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A system for converting a multilevel video signal into a binary video signal for use in an image processing machine such as a facsimile machine or digital copier includes a threshold setting circuit which first determines average density levels of a predetermined number of scanning lines and then a

A system for converting a multilevel video signal into a binary video signal for use in an image processing machine such as a facsimile machine or digital copier includes a threshold setting circuit which first determines average density levels of a predetermined number of scanning lines and then a digitally weighted average of these average density levels, said digitally weighted average being used as a threshold for the binary conversion processing.

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1. A system for converting multilevel video data into binary video data using an adjustable threshold, comprising: analog-to-digital converting means for converting analog video data obtained by scanning an original into multilevel video data; threshold setting means connected to receive said mu

1. A system for converting multilevel video data into binary video data using an adjustable threshold, comprising: analog-to-digital converting means for converting analog video data obtained by scanning an original into multilevel video data; threshold setting means connected to receive said multilevel digital video data for setting a threshold by taking a digitally weighted average of at least two averaged image densities each determined for each scanning line, said threshold setting means including a first arithmetic unit for determining an average image density for a single scanning line, a memory unit for storing two or more average image densities determined by said first arithmetic unit and a second arithmetic unit for determining a digitally weighted average of a predetermined number of average image densities stored in said memory unit, whereby said second arithmetic unit supplies said digitally weighted average as a threshold to be set; and binary converting means for converting said multilevel video data into binary video data using said digitally weighted average as a threshold. 2. A system of claim 1 wherein said binary converting means includes a digital comparator which is connected to receive said digitally weighted average from said threshold setting means as a threshold to be used in the binary conversion processing and to receive said multilevel digital video data as an input to be converted into binary video data using said threshold. 3. A system of claim 1 wherein said binary converting means includes an analog comparator connected to receive said analog video data as an input to be converted into binary video data and digital-to-analog converter for converting said digitally weighted average into an analog quantity before being supplied to said analog comparator as a threshold to be used in the binary conversion processing. 4. A system of claim 1 further comprising delaying means for delaying the supply of said multilevel video data to be converted into binary video data to said binary converting means. 5. A system of claim 4 wherein said delaying means includes a storing unit which temporarily stores said multilevel video data to be converted into binary video data and supplies it to said binary converting means after scanning a predetermined number of scanning lines. 6. A system for converting multilevel video data into binary video data using an adjustable threshold, comprising: analog-to-digital converter means for converting analog video data obtained by scanning an original into multilevel digital video data; threshold setting means connected to receive said multilevel digital video data for setting a threshold by taking a digitally weighted average of at least two averaged image densities each determined for each scanning line, said threshold setting means including a first processing unit for receiving said multilevel digital video data for determining an average image density of each scanning line, a plurality of serially connected latches each storing said average image density and a second processing unit connected to receive average image densities from said plurality of latches to determine a digitally weighted average of the average image densities; binary converting means for converting said multilevel video data into binary video data using said digitally weighted average as a threshold; and delaying means for delaying the supply of said multilevel video data to be converted into binary video data to said binary converting means. 7. A system for converting multilevel video data into binary video data using an adjustable threshold, comprising: analog-to-digital converting means for converting analog video data obtained by scanning an original into multilevel digital video data; threshold setting means connected to receive said multilevel digital video data for setting a threshold by taking a digitally weighted average of at least two averaged image densities each determined for each scanning line, said threshold setting means including a first processing unit for receiving said multilevel digital video data for determining at least two average image densities for predetermined sections of each scanning line, at least two sets of serially connected latches each set including a predetermined number of serially connected latches for each predetermined section of scanning line each latch storing said average image density of the corresponding section, a selector for allowing to pass the information from a selected set of serially connected latches and a second processing unit connected to said selector to determine a digitally weighted average based on the information supplied through said selector from said selected set of serially connected latches; binary converting means for converting said multilevel video data into binary video data using said digitally weighted average as a threshold; and delaying means for delaying the supply of said multilevel video data to be converted into binary video data to said binary converting means.