Video encoder and video encoding method

A video encoder for reducing the deterioration of picture quality of a compress coded video signal by implementing a method for deciding the number of picture elements needed to properly encode an inputted video signal. The decision is calculated based on the difficulty of compressing the inputted video signal and the number of picture elements needed to accurately reproduce a compressed inputted video signal. The decision circuitry decides the number of picture elements needed for compression of the inputted video signal allowing the number of codes generated in the encoding process to be decreased and quantization accuracy improved. The main advantage is a reduction in deterioration of the video signal usually found when reproducing a compress coded video signal.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a video encoder and video encoding method
 and more specifically, is preferably applied to a video signal encoder,
 for instance, used for a digital broadcasting system.
 2. Description of the Related Art
 Various kinds of compress-coding methods have been proposed for decreasing
 video and audio information. A method called moving picture experts group
 phase 2 (MPEG2) has been introduced as a representative one. A digital
 broadcasting system, which compress-encodes video and audio broadcasting
 data with employing the MPEG2 method and broadcasts the resultant via a
 ground wave and satellite wave, has been started.
 FIG. 1 generally shows a video signal encoder 50. A video signal D1 is
 inputted from a video signal supply device (not shown) such as a video
 tape recorder or the like, to a prefilter 51. The video signal D1 is
 subjected to a band restriction process corresponding to a frequency
 characteristic control signal D55 supplied from a quantization rate
 control section 55 in the prefilter 51. Thereby, the high frequency
 component of the video signal is reduced and the video signal is outputted
 to a picture element number converting section 52 as a band restricted
 video signal D51. Note that, the high frequency component of the video
 signal represents relatively minute parts of an image. The high frequency
 component is reduced to the degree that the minute section of the image is
 omitted, and the bandwidth of the video signal is reduced, while only a
 small influence is applied to the entire image.
 The picture element number converting section 52 executes a picture element
 number converting process on the inputted band restricted video signal
 D51. Assuming that the number of horizontal picture elements of the video
 signal D1 inputted to the encoder 50 is the number M of horizontal picture
 elements, the picture element number converting section 52 reduces the
 number of horizontal picture elements of the band restricted video signal
 D51 obtained by restricting the band of the video signal D1 to the number
 N of reduced horizontal picture elements with a relation of N&lt;M. The
 resultant signal is transmitted to a encoding section 53 as a picture
 element number converted video signal D52. The number N of the reduced
 horizontal picture elements is set to a large value in a program which
 requires a high picture quality: and to a small value in a program which
 does not require a high picture quality, based on the contents of the
 program in the video signal D1.
 The encoding section 53 executes on the picture element number converted
 video signal D52, a movement compensating process, a discrete cosine
 transform (DCT) process, a quantization process and a variable length
 coding (VLC) process, in order to transmit the resultant to a buffer 54 as
 variable length coded data D53. At this time, the encoding section 53
 adjusts a quantization rate in the quantization process based on a
 quantization control signal D56 supplied from a quantization rate control
 section 55. Further, encoding section control information D57 is supplied
 from a encoding section control circuit 57 to the encoding section 53. The
 encoding section 53 sets a coding timing and a movement vector search
 range in the movement compensating process according to the encoding
 section control information D57.
 When the movement of the video image in the video signal D1 is large, or
 the video image is complicated, the generated code of the variable length
 coded data D53 obtained by compress-coding the video signal D1 is
 increased. The video signal having the large number of generated codes by
 such a compress-coding is referred to as a video signal with a high degree
 of difficulty in compression. The degree of difficulty in compression
 varies based on the contents or scenes of the program in the video signal.
 Therefore, the quantization value of the quantization process in the
 encoding section 53 and the band restriction in the prefilter 51 are
 controlled based on the variable length coded data D53 occupied in the
 buffer 54. Thus, the number of generated codes for predetermined term, for
 example, each group of pictures (GOP) is controlled so as to be kept
 constant.
 In other words, the quantization rate control section 55 constantly
 monitors the accumulated state of the variable length coded data D53 in
 the buffer 54 in order to obtain an accumulated state as occupation rate
 information D54. Then, the quantization rate control section 55 generates
 the quantization control signal D56 and the frequency characteristic
 control signal D55 based on the occupation rate information D54 in order
 to supply them respectively to the encoding section 53 and the prefilter
 51. Thereby, the section 55 controls the number of generated codes of the
 variable length coded data D53 constant for each predetermined period.
