first , description will be given with respect to the fundamental principle of the system according to the present invention , by referring to fig1 a and 1b . when positions and magnitudes of maximum points a , c , e , and g and minimum points b , d , and f of a video signal in a composite video signal are extracted as information in order to reproduce the video signal , the information quantity can be described by &# 34 ;( position + magnitude )×( number of points )&# 34 ;. if one horizontal scanning period between horizontal synchronizing signals hs is 6 × 10 - 5 seconds , the chrominance level is described by 8 bits , and the sampling frequency is 8 mhz , one horizontal scanning period is sampled at 480 ( 6 × 10 - 5 × 8 × 10 6 ) points . accordingly , if the sampling points are described by 9 bits ( 512 points can be described by use of 9 bits ), and there are 7 maximum , minimum , and rapidly changing points as shown in fig1 a , 119 [( 9 + 8 )× 7 ] bits will be required upon extraction of these points . moreover , in a waveform such as that shown in fig1 b , a rapidly changing point of the voltage slope indicated by i is easily noticed , together with a maximum point h . thus , the rapidly changing points are also extracted together with the maximum and minimum points . in order to judge that a value m ( i ) sampled at an arbitrary point i is a maximum point or a minimum point , the following method is used . if the sampling points adjacent to the point i are i - 1 , i - 2 , . . . , i - n ( n is an arbitrary positive integer ) and i + 1 , i + 2 , . . . , i + n , and thresholds ( specific positive values ) are l1 and l2 , the following equations can be formed . if the equations ( 1a ) and ( 2 ) are satisfied , the point is a maximum point , and if the equations ( 1b ) and ( 2 ) are satisfied , the point is a minimum point . the equation ( 2 ) is used to eliminate error upon sampling due to noise and the like . next , when the above equation ( 2 ) is not satisfied and it cannot be judged whether the point is a maximum point or a minimum point , it is assumed that the point is a maximum point when the following equations ( 3a ) and ( 4 ) are satisfied , and that the point is a minimum point when the equations ( 3b ) and ( 4 ) are satisfied . furthermore , if even the above equation ( 4 ) is not satisfied and it cannot be judged whether the point is a maximum point or a minimum point , it is assumed that m ( i ) is a maximum point if the following equation ( 5a ) is satisfied and that m ( i ) is a minimum point if the following equation ( 5b ) is satisfied for all m ( m is a positive integer ) where m satisfies 3 ≦ m ≦ n . next , with respect to the rapidly changing points , m ( i ) is a rapidly changing point when the following equations ( 6 ) and ( 7 ) are satisfied , where l3 and l4 are thresholds ( specific positive values ). ## equ1 ## description will now be given with respect to a compression system of a first embodiment of a compression and expansion system for a composite video signal according to the present invention , which uses the above principle , by referring to the block system shown in fig2 . in fig2 a television camera 11 is in synchronism with a synchronizing signal supplied from a synchronizing signal generator 12 which will be described hereinafter , and supplies an analog composite video signal to an analog - to - digital ( a / d ) converter 13 . the a / d converter 13 samples the composite video signal by a clock pulse supplied from a clock pulse generator 14 . the clock pulse has a frequency which is twice the maximum frequency of the composite video signal . thus , the composite video signal is converted into an 8 - bit digital signal by the a / d converter 13 , and supplied to a shift register 15 . this shift register 15 comprises 2n + 1 ( n is an arbitrary positive integer ) blocks . one block is constructed from 8 bits . the shift register 15 stores the digital signal sampled by the a / d converter 13 , in units of sampling points . the sampled values stored in each block can be read out arbitrarily . the shift register 15 is supplied with the clock pulse from the clock pulse generator 14 , and shifts the sampled values downwards from the upper side in fig2 . the sampled values m ( i + 1 ) through m ( i + n ), and m ( i - 1 ) through m ( i - n ) in each block of the shift register 15 are respectively supplied to one input terminal of digital comparators 16al through 16an and 16bl through 16bn . the memory content m ( i ) of a block which is shifted by ( n + 1 ) pulses after being supplied to the shift register 15 , is supplied to the other input terminal of the digital comparators 16al through 16an and 16bl through 16bn , and also to a memory 25 . the digital comparators 16al through 16an and 16bl through 16bn respectively compare the two inputs supplied to the two terminals , and obtain a difference and the positive or negative sign . difference signals m ( i )- m ( i + 1 ) and m ( i - 1 )- m ( i ) obtained at the digital comparators 16al and 16bl , are respectively supplied to a digital comparator 17 . this digital comparator 17 judges whether m ( i ) is a maximum point , a minimum point , or a rapidly changing point satisfying the equations ( 1a ), ( 1b ), ( 2 ), ( 6 ), or ( 7 ), by using a horizontal position signal and a vertical position signal from horizontal and vertical counters 18 and 19 , the thresholds l1 , l2 , and l4 from a memory 20 , and the above difference signals . an output compared signal of the digital comparator 17 is supplied to a digital comparator 21 . the horizontal position signal and the vertical position signal from the horizontal and vertical counters 18 and 19 are supplied to the digital comparator 21 . the digital comparator 21 passes the compared signal from the digital comparator 17 as it is , as an output compared signal of this digital comparator 21 , when the compared signal from the digital comparator 17 indicates that the point is a maximum , minimum , or rapidly changing point . hence , in this case , the compared signal from the digital comparator 17 is supplied to a digital comparator 22 through the digital comparator 21 . on the other hand , when the compared signal from the digital comparator 17 does not indicate that the point is a maximum , minimum , or rapidly changing point , the digital comparator 21 judges whether the point is a maximum or minimum point which satisfies the above equations ( 3a ) or ( 3b ) and ( 4 ), by using the difference signals [ m ( i )- m ( i + 2 )] and [ m ( i - 2 )- m ( i )] supplied from the digital comparators 16a2 and 16b2 and the threshold l2 supplied from a memory 23 . an output compared signal of the digital comparator 21 is accordingly supplied to the digital comparator 22 . the horizontal and vertical position signals from the horizontal and vertical counters 18 and 19 , are supplied to the digital comparator 22 . the digital comparator 22 passes the compared signal from the digital comparator 21 as it is , as an output compared signal of this digital comparator 22 , when the compared signal from the digital comparator 21 indicates that the point is a maximum , minimum , or rapidly changing point . thus , in this case , the output compared signal of the digital comparator 22 is supplied to an address counter 24 and the memory 25 . on the other hand , when the compared signal from the digital comparator 21 does not indicate that the point is a maximum , minimum , or rapidly changing point , the digital comparator 22 judges whether the point is a maximum or minimum point which satisfies the above equations ( 5a ) and ( 5b ), by using the difference signals obtained from the digital comparators 16a3 through 16an and 16b3 through 16bn . an output compared signal of the digital comparator 22 is accordingly supplied to the address counter 24 and the memory 25 . in addition , the clock pulse generated by the clock pulse generator 14 , is supplied to the horizontal and vertical counters 18 and 19 . the horizontal counter 18 counts the above clock pulse , and produces a horizontal position signal which indicates where the sampling position i is in one horizontal scanning period . this horizontal position signal produced from the horizontal counter 18 is supplied to the synchronizing signal generator 12 , vertical counter 19 , digital comparators 17 , 21 , and 22 , and the memory 25 . the vertical counter 19 produces a vertical position signal which indicates the position of the horizontal scanning period of the sampling position i in one field . this vertical position signal produced from the vertical counter 19 is supplied to the synchronizing signal generator 12 , digital comparators 17 , 21 , and 22 , and the memory 25 . the horizontal and vertical position signals are respectively supplied to the digital comparators 17 , 21 , and 22 for the following reasons . that is , in the composite video signal compression and expansion system according to the present invention , the horizontal and vertical synchronizing signals within the composite video signal obtained from the television camera 11 , are not extracted . instead , the maximum , minimum , and rapidly changing points in the video signal are judged and extracted . accordingly , when the sampled value of the first sample of the video signal is supplied to the shift register 15 , and only the sampled value m ( i + n ) is in the shift register 15 , it is meaningless to perform judgement with respect to m ( i ). similarly , when the first sampled value of the video signal is the sampled value m ( i - 2 ) within the shift register 15 , the sampled values m ( i ) through m ( i + n ) are the subject of the above judgement , while the sampled values m ( i - 3 ) through m ( i - n ) are not . the digital comparators 17 , 21 , and 22 respectively perform judgement with respect to the sampled values which are the subject to this judgement , by use of the horizontal and vertical position signals . the synchronizing signal generator 12 is supplied with the above horizontal and vertical position signals , and produces the horizontal and vertical synchronizing signals . these horizontal and vertical synchronizing signals are supplied to the television camera 11 , to obtain synchronism between the television camera 11 and the circuits in the compression system . when there is an indication that the sampled value m ( i ) is a maximum , minimum , or rapidly changing point by the compared signal from the digital comparators 22 , the memory 25 writes the positional information of the horizontal and vertical position signals and the lightness ( luminance level ) information of the sampled value m ( i ) obtained from the shift register 15 , into addresses specified by the address counter 24 . moreover , when the compared signal from the digital comparator 22 indicates that the point is a maximum , minimum , or rapidly changing point , the address counter 24 advances the address as the above write - in operation of the memory 25 is completed , to specify the next write - in address . as described above , the positional information regarding only the maximum , minimum , or rapidly changing point , and the chrominance level information , are respectively and successively stored into the memory 25 . when the storing operation is completed for one field , the stored information is produced through an output terminal 26 in the order with which the information was originally stored , for transmitting or recording purposes . after this operation is completed , the horizontal and vertical counters 18 and 19 and the address counter 24 are reset , so that compression of the composite video signal can be performed with respect to the succeeding field . next , description will be given with respect to an expansion system for expanding the signal compressed by the above compression system , by referring to fig3 . in fig3 the compressed digital signal which has been reproduced or transmitted , is supplied to a memory 31 for one field , through an input terminal 30 . a