Stereophonic Sound - Vinyl Records

Vinyl Records

In 1958, the first group of mass-produced stereo two-channel vinyl records was issued, by Audio Fidelity in the USA and Pye in Britain, using the Westrex "45/45" single-groove system. Whereas the stylus moves horizontally when reproducing a monophonic disk recording, on stereo records, the stylus moves vertically as well as horizontally. One could envision a system in which the left channel was recorded laterally, as on a monophonic recording, with the right channel information recorded with a "hill and dale" vertical motion; such systems were proposed but not adopted, due to their incompatibility with existing phono pickup designs (see below).

In the Westrex system, each channel drives the cutting head at a 45-degree angle to the vertical. During playback, the combined signal is sensed by a left-channel coil mounted diagonally opposite the inner side of the groove and a right-channel coil mounted diagonally opposite the outer side of the groove. The Westrex system provided for the polarity of one channel to be inverted: this way large groove displacement would occur in the horizontal plane and not in the vertical one. The latter would require large up-and-down excursions and would encourage cartridge skipping during loud passages.

The combined stylus motion is, in terms of the vector, the sum and difference of the two stereo channels. Effectively, all horizontal stylus motion conveys the L+R sum signal, and vertical stylus motion carries the L−R difference signal. The advantages of the 45/45 system are that it has greater compatibility with monophonic recording and playback systems.

Even though a monophonic cartridge will technically reproduce an equal blend of the left and right channels, instead of reproducing only one channel, this was not recommended in the early days of stereo due to the larger stylus (1.0 mil vs 0.7 mil for stereo) coupled with the lack of vertical compliance of the mono cartridges available in the first ten years of stereo. These factors would result in the stylus ``digging into' the stereo vinyl and carving up the stereo portion of the groove, destroying it for subsequent playback on stereo cartridges. This is why one often notices the banner PLAY ONLY WITH STEREO CARTRIDGE AND STYLUS on stereo vinyl issued between 1958 and 1964.

Conversely, and with the benefit of no damage to any type of disc even from the beginning, a stereo cartridge reproduces the lateral grooves of monophonic recording equally through both channels, rather than through one channel. Also, it gives a more balanced sound, because the two channels have equal fidelity as opposed to providing one higher-fidelity laterally recorded channel and one lower-fidelity vertically recorded channel. Overall, this approach may give higher fidelity, because the "difference" signal is usually of low power, and is thus less affected by the intrinsic distortion of "hill and dale"-style recording.

Additionally, surface noise tends to be picked up in a greater capacity in the vertical channel, therefore a mono record played on a stereo system can be in worse shape than the same record in stereo and still be enjoyable. (See Gramophone record for more on lateral and vertical recording.)

This system was "invented" by Alan Blumlein of EMI in 1931 and was patented the same year, but it was not reduced to actual practice, as was a requirement for patenting in the U.S. and elsewhere at that time (Blumlein died during combat in WW-II). EMI cut the first stereo test discs using the system in 1933, but it was not applied commercially until a quarter of a century later, and by another company (Westrex). Stereo sound provides a more natural listening experience, since the spatial location of the source of a sound is (at least in part) reproduced.

In the 1960s, it was common practice to generate stereo versions of music from monophonic master tapes, which were normally marked "electronically reprocessed" or "electronically enhanced" stereo on track listings. These were generated by a variety of processing techniques to try to separate out various elements; this left noticeable and unsatisfactory artifacts in the sound, typically sounding "phasey". However, as multichannel recording became increasingly available, it has become progressively easier to master or remaster more plausible stereo recordings out of the archived multitrack master tapes.