 In the above video signal encoder 50, the bandwidth reduced by the
 prefilter 51 varies according to the degree of difficulty in compressing
 the video signal D1. On the other hand, the number N of horizontal picture
 elements reduced by the picture element number converting section 52 is
 constant according to the contents of the program in the video signal D1.
 Therefore, the number N of horizontal picture elements of the picture
 element number reduced video signal D52 happens to exceed the number of
 horizontal picture elements necessary for representing the video image of
 the band restricted video signal D51. In this case, the number of picture
 elements exceeding the requirement for representing the video image of the
 band restricted video signal D51 are to be encoded. Accordingly, the
 quantization value in the quantization process is unnecessarily increased
 and disadvantageously results in the deterioration of the picture quality.
 SUMMARY OF THE INVENTION
 In view of the foregoing, an object of this invention is to provide a video
 encoder and video encoding method for performing band restriction process
 and picture element number conversion process corresponding to the degree
 of difficulty in compressing an image.
 The foregoing object and other objects of the invention have been achieved
 by the provision of a video encoder having a signal band reducing means
 for reducing the signal band of a video signal and a picture element
 number converting means for converting the number of picture elements of
 the video signal, so that the number of picture elements can be converted
 in the picture element number converting means so as to be adapted to the
 number of picture elements corresponding to the signal band reduced by the
 signal band reducing means.
 A video encoder for compress-coding and transmitting an inputted video
 signal, comprises signal band reducing means for reducing the signal band
 of said inputted video signal; picture element number converting means for
 converting the number of picture elements of said inputted video signal
 based on said reduced signal band; and coding means for compress-coding
 said inputted video signal, the number of picture elements of which is
 converted by said picture element number converting means.
 A video encoding method of compress-coding and transmitting an inputted
 video signal, comprises the steps of reducing the signal band of said
 inputted video signal; converting the number of picture elements of said
 inputted video signal based on said reduced signal band; and
 compress-coding said inputted video signal, the number of picture elements
 of which is converted by said picture element number converting step.
 The nature, principle and utility of the invention will become more
 apparent from the following detailed description when read in conjunction
 with the accompanying drawings in which like sections are designated by
 like reference numerals or characters.

DETAILED DESCRIPTION OF THE EMBODIMENT
 Preferred embodiments of this invention will be described with reference to
 the accompanying drawings:
 FIG. 2 generally shows a video signal encoder 10. A video signal D1 of an
 high definition television (HDTV) standard is inputted from a video signal
 supply device (not shown) such as a video tape recorder, to a prefilter
 14.
 The prefilter 14 is a horizontal frequency low-pass filter. The prefilter
 14 performs a band restriction process for reducing the high frequency
 component of the video signal D1 according to a frequency characteristic
 control signal D18 supplied from a quantization rate control section 18.
 Thereby, the bandwidth of the image signal is reduced, however, to the
 degree that the entire video image is kept unchanged, by omitting minute
 parts of a video image, so that the resultant band restricted video signal
 D2 is outputted to a picture element number converting section 15.
 The picture element number converting section 15 reduces the number of
 horizontal picture elements of the inputted band restricted video signal
 D2 based on a picture element number control signal D17 supplied from a
 picture element number conversion deciding section 17, and outputs the
 resultant as a picture element number converted video signal D3 to a
 encoding section 20. For example, assuming that the number of horizontal
 picture elements of the video signal D1 inputted to the encoder 10 is
 1920, the picture element number converting section 15 reduces the number
 of horizontal picture elements of the band restricted video signal D2,
 which is obtained by restricting the band of the video signal D1, to the
 number N of reduced horizontal picture elements with a relation of N&lt;1920.
 Then, the section 15 transmits the resultant to the encoding section 20 as
 a picture element number converted video signal D3.
 In this case, the picture element number conversion deciding section 17
 generates the picture element number control signal D17 based on the
 frequency characteristic control signal D18 transmitted from the
 quantization rate control section 18. Initially, the picture element
 number conversion deciding section 17 obtains the number N' of required
 horizontal picture elements, which is necessary for producing the image of
 the band restricted video signal D2, based on the bandwidth, which is
 indicated by the frequency characteristic control signal D18, acquired by
 the high frequency component reducing process. Assuming that the bandwidth
 is FHz and the horizontal scanning time of the band restricted video
 signal D2 is Th seconds, N' picture elements is expressed by the following
 equation.
