New possibilities in speech transmission

Frequency compression makes it possible to transmit intelligible speech in very much less than the usual waveband of 2000?3000 c/s. Two methods are discussed, of which only the first has been experimentally tested. In the first method, frequencies are compressed or expanded by the Doppler effect which arises when a continuously moving record, such as a sound film, is scanned by slits or the like which are themselves running. An experimental device is described which has been constructed for testing the quality of compressed and re-expanded speech. Compression to one-half, followed by re-expansion gives full intelligibility, but some roughness. Speech compressed to one-quarter, and even to one-sixth, remains intelligible, but becomes monotonous, as the inflections of the pitch are not transmitted. The intelligibility is lost only at about eightfold compression. Analysis of the causes of roughness and monotony leads to an improved principle of speech transmission by compression. In the original method only certain ?preferred frequencies? are correctly transmitted, and the reproduction is good only if one of these happens to coincide with the pitch of the speech. In the improved method the preferred frequencies are not fixed but are variable, and adjust themselves automatically to the pitch, which is always correctly transmitted. This also ensures automatically the correct and undistorted transmission of its overtones. Suggestions are made for realizing this principle, which eliminates both roughness and monotony. Frequency-compression systems, however perfect, utilize only one-half of the capacity of a transmission channel, as they do not make use of the information contained in the phase of the spectral components. The concept of ?phase? is analysed, and it is shown that it is possible, at least in principle, to transmit two independent messages through the same channel by a system called ?quadrature transmission? or ?sine-cosine duplex,? but that there are great tec- hnical difficulties in the way of realizing this system.