For some time work has been going on behind the scenes in most of the major gramophone recording companies of the world on the development of discs in which sounds from two separate sources are recorded in the same groove. These recordings can be extracted by a special pickup, with two degrees of freedom, and reproduced through separate amplifiers and loudspeakers to synthesize a sound field which, to the listener, has directional properties similar to those of the original.
The foundations of stereophonic recording on disc were firmly laid by A D Blumlein of EMI in a British patent (394,325) granted in 1933. He showed that modulations of the groove at right angles to each other could be vertical and horizontal or both at 45° to the surface of the disc, and that one system could be converted to the other by taking the sum and difference of the electrical outputs from the two channels. Shellac records at 78 rpm made by the Columbia Graphophone Company on this principle in 1933, were demonstrated recently by H A M Clark at the Institution of Electrical Engineers as a prelude to examples of the latest experimental recordings by EMI. We have also been privileged to hear the Decca records which made a profound impression when they were demonstrated recently in America. Both these demonstrations left little doubt that the problems of cutting at groove with dual modulation and extracting the signals without noticeable distortion or crosstalk have been successfully solved by the use of modern techniques of recording with feedback cutters, and reproduction with lightweight pickup movements. The results from discs are practically indistinguishable from the master dual-track magnetic tapes from which they are transcribed.
The record manufacturers have been alive to the necessity for standardization before stereo discs are marketed on a large scale. We understand that agreement has been reached, independently by groups of companies in Europe and in America, on the same basic standards which, subject to confirmation, will be as follows:
- Modulations to be mutually at right angles and inclined at 45°/45° to the surface of the record;
- Playback to be with a stylus tip of 0.0005 in radius
- Relative channel polarity to give predominantly lateral movement of the stylus when the inputs are approximately in phase
- Modulation of the outer groove wall to correspond with the right-hand sound channel, ie if the axis of the right-hand modulation is extended it cuts the axis of rotation above the record
- Frequency response to IEC fine-groove playback standard (B.S. 1928: 1955).
There are fundamental differences in the processes of tracing lateral and vertical modulations of the groove, and the 45°/45° system has been chosen because it gives better symmetry in this respect between the two channels. The smaller stylus radius is necessary because with dual modulation the groove will at times be narrow and shallow, but it is also of advantage in giving a reduction of tracing distortion. Distortion is also reduced by the relative channel recording polarity which has been chosen.
The qualities of stereophonic sound reproduction as exemplified by HMV 'Stereosonic' magnetic tape records have made many converts, even among those with full knowledge of the very realistic results which have been achieved with multi-microphone and other studio techniques on single-channel LP records. Quite apart from the obvious spatial effects, there is, in the best stereophonic orchestral recordings, a clarity and definition which is immediately appreciated by musicians. We say 'the best' advisedly for pseudo-stereophony can also introduce ludicrous incongruities such as levitations and expansions and contractions in the apparent size of musical instruments according to the registers in which they are playing! Much will depend on the skill of the studio managers and recording engineers, and what between them they put on the record. The qualities of some ordinary LP records seem to shine through all kinds of distortion in indifferent reproducers, and it is to be hoped that similar foolproof qualities will emerge from the stereo discs which are offered to the general public. The crude effects of marching bands, railway sounds, and even antiphonal choral singing can be safely left to look alter themselves, but it remains to be seen whether the more subtle qualities will survive the vagaries of general usage. The purely spatial effects are likely soon to lose their novelty, and stereophony will displace the single-channel high-quality reproducing system only if it shows a gain in realism on all kinds of programme material which is commensurate with the extra cost of the equipment.