1. In a real-time text-to-speech conversion system in which waveforms are encoded for storage in terms of digital samples, the frequency of occurrence of the various possible sample levels being non-uniform, the improvement comprising encoding corresponding samples of a pair of successive waveforms for storage by a Huffman coding in which the sample values themselves are encoded for the first waveform of said pair, and the value of the difference between each sample and the corresponding sample of said first waveform is encoded for the second waveform of said pair.