1. In a method of digitally converting text to speech, said method including the step of encoding waveforms representing sounds in the form of digital indices representing addresses in a table containing selected values of .DELTA., and the successive samples of the waveform are computed by the formula EQU S.sub.t =aS.sub.t-1 +bS.sub.t-2 +.DELTA. where S.sub.t is the sample being computed, S.sub.t-1 is the next preceding sample, S.sub.t-2 is the second preceding sample, a and b are constants, and .DELTA. is the value stored in said table at the address defined by the index corresponding to S.sub.t, the improvement comprising encoding said indices by a Huffman coding in which the shortest codes of said Huffman coding represent the addresses of the .DELTA. values occurring most frequently in the computation of said waveform.