fig1 is a block diagram showing an encoding circuit 1 . an input image signal is a digitized image signal ( luminance / color - difference signal or rgb signal ). an image signal passing through a low - pass filter ( lpf ) 10 , which is a digital filter , and a sampling - rate decreasing portion 11 is input to the first input terminal of a switch 12 and an image signal is directly input to the second input terminal of the switch 12 . the switch 12 selects an image signal in accordance with a comparison result by a comparator 16 to be described later . the low - pass filter 10 passes only the low frequency band of an input signal . for example , in the present embodiment , the frequency band of an image signal is narrowed to half . moreover , the sampling - rate decreasing portion 11 outputs only every other value output from the low - pass filter 10 . that is , the portion 11 discards every other output value , thereby for decreasing the sampling rate by one half . an image signal selected by the switch 12 is input to the encoder 13 . the encoding method employed by the encoder 13 is not restricted by the present invention . known encoding methods include orthogonal transforming methods such as dct ( discrete cosine transformation ) and hadamard transformation . in the present example , the encoder 13 is described by assuming that the encoder 13 performs the hadamard transformation . the encoder 13 for performing the hadamard transformation is shown in fig3 as an example . an image signal delayed by a frame delay memory 13 a is input to a first hadamard transformation circuit 13 b of the encoder 13 . moreover , an image signal is directly input to a second hadamard transformation circuit 13 c . the hadamard transformation circuits 13 b and 13 c respectively apply two - dimensional hadamard transformation to the image signals and output the signals to an adder 13 d and a subtracter 13 e . the number of components of the two - dimensional hadamard transformation becomes m × n in the case of a block of rows ( m ) and columns ( n ). for example , in the case of m = 2 and n = 2 , four components are output . the adder 13 d and subtracter 13 e add and subtract components of the two - dimensional hadamard transformation from the first hadamard transformation circuit 13 b and the second hadamard transformation circuit 13 c . in this case , when the components for a signal having no frame delay are s ( 0 ) to s ( m × n − 1 ) and components for a signal having a frame delay are fs ( 0 ) to fs ( m × n − 1 ), the adder 13 d and subtracter 13 e at the next stage execute the following operations : quantizing portions 13 f and 13 g respectively apply quantization processing to each computation result . both quantization results are connected in one - dimensional arrangement and input to a huffman encoding circuit 13 h . the huffman encoder 13 h performs widely - known huffman encoding . for example , a not - illustrated transmitting portion is set to the rear stage of the huffman encoder 13 h . when the encoder 13 is set to , for example , a tv receiver , a received program can be transmitted from the transmitting portion . a child tv set which is an accessory of the tv receiver displays an image by receiving a transmission signal from the transmitting portion , performing demodulation processing , and moreover performing decoding processing . in the case of the encoder 13 , the encoding unit period is set to a two - frame period ( four - field period ) of an image signal . moreover , delay time is set to a two - field period by a frame delay memory . specifically , in the case of a frame - delayed signal and a signal having no frame delay , a first field corresponds to a third field . hadamard transformation is applied to blocks to which the both fields correspond and the above addition and subtraction are performed . similarly , a second field corresponds to a fourth field . hadamard transformation is applied to blocks to which the both fields correspond and the above addition and subtraction are performed . moreover , in the next encoding unit period , a fifth field corresponds to a seventh field and a sixth field corresponds to an eighth field . a number - of - generated - codes accumulating and calculating portion 14 accumulates the number of codes output from the encoder 13 by a predetermined period and outputs it . the accumulated value ( hereafter referred to as actual value ) of actual number of generated codes at constant time intervals in the encoding unit period output from the encoder 13 is calculated . that is , the number of codes output from the encoder 13 is accumulated for the encoding unit period and the value is output as an actual value for every constant time interval . a reference value generating portion 15 outputs a reference number of generated codes ( hereafter referred to as reference value ) preset synchronously with a time interval in which an actual value is output . for example , the reference value generating portion 15 is constituted of , for example , a memory and sequentially outputs holding data ( r 1 , r 2 , r 3 , . . . , rn ) by inputting a counted value ( time ) from a not - illustrated counter as an address . a comparator 16 receives an actual value from the number - of - generated - codes accumulating and calculating portion 14 and a reference value from the reference value generating portion 