EQU N'=2F.multidot.Th
 Subsequently, the picture element number conversion deciding section 17
 selects the minimum value of set value Ns of horizontal picture elements,
 which satisfies the condition of N'&lt;Ns from a plurality of set value Ns of
 horizontal picture element which are previously set. The selected minimum
 set value is determined as the number N of reduced horizontal picture
 elements. In this embodiment, the following four kinds of 1440 pixels,
 1280 pixels, 1152 pixels and 960 pixels are previously set as set values
 Ns of horizontal picture elements. Then, the picture element number
 conversion deciding section 17 transmits an identification number
 corresponding to each set values of horizontal picture elements as the
 picture element number control signal D17, to the picture element number
 converting section 15 and a encoding section control circuit 16. Assuming
 that the number of set values of horizontal picture elements is j, the
 picture element number control signal D17 is expressed by a signal of k
 bits (j.ltoreq.2.sup.k).
 Thus, the band restricted video signal D2, the number of picture elements
 of which is reduced to the number N of reduced horizontal picture elements
 sufficient to express the band restricted video signal D2 in the picture
 element number converting section 15, is transmitted to the encoding
 section 20 as the picture element number converted video signal D3.
 The encoding section control circuit 16 generates a encoding section
 control signal D16 based on the picture element number control signal D17
 and supplies the signal S16 to the encoding section 20. The encoding
 section 20 executes encoding process corresponding to the number of the
 horizontal picture elements of the picture element number converted video
 signal D3, based on the encoding section control signal D16. More
 specifically, the picture elements of the band restricted video signal D2
 having the number M of horizontal picture elements shown in FIG. 3A is
 converted in the picture element number converting section 15, and the
 resultant is transmitted to the encoding section 20 as a picture element
 number converted video signal D3 having the number N of horizontal picture
 elements shown in FIG. 3B. Note that, the picture element number converted
 video signal D3 is not continuously transmitted. As shown in FIG. 3B,
 after a number N of picture elements are continuously transmitted, the
 transmission of picture elements is stopped for the period of the number
 (M-N) of picture elements. That is, the encoding section 20 performs an
 encoding operation during the period of number N of picture elements based
 on the encoding section control signal D16, and stops the coding operation
 during a period of the number (M-N) of picture elements. Thereby, keeping
 the operating clock constant, the encoding section 20 executes the
 encoding process corresponding to the number of horizontal picture
 elements of the picture element number converted video signal D3. Further,
 the encoding section 20 sets a movement vector search range in a movement
 compensating process based on the encoding section control signal D16.
 In the encoding section 20 (shown in FIG. 2), the picture element number
 converted video signal D3 is inputted to a preprocess section 21. The
 preprocess section 21 classifies each frame picture of the
 sequentially-inputted picture element number converted video signal D3,
 into three picture types: an I-picture, a P-picture and a B-picture, and
 selects a suitable process for each frame based on the picture type which
 the frame classified into. Then, the preprocessing section 21 rearranges
 the frame pictures in the order of encoding based on the classified
 picture type. Further, the preprocess section 21 divides the frame
 pictures into macro blocks comprising luminance signals of 16 picture
 elements.times.16 lines and color difference signals corresponding to the
 luminance signals. Thereby, the section 21 generates macro block data D4
 in order to supply to an arithmetic circuit 22 and a movement vector
 detecting section 31.
 The movement vector detecting section 31 calculates the movement vector of
 each macro block of the macro block data D4, based on the macro block data
 D4 and reference picture data D28 stored in a frame memory 29. Then, the
 section 31 transmits the resultant as movement vector data D31 to a
 movement compensating section 30 and a variable length coding (VLC)
 section 25.
 The arithmetic circuit 22 performs movement compensation in any of the
 following predictive modes: an intra mode, a forward direction predictive
 mode, a backward direction predictive mode and a mutual direction
 predictive mode, on the macro block data D4 supplied from the preprocess
 section 21, based on the image type of each macro block of the macro block
 data D4. Note that, the intra mode refers to a method for dealing frame
 pictures to be encoded as transmission data as it is. The forward
 direction predictive mode is a method for dealing the predictive remainder
 between the frame pictures to be encoded and past reference pictures as
 transmission data. The backward direction predictive mode is a method for
 dealing the predictive remainder between the frame pictures to be encoded
 and future reference pictures as transmission data. The mutual direction
 predictive mode is a method for dealing the predictive remainder between
 the frame pictures to be encoded and the average value of two predictive
 pictures of past reference pictures and future reference pictures as
 transmission data.
