GB1488434A - Fsk modulator - Google Patents

Abstract

1488434 Data transmission; FSK modulators INTERNATIONAL BUSINESS MACHINES CORP 21 Aug 1975 [21 Nov 1974] 30551/76 Divided out of 1488433 Heading H4P Description of the embodiments is identical to the parent Specification, but the claims are directed to an FSK modulator of the type in which sinusoidal waves are synthesized digitally by successively summing, sets of incremental digital angular values ##, also increments of fractional baud time, then performing an ### to amplitude (sin#) translation through a previously calculated look up table, the digital amplitude values being converted to analogue form by conventional techniques. A clock 30 of frequency fs supplies anti phase outputs C1, C2 which are applied to address generator 31 also receiving three high order bits on buffer 32 and input data (D0). During C1 times, 8 values #T (T=baud time) are sequentially placed in adder 34 operating in conjunction with circulatory register 36 controlled through read/write memory 35 (clocked by C1, C2) hence 36 is sequentially "pumped up" by successive increments of #T. During C2 times angular 8 increments ##(T) are similarly treated with respect to register 37; after each addition, content of 36, 37 is inserted in 32. Add/subtract function is controlled by 38 so that: Output of 34 is passed through a 0 to sin # converter (i.e. ROM table) 41 to register 42 strobed by C2 pulses controlling a d/a converter, output being passed through a RC filter to line. The register 36 is prevented from over/under running when #T is added/substracted by arranging the 8 values to start at 000 with intermediate values representing equal steps of fractional baud time and terminating at 111. Register 36 may then be reset to 000. Similar considerations apply to the ##(T) values with respect to register 37. Frequency changes at other than zero point are inherently averaged during the particular cycle which is illustrated in Fig. 1B (not shown) so that frequency changes occur in substantially continuous manner within a single baud time. The modulator may be changed to another group of frequencies by storing appropriate values in ROM 33 which are selectable by a control word LCW.