1. A signal conversion system for converting a picture signal into a pulse train signal of a low frequency band, comprising: analog to digital converting means for converting a picture signal into a coded signal by sampling the picture signal; a digital memory having a capacity capable of storing at least one frame of a picture signal converted into the digital signal, in and from which the digital signal can be written and read by the application of a pulse signal thereto; means for producing clock pulses having periods equal to the sampling periods by the analog to digital converting means; a hold circuit for temporarily storing one coded sample value read from the digital memory; means for producing a reference pulse signal having a predetermined period; counting means for counting the number of pulses of the reference pulse signal produced by the pulse signal producing means; a coincidence circuit for producing a coincidence pulse when the coded signal stored in the hold circuit and the coded signal corresponding to the number counted by the counting means are in agreement with each other; means for writing the picture signal of one frame coded by the analog to digital converting means, successively, sample value by sample value, in the digital memory by the application thereto of the clock pulses from the clock pulse producing means; means for producing, upon completion of writing the picture signal of one frame in the digital memory, a pulse to output it as one of the pulses of an output pulse train signal, and to apply it to the digital memory to read one sample value of the stored coded signal therefrom, and for storing the read out one sample value in the hold circuit; means for supplying the coincidence circuit with the coded signal stored in the hold circuit and the coded signal representative of the number of pulses of the reference pulse signal counted by the counting means to compare them; and means for providing the coincidence pulse produced by the coincidence circuit as the pulse train signal, for resetting the counting means and the hold circuit by the coincidence pulse, and for applying the coincidence pulse to the digital memory to read therefrom the next one sample value of the stored coded signal which is stored in the hold circuit.

2. A signal conversion system for converting a picture signal into a pulse train signal of a low frequency band, comprising: memory means having a memory capacity sufficient for storing a picture signal of one frame, capable of having written therein and read out therefrom, by being supplied with a pulse signal and capable of storing in analog form, the values of the picture signal sampled with the period of the pulse signal; means for producing clock pulses for sampling the picture signal and writing in the memory means; a hold circuit for temporarily storing one sample value of the signal read from the memory means; a comparator circuit having two terminals for comparing the amplitudes of two input signals applied thereto and for producing a coincidence pulse when the amplitudes are in agreement with each other; a saw tooth wave signal producing means for producing a ramp signAl with a predetermined slope; means for applying the clock pulses from the clock pulse producing means to the memory means to write the picture signal therein while sampling the picture signal with the periods of the clock pulses and for reading the stored first one sample value, after the completion of the writing-in of the picture signal of one frame, to store it in the hold circuit; means for applying the signal held in the hold circuit to one input of the comparator circuit, for applying the output signal of the saw tooth wave signal producing means to the other input of the comparator circuit, and for causing the saw tooth wave signal producing means to start the production of the ramp signal when the signal read from the storing means is written in the hold circuit; and means for providing the coincidence pulse produced by the comparator circuit as the pulse of the pulse train signal of the picture signal, for causing the output signal of the saw tooth wave signal producing means to fly back to the starting value of the ramp signal by triggering with the coincidence pulse, and for applying the coincidence pulse to the memory means to read therefrom the next one sample value which is written in the hold circuit.

3. A signal conversion system for converting a picture signal into a pulse train signal of a low frequency band, comprising: memory means having the memory capacity sufficient for storing a picture signal of one frame, capable of having written therein and read out therefrom, by being supplied with a pulse signal and capable of storing in analog form, the values of the picture signal sampled with the period of the pulse signal; means for producing clock pulses for sampling the picture signal and writing in the memory means; a hold circuit for temporarily storing one sample value of the signal read from the memory means; a comparator circuit having two terminals for comparing the amplitudes of two input signals applied to the terminals and for producing a coincidence pulse when the amplitudes are in agreement with each other; a pair of saw tooth wave signal producing means for producing alternately ramp signals with a predetermined slope; means for applying the clock pulses from the clock pulse producing means to the memory means to write the picture signal therein while sampling the picture signal with the periods of the clock pulses and for reading the stored first one sample value, after the completion of the writing-in of the picture signal of one frame, to store it in the hold circuit; means for applying the signal held in the hold circuit to one input of the comparator circuit, for applying the output signals of the pair of saw tooth wave signal producing means to the other input of the comparator circuit, and for causing one of the pair of saw tooth wave signal producing means to start the production of the ramp signal when the signal read from the memory means is written in the hold circuit; means for delivering the coincidence pulse produced by the comparator circuit as the pulse of the pulse train signal of the picture signal; means for applying the coincidence pulse to the memory means to read therefrom the next one sample value which is written in the hold circuit; and means for resetting the output signal of the saw tooth wave signal producing means being producing the output signal to the starting value of the ramp signal and for causing the other saw tooth wave signal producing means to start the production of the ramp signal.

