc
`encoded to ‘00’ and ‘10’, respectively. The gray portion G is
`also partitioned into four blocks whichare, in turn, scanned
`from the left uppermost end, in clockwise. The white portion
`W is encoded to ‘01’. A differential signal of each pixel in
`the white portion W is DPCM-encoded. The white portion
`W is a block determinedto have the numberof edgesnotless
`than a predetermined threshold, that is, a large amount of
`fine information.
`In this case, the DPCM codes are assumedto be of four
`On the other hand, when the numberof edge information
`bits. Also, the Quadtree-encoding direction is assumed to be
`is not less than the first predetermined threshold t-black, but
`clockwise. That is, the encoding is carried out in a recursive
`not more than the second predetermined threshold t-white,it
`manner, according to the order of the left upper block, the
`is determined that the block has moreorless edge informa-
`right upper block, the right lower block, and the left lower
`tion. In this case, the block is partitioned into four blocks,
`block.
`each of which will be repeatedly subjected to the above
`At this time, the formatter 67 counts the numberofbits of
`process.
`the encoded differential signal from the DPCM encoder 66
`The two thresholds t-black and t-white needed in the
`and then send a bit count signal to the threshold calculator
`Quadtree encoder 63 are defined in the following manner.
`64 which, in tum,varies the threshold for the edge detection,
`in reversely proportional to the bit count value.
`Assuming that one video block has a longitudinal length
`This operation will now be described in detail.
`X anda lateral length Y and the number of Quadtree levels
`is four,
`The amount ofbits encoded by the second encoderunit 60
`of FIG. 5 is not varied, depending on the kind of frame(that
`the size of the first Quadtrce level is X * Y,
`is, an intraframe coding or an interframe coding). It is fixed
`the size of the second Quadtree level is (X/2) * (Y/2),
`for frames to meetatarget rate.
`the size of the third Quadtree level is (X/4) * (Y/4), and
`For example, when the target rate and the framerate are
`and
`40
`a bits/sec and 30 Hz, respectively,
`the number ofbits
`allocated in each frameis a/30 bits. This bit number can be
`controlled, using the edge detector 61 of FIG. 5.
`That is, as the amount of edge information is control led
`by varying the threshold for the edge detection, the number
`of allocated bits. However, this methodis difficult to meet
`the numberof allocated bits precisely.
`Where the numberofbits of a differential signal encoded
`in the above-mentioned manneris less than the number of
`allocatedbits,it is stuffed with dummy bits corresponding to
`the shortageof bits. On the other hand, when the number of
`bits of the differential signal is less than the number of
`allocated bits, the surplus bits are cutoff.
`The threshold calculator 64 calculates thresholds for next
`frame, based on the bit count value obtained by continuously
`counting the number of encoded bits in the present frame.
`Such a calculation can be expressed by the following
`equations:
`
`the size of the fourth Quadtree level is (X/8) * (¥/8). That
`is, each lower level size is reduced in the number of
`pixels at a rate of 14, as compared with the just higher
`level size. Accordingly,
`it is required to adjust
`the
`thresholds so that they meet the reduction rate.
`For example, when the thresholds t-black and t-white of
`the first Quadtree level are t, and t,, respectively,
`the
`thresholds at the second Quadtree level becomet,/4 and t,/4,
`the thresholds at the second Quadtree level becomet,/16 and
`t/16, and the thresholds at
`the second Quadtree level
`become t,/64 and t,/64.
`When the Quadtree encoder 63 determines that a suffi-
`cient amount of white portions, namely, fine information are
`presentin a block of a certain level, it sends a control signal
`to the switch 65,so as to switch the switch 65to its ON state.
`Accordingly,
`the DPCM encoder 66 encodes respective
`differential signals from all pixels in the blocks determined
`to havea sufficient amountoffine information, in the DPCM
`coding manner.
`As the DPCM coding method, a nonlinear quantization
`method is used, in accordance with the present invention.
`The nonlinear quantization utilizes a test video, to obtain a
`histogram for a differential signal indicative of a difference
`between an original video portion corresponding to the
`white portion ofthe test video and a video portion obtained
`by encoding the original video portion and then recovering
`it. From the obtained histogram, a variance is also calcu-
`
`45