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Standards for Stereo Disc Records

Wireless World, June, 1958.
    
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A Statement Issued by the Companies Represented at the Zurich Conference

The present-day gramophone enjoys the advantage of an international dimensional standard and it is extremely important that any major development which necessitates a departure from these existing standards should be discussed in the broadest possible manner in order to establish the new set of standards which may also be acceptable to everyone interested in the development and manufacture of discs and replay equipment throughout the world. The development of the stereo disc has now passed the stage of possibility and is now a clear probability for the very near future. In order to avoid ambiguity with regard to the technical intentions of the various companies a meeting was held in Zurich last November with representatives of the European record manufacturers known to be active in the field of stereo disc recording. This ad hoc committee came to a unanimous decision to recommend the adoption of what is now defined as the 45/45 system, but withheld the publication until it was quite clear that these were in accord with the recommendations of the Record Industry Association of America. It is now certain that the USA recommendations will be in complete agreement with those arrived at independently at Zurich and therefore it is now considered safe to release details of the standards for the 45/45 system for publication.

Wherever possible the existing standards for micro-grooves as laid down by the British and International Electro Technical Commission standards will be maintained. Thus the diameter of the disc, the speed and the recording characteristics will remain unchanged. It is however, necessary to reduce the radius of the reproducing point and the recommended limits are 12.5 microns minimum radius and 15 microns maximum radius. The angle of the reproducing point will remain the same as at present. The maximum radius of the bottom of the groove will be five microns, but the included angle will remain unchanged. For the better understanding of the definitions which are to follow, the notation with regard to left and right channels will be as laid down in the IEC publication No. 94 'Recommendations for magnetic tape recording and reproducing systems' in which the right-hand channel is defined as that which supplies the right-hand speaker as viewed by the audience.

The stereo disc will carry two channels of information arranged as follows:-

  1. The two channels are recorded in such a manner that they can be reproduced by movement of the reproducing stylus in two directions at 90 degrees to each other.
  2. The right-hand loudspeaker is actuated only when there is movement on the axis which is inclined at 45 degrees to the disc surface and intercepts the axis of rotation of the disc above the surface. In normal practice the groove angle is nominally 90 degrees and for this particular case the two channels can be said to be recorded in such a manner that the right-hand loudspeaker is actuated by a modulation normal to the surface of the groove wall which faces the axis of the disc, and the left-hand loudspeaker is actuated by a modulation of the groove wall which faces away from the axis of the disc. The surface modulations of each groove wall will be 45 degrees to the plane of the disc and 90 degrees to each other.
  3. A movement of the stylus point in a direction parallel to the surface of the disc shall provide equal, in-phase, acoustical signals from the loudspeakers.

It follows, from clause 3 above, 'that pickups designed' for stereo reproduction will be suitable for playing normal microgroove single channel records. However, the majority of present day pickups are designed for single channel operation and are fitted with a larger stylus point than is recommended for stereo discs and many of them offer a high mechanical impedance to the vertical movement of the stylus and hence are not suitable for playing stereo discs.

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