4. A signal conversion system for converting a pulse train signal pulse interval modulated by a picture signal into an analog picture signal, comprising: reference pulse producing means for producing a reference pulse signal of a predetermined constant period; counting means for counting the number of pulses of the reference pulse signal; a digital memory capable of having written therein and read out therefrom supplied pulse signals at a rate corresponding to the rate of the pulse signals and capable of storing signals of one frame of sampled picture signal; means for producing clock pulses of periods equal to the periods of the sampling; a digital-to-analog converter means for converting a digital signal into an analog signal; means for causing the counting means to count the number of pulses of the reference pulse signal supplied from the reference pulse producing means each time a pulse of a pulse train signal is applied, to convert the pulse intervals of the pulse train signal into the pulse number of the reference pulse signal and for writing the pulse intervals in the digital memory as a coded signal representative of the pulse number, each time the pulse of the pulse train signal is applied; and means for reading the stored coded signal from the digital memory by applying clock pulses from the clock pulse producing means to the digital memory when the writing of signals for the memory capacity in the digital memory is completed, to supply the read out coded signal to the digital-to-analog converter means.

5. A signal conversion system for converting a pulse train signal pulse interval modulated by a picture signal into an analog picture signal, comprising: saw tooth wave signal producing means for producing ramp signals of a predetermined slope; a hold circuit for holding the amplitudes of an input pulse signal for a predetermined time; memory means capable of having written therein and read out therefrom an input analog signal depending on the period of a supplied pulse and having a memory capacity capable of storing a sampled picture signal of one frame; means for producing clock pulses of a constant period equal to the sampling period; means for applying the pulse train signal to the saw tooth wave signal producing means to start the production of a ramp signal by the first pulse of the pulse train signal, and causing, by the next pulse, the ramp signal to fly back to the start point and again start the production of a ramp signal, thus producing a saw tooth wave of ramp signals equal to the pulse intervals, respectively, of the pulse train signal, and for causing the hold circuit to hold the maximum amplitude of a ramp signal each time a pulse of the pulse train signal is applied; and means for writing the amplitude held by the hold circuit in the memory means each time a pulse train signal is applied until the memory capacity is filled, and for reading the stored signal by applying clock pulses from the clock pulse producing means.

6. A signal conversion system for converting a picture signal of a low frequency band into a train of pulse signals comprising: means for sampling a picture signal and encoding the values of the amplitudes of sampled signal elements; digital memory means for memorizing the coded signals produced by said encoding means; means for producing a reference pulse train of a predetermined constant frequency; counter means for counting the pulses of said reference pulse train; means for reading out from said memory means said coded signals, successively, each corresponding to one of said sampled signal elements and comparing each of said coded signals thus read out with the number of pulses of said reference pulse train counted by said counter means to thereby produce a reset pulse signal each time said number of pulses reaches the value represented by said each coded signal; and means for resetting said counter means each time said reset pulse is produced.

7. A signal conversion system for converting a picture signal into a train of pulse signals of a low frequency band comprising: means for sampling signal elements from a picture signal at a predetermined frequency; analog memory means adapted to have written thereinto, successively, signals representative of the amplitudes of said sampled signal elements in accordance with a first clock signal having a frequency substantiAlly the same as the frequency of said sampling and to have read out, successively therefrom, said signals in accordance with a second clock pulse; means for producing a ramp signal whose amplitude changes progressively with a predetermined slope; means for reading out said signals, successively, written in said analog memory means and comparing the value of each of said signals thus read out with the amplitude of said ramp signal to thereby produce a reset pulse each time said amplitude of said ramp signal reaches a value equal to the value of said each read out signal; and means for resetting the amplitude of said ramp signal to its original value each time said reset pulse is produced, and causing said reading out means to read out the next one of said signals written in said analog memory means.

8. A signal conversion system for converting a train of pulse signals, which are spaced one another by intervals of time, respectively, proportional to the values of sampled elements of a picture signal, into an analog picture signal of a video signal frequency comprising: memory means adapted to have written therein and read out therefrom signals at a rate corresponding to the rate of clock pulses applied thereto; means for producing a train of reference pulses at a predetermined constant frequency; counter means for counting the number of said reference pulses, said counting starting upon receiving one of the pulse signals to be converted into an analog picture signal and ending upon receiving the next one thereof; means for writing into said memory means code signals each representative of the instant counter number of said counter means each time out of said pulse signals to be converted into the analog picture signal is applied thereto by applying said pulse signals, as clock pulses, to said memory means; means for producing clock pulses having a predetermined constant frequency which is the same as that used in sampling of the picture signal; means for reading out said code signals written into said memory means, successively, in accordance with said clock pulses having said constant frequency, and means for converting code signals thus read out into analog signals, respectively.