 Initially, a case that the macro block data D4 is composed of I-pictures
 will be described. The macro block data D4 is processed in the intra mode.
 That is, the arithmetic circuit 22 transmits the macro blocks of the macro
 block data D4 to a discrete cosine transform (DCT) section 23 as
 arithmetic data D5 as it is. The DCT section 23 performs a DCT conversion
 process on the arithmetic data D5 in order to obtain a DCT coefficient,
 and transmits the resultant to a quantization section 24 as DCT
 coefficient data D6. The quantizaion section 24 executes a quantization
 process on the DCT coefficient data D6, and transmits the resultant to the
 VLC section 25 and an inverse quantization section 26 as quantization DCT
 coefficient data D7. At this time, the quantization section 24 adjusts the
 quantization value based on a quantization control signal D20 supplied
 from the quantization rate control section 18, so that the amount of
 generated code is controlled.
 The inverse quantization section 26 executes inverse quantization process
 on the received quantization DCT coefficient data D7 and transmits the
 resultant to an inverse DCT section 27 as DCT coefficient data D26. Then,
 the inverse DCT section 27 executes inverse DCT process on the DCT
 coefficient data D26 and transmits the resultant to an arithmetic circuit
 28 as arithmetic data D27. In this case, the data D27 is transmitted via
 the arithmetic circuit 28 without any process, to be stored in the frame
 memory 29 as reference picture data D28.
 Next, a case the macro block data D4 is composed of P-pictures will be
 described. The arithmetic circuit 22 executes a movement compensating
 process on the macro block data D4 in either of the predictive mode: that
 is, the intra mode or the forward direction predictive mode.
 When the predictive mode is the intra mode, the arithmetic circuit 22
 transmits the macro blocks of the macro block data D4 as it is to the DCT
 section 23 as arithmetic data D5, as well as the case of macro block data
 D4 comprising I-pictures.
 On the other hand, when the predictive mode is the forward direction
 predictive mode, the arithmetic circuit 22 performs subtraction process on
 the macro block data D4 by using forward direction predictive picture data
 D30F supplied from the movement compensating section 30.
 Note that, the forward direction predictive picture data D30F is obtained
 by performing movement compensation on the reference picture data D28
 stored in the frame memory 29 based on the movement vector data D31. That
 is, in the forward direction predictive mode, the movement compensating
 section 30 shifts the read address of the frame memory 29 based on the
 movement vector data D31, in order to read the reference picture data D28.
 Then, the section 30 supplies the read data 28 to the arithmetic circuit
 22 and the arithmetic circuit 28, as the forward direction predictive
 picture data D30F. The arithmetic circuit 22 subtracts the forward
 direction predictive picture data D30F from the macro block data D4 in
 order to obtain difference data as a predictive remainder, and transmits
 the resultant to the DCT section 23 as the arithmetic data D5.
 Further, the forward direction predictive picture data D30F is supplied to
 the arithmetic circuit 28 from the movement compensating section 30. The
 arithmetic circuit 28 adds the data D30F with the arithmetic data D27, so
 that the reference picture data D28 (P-picture) is partly reproduced and
 the resultant is stored in the frame memory 29.
 Next, a case the macro block data D4 comprises B-pictures is supplied to
 the arithmetic circuit 22 from the preprocess section 21 will be
 described. The arithmetic circuit 22 executes on macro block data D4, a
 movement compensating process in any of the following modes: the intra
 mode, the forward direction predictive mode, the backward direction
 predictive mode or the mutual direction predictive mode.
 When the predictive mode is the intra mode or the forward direction
 predictive mode, the macro block data D4 undergoes the same process as in
 the case of the P-pictures. Note that, as the B-pictures are not employed
 as other predictive reference pictures, the reference picture data D28 is
 not stored in the frame memory 29.
 On the other hand, when the predictive mode is the backward direction
 predictive mode, the arithmetic circuit 22 performs subtraction process on
 the macro block data D4 by using backward direction predictive picture
 data D30B supplied from the movement compensating section 30.
