Logic Synthesis of Some High-Speed Digital Comparators

l.i Applications A digital comparator compares two numbers presented in digital form and obtains a measure of the difference between them. Comparison may consist of detecting only the sign of the difference or the direction of mismatch of the two numbers, or the result of the comparison may be both magnitude and sign of the difference. The comparator is essentially a subtracter suitably modified to fulfill requirements of the intended application. An immediate need for a dependable, high-speed digital comparator is in the feedback control loop for the flying spot store of an experimental electronic switching system. 1 R. W. Ketchledge has derived several methods of implementing such comparators." In this application the comparator functions as an error detector, giving an output depending on the difference between the desired input address position and the fed19 20 THK B E L L SYSTEM T E C H N I C A L J O U R N A L , J A N U A R Y 1959 back present position. By servo action, this difference then acts to eliminate the positional error. The input address is presented in parallel digital form and the present position is digitally encoded in parallel. For proper holding action of the servo the comparator output should be a linear error signal when close to a zero difference. A similar technique could be used in applications requiring fast and accurate positioning through a precise digital servo action. The use of reliable, high-speed comparators in a digital computer allows greater flexibility of operation.