9. A signal conversion system for converting a train of pulse signals, which are spaced one another by intervals of time, respectively, proportional to the values of sampled elements of a picture signal, into an analog picture signal of a video signal frequency comprising: analog memory means adapted to have written therein and read out therefrom signals at a rate corresponding to the rate of clock signals applied thereto; means for producing repeatedly a saw-tooth wave signal which begins to change progressively from a predetermined reference value with a predetermined value upon receiving each of the pulse signals to be converted into the analog picture signal and then returned to the reference value upon receiving the next one of said pulse signals; means for writing into said analog memory means signals, respectively, corresponding to the peak values of said saw-tooth wave signals thus produced repeatedly, by using said pulse signals as clock pulses; means for producing clock pulses having a predetermined constant frequency which is the same as that used in sampling of the picture signal; and means for successively reading out the signals written into said analog memory by applying said clock pulses having said constant frequency to said memory.

10. A signal conversion system for converting a picture signal into a pulse train signal of a low frequency band, comprising: storing means adapted to store input signals successively in accordance with a first clock signal of a predetermined frequency and to read out said stored signals successively in accordance with a second clock signal; means for sampling a picture signal and applying to said storing means signals respectively representative of the amplituDes of the sampled elements of said picture signal in accordance with said first clock signal; means for producing clock pulses successively, which are spaced one another by intervals of time, respectively, proportional to the amplitudes of the sampled elements represented by the signals read out from said storing means and ready for read out; means for reading out the signals stored in said storing means, successively, in accordance with said clock pulses as the second clock signal; and means for producing a pulse each time one of the signals is read out from said storing means, thereby producing a train of pulses which are spaced from one another by intervals of time, respectively, proportional to the amplitudes of the sampled elements of said picture signal.

11. A signal conversion system for converting a signal of a train of pulses, of a low frequency band, spaced one another by intervals of time, respectively, proportional to values sampled with a predetermined constant frequency, successively, from a picture signal, into an analog picture signal of a video signal frequency comprising: memory means adapted to have written therein and read out therefrom signals at a rate corresponding to the rate of clock pulses applied thereto; means for producing signals having amplitudes respectively proportional to the successive intervals of time with which trains of pulse signals are applied thereto; means for writing into said memory means the signals produced by said producing means by using said train of pulse signals applied to said producing means as the clock pulses to be applied to said memory means; means for generating clock pulses with a frequency substantially equal to a constant frequency used in sampling from the picture signal; and means for reading out the signals written into said memory means by applying said clock pulses produced by said clock pulse generating means to said memory means.

12. A signal conversion system for converting an analog signal having a low frequency band into a train of pulses representative of said analog signal comprising: first means for sampling the amplitude of an analog signal and encoding each respective sampled amplitude value of the analog signal; second means, coupled to said first means, for storing the respective encoded sampled amplitude values of the analog signals; third means, coupled to said second means, for generating respective sequences of pulses, the number of pulses in each sequence being proportional to the respective encoded sampled amplitude value of the analog signal; and fourth means, coupled to said third means, for generating an output signal made up of a train of pulses the respective intervals between which are proportional to the respective numbers of pulses in the sequences of pulses generated by said third means; whereby the intervals between the pulses in said output signal are representative of the respective sampled amplitude values of said analog signal.

16. A signal conversion system for converting a train of pulses, the spacing between which is variable, into an analog signal, the amplitude of which is produced in accordance with said spacing, comprising: first means, coupled to receive said train of pulses for generating respective sequences of pulses, the number of pulses in each sequence respectively being proportional to the spacing between the pulses in said train; second means, coupled to said first means, for counting the number of pulses in each sequence; and third means, coupled to said second means, for decoding the number of pulses counted by said second means, and converting the decoded number into an analog signal, the amplitude value of which at sequential instants of time is proportional to the respective decoded numbers.

19. A signal conversion system for converting an analog signal into a train of pulses representative of said analog signal comprising: first means for sampling the amplitude of an analog signal and storing each respective sampled amplitude value; second means for generating a first signal the amplitude of which changes in proprotion to the lapse of time from the beginning of the signal; and third means, coupled to said first means and said second means, for comparing the respective sampled amplitude values of said analog signal with the amplitude of said first signal and for generating a pulse upon the amplitude of said first signal corresponding to a respective sampled amplitude value of said analog signal and for resetting said second means to initiate the regeneration of said first signal, whereby a train of pulses is generated, the time intervals between which is representative of the respective sampled amplitude values of said analog signal.

22. A signal conversion system for converting a train of pulses, the spacing between which is variable, into an analog signal, the amplitude of which is produced in accordance with said spacing, comprising: first means, coupled to receive said train of pulses, for generating a respective first signal, the amplitude of which changes in proportion to the lapse of time from the beginning of the signal in response to each successive pulse in said train; second means, coupled to said first means, for sampling the amplitude of said first signal generated in response to each successive pulse in said train; third means, coupled to said second means, for storing each respective first signal sampled amplitude for a prescribed period of time; and fourth means, coupled to said third means, for reading out the stored first signal sampled amplitudes in sequence, thereby providing said analog signal.