in an apparatus for executing an algorithm for realizing an orthogonal transform operation such as the 8 points fast cosine transform , by operating on successive sets of data values of a digital signal such as a digital video signal in such applications as high efficiency coding of a digital video signal , a plurality of multiplication operations that are executed during processing of each set of data values are executed sequentially by time division multiplex operation of a single multiplier which is capable of executing a multiplication operation within one sample period of the digital signal , with input and output data values being transferred by selector units between the multiplier and other sections of the apparatus at appropriate times during processing of each set of the input digital signal values . the scale of hardware required for the apparatus is thereby reduced by comparison with an apparatus which employs a plurality of separate multipliers .