 The backward direction predictive picture data D30B is calculated by
 performing movement compensation on the reference picture data D28 stored
 in the frame memory 29 based on the movement vector data D31. More
 specifically, in the backward direction predictive mode, the movement
 compensating section 30 shifts addresses read by the frame memory 29,
 based on the movement vector data D31 in order to read the reference
 picture data D28. Then, the section 30 supplies the resultant to the
 arithmetic circuit 22 and the arithmetic circuit 28 as the backward
 direction predictive picture data D30B. The arithmetic circuit 22
 subtracts the backward direction predictive picture data D30B from the
 macro block data D4 in order to obtain difference data as a predictive
 remainder, and transmits the resultant to the DCT section 23 as the
 arithmetic data D5.
 Further, the backward direction predictive picture data D30B is supplied
 from the movement compensating section 30 to the arithmetic circuit 28.
 The arithmetic circuit 28 adds the backward direction predictive picture
 data D30B to the arithmetic data D27, so that the circuit 28 partly
 reproduces the reference picture data D28 (B-picture). However, as
 B-pictures are not used as other predictive reference pictures, the
 reference picture data D28 is not stored in the frame memory 29.
 When the predictive mode is the mutual direction mode, the arithmetic
 circuit 22 subtracts the average value of the forward direction predictive
 picture data D30F and the backward direction predictive picture data D30B
 supplied from the movement compensating section 30, from the macro block
 data D4. Thereby, the circuit 22 obtains difference data as a predictive
 remainder and transmits the resultant to the DCT section 23 as the
 arithmetic data D5.
 Further, the forward direction predictive picture data D30F and the
 backward direction predictive picture data D30B are supplied from the
 movement compensating section 30 to the arithmetic circuit 28. The
 arithmetic circuit 28 adds the average value of the forward direction
 predictive picture data D30F and the backward direction predictive picture
 data D30B with the arithmetic data D27 in order to reproduce partly the
 reference picture data D28 (B-picture). However, as the B-pictures are not
 employed as other predictive reference pictures, the reference picture
 data D28 is not stored in the frame memory 29.
 Thus, the picture element number converted video signal D3 inputted to the
 encoding section 20, is subjected to a movement compensating process, a
 DCT process and a quantization process in order to be supplied to the VLC
 section 25 as the quantization DCT coefficient data D7.
 The VLC section 25 performs a variable length encoding process based on a
 prescribed conversion table on the quantization DCT coefficient data D7
 and the movement vector data D31 in order to transmit the resultant to a
 buffer 19 as variable length coded data D8. In the buffer 19, the variable
 length coded data D8 is temporarily stored and then, read out sequentially
 as variable length coded data D10.
 The quantization rate control section 18 successively monitors the
 accumulated state of the variable length coded data D8 stored in the
 buffer 19, and deals the resultant as occupation rate information D19.
 Then, the quantization rate control section 18 generates a frequency
 characteristic control signal D18 and a quantization control signal D20
 based on the occupation rate information D19, in order to transmit the
 resultant respectively to the prefilter 14 and the quantization section
 24. Thereby, the section 18 adjusts the bandwidth in the band reducing
 process and the quantization value in the quantization process.
 The number of picture elements in the picture element number converted
 video signal D3 is reduced to less than that of the video signal D1.
 Therefore, the number of macro blocks of the macro block data D4 formed by
 dividing the picture image of the signal D3 into 16 pixels.times.16 lines
 is also reduced. The total movement vector in the movement compensating
 process is substantially proportional to the number of macro blocks. Thus,
 the number of macro blocks of the macro block data D4 decreases and
 accordingly, the number of generated code of the movement vector data D31
 also decreases. Therefore, the number of generated code of the variable
 length coded data D8 obtained by performing a variable length encoding
 process on the DCT coefficient data D7 and the movement vector data D31 is
 decreased. Accordingly, the variable length coded data D8 stored in the
 buffer 19 is also decreased.
 Therefore, assuming that the number of generated code of the variable
 length coded data D10 is constant, the quantization rate control section
 18 minutely controls the quantization value in the quantization section 24
 in relation to the decrease in the variable length coded data DS stored in
 the buffer 19. Thereby, the DCT coefficient data D6 can be quantized by a
 more minute quantization value in accordance with the decrease in the
 generated code of the movement vector data D31 due to the decrease in the
 number of macro blocks. Therefore, the quantize accuracy can be improved.
 In the above configuration, the video signal D1 inputted to the video
 signal encoder undergoes the band reducing process in the prefilter 14,
 and the resultant is transmitted to the picture element number converting
 section 15 as the band restricted video signal D2. At this time, the
 quantization rate control section 18 controls the band reduction in the
 prefilter 14 according to the degree of difficulty in compressing the
 video signal D1.
