Survey of stereophony

The first experiments in stereophony took place at the end of last century, but no serious developments occurred until the advent of the ?talkies? about 30 years ago when the cinema industry began actively to investigate the problem. About that time, when developments of cinema stereophony were in progress, Blumlein investigated the position very fully and patented microphone and loudspeaker systems for stereophony, together with methods of recording the information in a single groove on a gramophone record. He also considered domestic stereophony. Lack of satisfactory commercial methods of recording and reproduction, however, prevented developments in the domestic field. The present great interest in stereophony is entirely due to the fact that, by the improvement of recording techniques, it is now possible to make gramophone records with the stereophonic information on the opposite sides of the same groove. The standards have been chosen so that a monophonic pick-up will add the left- and right-hand information to produce a result which is asserted to be ?compatible? for monophonic purposes. The implications behind the assumption that the addition of left- and right-hand signals gives ?compatibility? are not well appreciated. Monophonic microphone techniques are therefore discussed, followed by the corresponding techniques for stereophony, so that comparisons can be made. It is then revealed that the best microphone positions for monophony and stereophony frequently differ considerably and are often quite incompatible. This poses problems for recording companies and broadcasting organizations for which there may be no easy solution, although undoubtedly some compromise may be found which will be acceptable to both monophonic and stereophonic listeners. There will, however, remain certain types of programme material, especially in broadcasting, in which such compromises may be impossible. Various workers have investigated the requirements for stereophonic transmiss- - ion, and most of them appear to be in favour of coincident microphones on the ground that the images produced are sharper than with spaced microphones. However, some organizations use spaced microphones, and, in these circumstances, it is claimed that, although the images may not be so sharp as those given by coincident microphones, the position of the listener with respect to the loudspeakers is less critical. In view of the obvious need to monitor the programme material at source, consideration is given to the listening conditions, and some related work in connection with monophonic listening rooms is described. With regard to loudspeakers, various investigators have claimed that it is possible to devise loudspeaker polar characteristics which will keep unchanged the ratio of sound level from both loudspeakers irrespective of the position of the observer in the listening room.