clock pulse generated by a clock pulse generator 32 , is supplied to horizontal and vertical counters 33 and 34 . the horizontal counter 33 counts the above clock pulse , and produces a horizontal position signal indicating a position in one horizontal scanning period . this horizontal position signal is supplied to the vertical counter 34 , a comparator 35 , and a synchronizing signal generator 36 . the vertical counter 34 produces a vertical position signal indicating the horizontal scanning period in one field , by use of the clock pulse and the horizontal position signal . this vertical position signal is supplied to the comparator 35 , the synchronizing signal generator 36 , and an address counter 37 . when the compressed video signal is supplied to the memory 31 , the address counter 37 is in a reset state . hence , the positional information and the lightness information in the first horizontal scanning period of the compressed video signal , are read out from the memory 31 . the positional information thus read out , is supplied to the comparator 35 and a connecting linear function generator 38 through a terminal 31a . the lightness information thus read out , is supplied to the connecting linear function generator 38 through a terminal 31b . the connecting linear function generator 38 hence supplied with these positional information and the lightness information which are digital signals , produces an analog signal for performing linear interpolation by use of the previously supplied positional information and the lightness information ( the position and lightness of the previous information are zero , for example , with respect to the positional information and the lightness information first supplied in the horizontal scanning period ). the above analog signal produced by the connecting linear function generator 38 , is supplied to a mixer 39 . in addition , the comparator 35 compares the positional information from the memory 39 and the horizontal and vertical position signals from the counters 33 and 34 . when these signals coincide , a coincidence signal is supplied to the address counter 37 and the memory 31 from the comparator 35 . accordingly , the address counter 37 advances the address , and the positional information and the lightness information are read out from the memory 31 from the addresses specified by the address counter 37 , to perform the above interpolation . the synchronizing signal generator 36 is supplied with the horizontal and vertical position signals , and generates a horizontal synchronizing signal and a vertical synchronizing signal at positions corresponding to these horizontal and vertical position signals . the above horizontal synchronizing signal and the vertical synchronizing signal are supplied to the mixer 39 . the mixer 39 mixes the video signal which is converted into the analog form through interpolation , and the horizontal and vertical synchronizing signals , to form the original composite color video signal . the composite video signal thus formed , is supplied to a television receiver 40 . when the vertical counter 34 indicates that the expansion operation for one field is completed , the address counter 37 is accordingly reset . then , the following compressed digital signal is supplied to the input terminal 30 , and the above described operations are repeated . furthermore , since the connecting linear function generator 38 performs linear interpolation , the pseudo - contours which will be described hereinafter will not be introduced , when the expanded composite video signal is monitored by a television receiver . next , description will be given with respect to a compression system of a second embodiment of a system according to the present invention , by referring to fig4 through 6 . in the above described compression system of the first embodiment of the system according to the present invention , the lightness is converted into 8 bits ( 256 steps ) at the extracting points of the maximum , minimum , and rapidly changing points . in the normal video signal , the quantization noise is increased when the number of bits describing the above lightness is decreased , and further , pseudo - contours are introduced in the reproduced picture . however , if the level difference between the lightness of the extracted point and the previously extracted point on the scanning line is to be stored , the lightness difference can generally be described by 7 bits . in addition , by storing the logarithm and the sign of the absolute value of the above lightness difference , the information quantity can be decreased . as the level difference between the extracted point and the previously extracted point j increases , there is a limit in storing the large level difference within a certain number of bits . however , as shown in fig4 if the logarithm of the level difference is stored , the large level difference can be stored within the same number of bits with higher accuracy . the quantitative sense of the human eye is high when the lightness difference between adjacent picture is small , and low when the lightness difference is large , that is , when the lightness drastically changes . therefore , when the lightness is changing drastically , the eye can judge that there is a change in the lightness , however , it is impossible to judge whether the lightness is increasing to twice or three times the original lightness . therefore , even when the logarithm of the lightness difference is used , to give an exponential stop , the information can be compressed without introducing degradation in the reproduced picture quality . next , description will be given with respect to a compression system of a second embodiment of a system according to the present invention , which was devised based on the above points , by referring to fig5 . in fig5 those parts which are the same as those corresponding parts in fig2 are designated by the same reference numerals , and their description will be omitted . the sampled value