The E. M. I. Stereo Recording and Reproducing Systems

Abstract

The development of this system was originated by Blumlein in 1931 and the basic principles were published in the British Patent Specification 394,325. This system attempts to reproduce at the listener's ears the same relative sound pressures and time phases as the listener would have experienced in the original sound field. This is accomplished, to a very good approximation, by using only two recording channels and two loudspeakers. It is recognized that it is not possible to produce the required phase differences at the ears by controlling the phase differences at the loudspeakers. If, however, the loudspeakers are supplied with signals in phase, the required phase differences at the ears can be effected by a control of the intensity differences at the loudspeakers, and if these are properly related to the direction of the sound arriving at the transmitting end of the system, localization of sounds over the whole frequency range can be effected by the listener. The two channels are interconnected in such a way that the final pair of outputs control the necessary magnitude differences at both low and high frequencies to drive the loudspeakers. An important feature of the arrangement is that, although two microphone channels are employed, these are used at such close spacing that the microphone system is to all intents and purposes a single transducer. One method of operating the system is to use two directional microphones in juxtaposition and inclined at equal angles to the central direction. The two microphone outputs may be taken direct to their respective loudspeakers or they may be modified by taking the sum and difference of the two microphone outputs and adjusting, if necessary, the gain of the difference channel relative to the sum channel and recombining to form two loudspeaker outputs. In 1934, this system was applied to a single-track disk recording, one channel being recorded “hill and dale,” and the other lateral cut. At that date the frequency range was limited to below 5 kc/sec. The advent of twin channel domestic tape records allowed this stereosystem to be operated on a commercial scale and this was launched on the market in August, 1955.