15 , compares the actual value with the reference value , and outputs the results of the comparison . specifically , when the actual value is smaller than the reference value , the comparator 16 outputs low ( 0 ) to the switch 12 and high ( 1 ) to the switch 12 when the actual value is larger than the reference value . when receiving low ( 0 ), the switch 12 directly encodes an input image signal and supplies it to the encoder 13 but when receiving high ( 1 ), the switch 12 supplies an input image signal passing through the low - pass filter 10 to the encoder 13 . fig2 is an illustration showing elapsed time versus accumulated generated codes . r 1 , r 2 , r 3 , . . . , rn in fig2 show reference values output from the reference value generating portion 15 for every certain time . in the example of this embodiment , the reference value is value on a straight line shown by a linear expression to time . however , it is also possible to set the reference value as a value on a curve . in this case , when an encoding unit period is a period from time 0 to time n , a reference value supplied to the comparator 16 becomes r 0 at the time 0 , r 1 at the time 1 , and rn at the time n . moreover , a thick line in fig2 shows the accumulated value ( actual value ) of number of generated codes when number - of - codes control is not performed . the comparator 16 outputs low ( 0 ) while the actual value is smaller than the reference value . therefore , the switch 12 directly supplies an input image signal to the encoder 13 . at the position shown by the point p in fig2 , it is assumed that the actual value exceeds the reference value . in this case , an output of the comparator 16 is set to high ( 1 ) and an input image signal passing through the low - pass filter 10 and sampling rate decreasing portion 11 is input to the encoder 13 by the switch 12 . because the input image signal passing through the low - pass filter 10 and sampling rate decreasing portion 11 is a signal from which a frequency band and sampling rate are removed , the number of generated codes in the encoder 13 thereafter is decreased to show the change shown by the thick dotted line in fig2 . when considering a case in which the frequency band of an input image signal is restricted to one half , and the sampling rate of the signal is decreased to one half by the low - pass filter 10 , the number of codes generated in the encoder 13 is halved . in this ease , when the number of codes generated is not adjusted and it is assumed that an actual value ( accumulated number of codes ) on the extension line of a straight line connecting the point q with the original in fig2 is increased . when adjustment of the number of codes generated is staffed at time x q , the number of codes is halved due to the decrease of the frequency band and decrease of the sampling rate . in other words , the increase rate of the actual value ( accumulated number of codes ) is halved . a condition for an actual value to which encoding control is applied not to exceed rn in an encoding period ( from time 0 to time n ) can be expressed as follows . when the coordinates of the point q are ( x q , y q ), the following expression ( 1 ) can be obtained . ( y q / 2 x q )× n +( y q / 2 )& lt ; rn ( 1 ) in this case , x q = 0 , 1 , 2 , . . . , n . that is , by obtaining reference values of r 0 to rn in accordance with the value y q shown by the expression ( 2 ), the accumulated number of codes does not exceed rn . moreover , the condition is an assumption that the subsequent number of codes increases at the same gradient at the point q . the same condition can be obtained even when the number of codes increases at two times gradient . the conditional expression in such a case becomes the data rate of an output of the encoder 13 in the above encoding control can be shown by rn / n ( bps ). the transmission band of a transmission route is set in accordance with the data rate to transmit an encoding signal to the transmission route . or , when a transmission band is specified , other reference value is set by setting the reference value rn in accordance with the transmission band . while an input image signal from the low - pass filter 10 is selected , the resolution of the image signal may be halved . for example , when the encoding unit period of the encoder 13 is equal to a two - frame period of an image signal , periods in which input image signals passing through the low - pass filter 10 are selected are stochastically concentrated in the latter half of an encoding period . therefore , there is an advantage that the periods are not recognized as image quality deterioration . as an extreme example , in a two - frame ( four - field ) period , when the period in which an input image signal passing through the low - pass filter 10 is selected is the final fourth field , three images at a high resolution are displayed , and thereafter only one image at a low resolution is presented . therefore , there is little recognizable deterioration in image quality . in the case of the above example , the switch 12 is operated based on comparison between the accumulated value and reference value of the actual number of generated codes . however , it is also possible to operate the switch 12 when a change of the number of codes ( increase gradient ) is detected or when the detected change exceeds a predetermined value .