m ( i ) in the shift register 15 , which is used to judge whether the point should be extracted because the point is a maximum , minimum , or rapidly changing point , is supplied to one input terminal of a memory 50 and a subtraction circuit 51 . the memory 50 is supplied with the compared signal from the digital comparator 22 , and when this compared signal indicates that the sampled value m ( i ) is a point to be extracted , the memory 50 stores this sampled value m ( i ). the stored sampled value m ( i ) is supplied to the other input terminal of the subtraction circuit 51 . then , a sampled value m ( i ) a which is judged as being the succeeding point which is to be extracted , is supplied to the memory 50 and the subtraction circuit 51 . the subtraction circuit 51 obtains a difference between the previously extracted sampled value m ( i ) from the memory 50 and the extracted sampled value m ( i ) a from the shift register 15 , and supplies this difference to the memory 25 . in the first embodiment of the invention , the memory 25 stores the sampled value 25 . however , in the present embodiment of the invention , the memory 25 stores the above difference [ m ( i ) a - m ( i )]. further , the memory 50 simultaneously stores the sampled value m ( i ) a . fig7 shows a part of a modification of the system shown in fig5 . in fig7 those parts which are the same as those corresponding parts in fig5 are designated by the same reference numerals , and their description will be omitted . in fig7 instead of supplying the difference [ m ( i ) a - m ( i )] to the memory 25 from the subtracting circuit 51 , the difference [ m ( i ) a - m ( i )] is supplied to a logarithm rate multiplier 52 . in this case , the logarithm of the difference [ m ( i ) a - m ( i )] is obtained at the logarithm rate multiplier 52 , and the logarithmic value [ log ( m ( i ) a - m ( i ))] is supplied to the memory 25 , to store this logarithmic value in the memory 25 . generally , in adjacent scanning lines of the video signal , the mutual correlation coefficient which is calculated by multiplication and addition of picture elements corresponding to each other along the direction towards the adjacent scanning line , is generally over 0 . 9 . if the picture is in the form of stripes which are perpendicular to the horizontal scanning direction , the mutual correlation coefficient is close to 1 , however , in a picture where the stripes are inclined , the mutual correlation coefficient rapidly decreases . in these cases , the lightness , the lightness difference , or the logarithm of the lightness difference and the like of the maximum , minimum , and rapidly changing points extracted by the compression systems in the first and second embodiments of the present invention , that is , the lightness information is the same in the adjacent scanning lines , and only the positional information differ between adjacent scanning lines . in these cases , it is possible to more effective perform compression of the information , by only extracting the positional information and using the lightness information obtained from the previous scanning line . moreover , the above compression method can also be applied to scanning lines in substantially the same position of the picture at differing times , as in the case shown in fig6 . in fig6 points p and q respectively correspond to points r and s . the above method can be applied regardless of whether there is interlace between fields . generally , the mutual correlation coefficient of the scanning lines , is also quite high between fields . however , when panning is performed with respect to an object , the mutual correlation coefficient decreases rapidly . however , only the positional information changes and the lightness information of the maximum , minimum , and rapidly changing points does not change in this case . accordingly , as in the above described case , only the positional information need to be extracted , and the lightness information of the scanning line in the previous field can be used . to judge whether this compression method can be applied , the number of maximum , minimum , and rapidly changing points on scanning lines of corresponding fields are counted , and measures may be taken so that only the positional information is extracted if the number of the points are the same . in the above described embodiments of the present invention , description was given with respect to a black - and - white video signal . however , it is also possible to apply the system according to the present invention with respect to a composite color video signal . in this case , the compression and expansion system of the present invention is applied to the three primary color signals , or the luminance signal and the chrominance signal such as the i signal and the q signal . there is high correlation among the color phase , the color saturation , and the lightness of a color picture . moreover , when the lightness changes , the color phase and the color saturation changes with this change in lightness . hence , the lightness seldom remains constant when the color phase and the color saturation change . the changing points ( maximum , minimum , and rapidly changing points ) of the lightness include the changing points of the color phase and the color saturation . accordingly , by adding the color phase and color saturation information ( chrominance information ) to the lightness information which is sampled at the compression system according to the present invention , it is also possible to compress a composite color video signal . furthermore , the color phase and the color saturation information can be the i - axis and the q - axis of the ntsc system signal which indirectly describe the color phase and color saturation information , or the color difference signal . further , the present invention is not limited to these embodiments , but various variations and modifications may be made without departing from the scope of the present invention .