 The picture element number converting section 15 reduces the picture
 element number in the band restricted video signal D2, to the minimum
 number of picture elements necessary for representing the band restricted
 video signal D2, in order to transmit the resultant to the encoding
 section 20 as the picture element number reduced video signal D3. At this
 time, since the number of picture elements is decreased, the number of
 macro blocks is also decreased.
 The encoding section 20 executes on the picture element number reduced
 video signal D3, movement compensating process, a DCT converting process,
 a quantization process and a variable length encoding process and outputs
 the resultant to the buffer 19 as the variable length coded data D8. At
 this time, the quantization rate control section 18 controls the
 quantization value at the quantizaion process in the encoding section 20
 according to the degree of difficulty in compressing the video signal D1.
 The picture element number reduced video signal D3 has the less number of
 macro blocks than that of the band restricted video signal D2. Since the
 generated code of the movement vector under the movement compensating
 process is substantially proportional to the number of macro blocks, the
 generated code of the movement vector of the variable length coded data D8
 obtained by encoding the picture element number reduced video signal D3 is
 also decreased. When the generated code of the movement vector is reduced,
 a DCT coefficient can be quantized by more minute quantization value.
 Hence, deterioration in picture quality can be avoided.
 According to the above configuration, the prefilter for reducing the signal
 band of the video signal, and the picture element number converting
 section for converting the number of picture elements of the video signal
 are provided. The picture element number converting section executes
 picture element number conversion on video signals, so as to correspond to
 the number of picture elements according to the signal band reduced by the
 prefilter and to encode the minimum number of required picture elements.
 Thus, the quantization value in the quantization process can be prevented
 from becoming unnecessarily large, deterioration of the picture quality by
 the encoding process can be prevented.
 In the above embodiment, the picture element number conversion deciding
 section 17 transmits the picture element number control signal D17 in
 accordance with the frequency characteristic control signal D18 sent from
 the quantization rate control section 18. However, the present invention
 is not limited to this. The frame of the video signal D1 can be detected
 and the picture element number control signal D17 can be sent
 synchronously with a frame cycle. More specifically, in FIG. 4 the same
 sections having the same reference numerals as in FIG. 2, 11 generally
 shows an encoder. A video signal D1 is supplied to a prefilter 14 and a
 synchronizing signal generating section 35 from a video signal supply
 device (not shown). A synchronizing signal generating section 35 detects
 the frame cycle of the video signal D1 and transmits a synchronizing
 signal D35 synchronized with the frame cycle to a picture element number
 conversion deciding section 36. The picture element number conversion
 deciding section 36 transmits a picture element number control signal D17
 to a picture element number converting section 15 synchronously with the
 synchronizing signal D35. The picture element number converting section 15
 performs a picture element number converting process on a band restricted
 video signal D2 by the frame based on the picture element number control
 signal D17.
 Further, according to the above embodiment, the number of horizontal
 picture elements to be converted in the picture element number converting
 process is set at 1440 pixels, 1280 pixels, 1152 pixels and 960 pixels.
 However, the present invention is not limited to this. Other Numbers of
 horizontal picture elements can be used.
 Additionally, according to the above embodiment, the video signal D1 is a
 video signal of the HDTV standard. However, the present invention is not
 limited to this. A video signal of other standard can be inputted.
 Furthermore, according to the above embodiment, the number of horizontal
 picture elements is reduced according to the degree of difficulty in
 compressing a picture image. However, the present invention is not limited
 to this. The number of vertical picture elements; that is, the number of
 scanning lines can be reduced. Also, the number of vertical and horizontal
 picture elements; that is, the number of scanning lines and the number of
 horizontal picture elements, can be simultaneously reduced.
 As described above, according to the present invention, a signal band
 reducing means for reducing the signal band of the video signal and a
 picture element number converting means for converting the number of
 picture elements of the video signal are provided. Picture element number
 conversion is executed in the picture element number converting means so
 as to correspond to the number of picture elements corresponding to the
 signal band reduced by the signal band reducing means. Thereby, the number
 of generated code in the encoding process can be decreased, and the
 quantization accuracy in the quantization process can be improved. Thus,
 the deterioration of picture quality in the encoding process can be
 prevented.
 While there has been described in connection with the preferred embodiments
 of the invention, it will be obvious to those skilled in the art that
 various changes and modifications may be aimed, therefore, to cover in the
 appended claims all such changes and modifications as fall within the true
 spirit and scope of the invention.