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the resulting disc by widening thc fre-qucncy response and increasing the dynamie rangę. The difficulties in-volved bccame apparent when it is remembered that the production of any record is a complicated combination of science and alchemy. In the case of the stereo disc, complications are multi-piied. When one considers the simple fact that with a stereo recording. two separate and distinct signals have to be inscribed in a groove less than .003 inch wide, the scope of the problem becomes apparent. Add to this the fact that fuli dynamie rangę and wide frequency response for each of these channels has to be crammed into this microscopic space. it is a wonder that such an en-gineering feat was ever attempted in the first place.

Another problem confronting the record manufacturcr is the difficulty involved in plating master recordings for stereo dises. After the record is cut. the master is nlated in an elcctro-gal-vanic plating bath before it is used to manufacture the stampers which press the actual vinyl record. Because the stereo cartridge is as sensitive to ver-tical motion as it is to horizontal mo» tion and because the Stylus rides so close to the bot tom of the groove. materia! lodging in the bottom of the groove will create ticks, pops. and noise. For this rcason the plating of the masters must be very carefully done sińce the slightest bit of foreign matter in the plating bath can ruin the quality of the record.

Yet another problem, inherent in the records, is that of wear. This problem is aggravated in the stereo disc mainly because of the character of the stereo cartridge. Most stereo tips are .7 mil in radius with one or two as smali as .5 mil. For this rcason the Stylus cxcrts that much morę force per unit area for the same cartridge weight as would a 1 mil monophonic Stylus. This is offset, to some extent, by the fact that the pressures used are below those of monophonic pickups.

Dises and Tapes

From the standpoint of quality, to-day‘s stereo dises can be madę as good a sourcc of musie as any medium on the market. When one considers all the factors: dynamie rangę, frequency response. signal-to-noise ratio, rcalism. and ease of handling. no recorded source can surpass dises for stereo. Perhaps this is a rather broad State-ment but it is my personal conviction that many of the facts will bear me out in this ccntention. Thorc are others, whose primary interests lie in the tape field, who will claim that the best home method for achieving stereo playback is on 7.5 ips reel-to-reel pre-recorded tape. There are certainly some features of this medium that cannot be denied and do tip the scalę as far as quality is concerned. However, when it comes to ease of handling. no one can doubt that the disc is far out in front.

It is not our intention to imply that all stereo dises on the market today are of top quality. As we all know,

October, 1959

Cutting a Stereo Disc

OUR cov«r this month shows a lacquer master of a special stereo disc being cut at Olmstead Sound Słudios in New York City, as arranged by Audio Fidelity. The cutting head being used is a Westrei 3C that cosłs over $4000. Ił is carefully mounłed in a counter-balanced and spring-loaded carriage on the cutting lathe. Notę the two sets of leads that connecł the right- and lefł-channel audio signals to the cutting head. These leads come from recording amplifiers rated at 150 wałłs per channei. The rubber sucłion hose (at the eitreme right) draws off the "chip" formed as the disc is cut.

The disc itself is mounłed on the heavy, weighłed łurnłable of a Neumann fully auło-matlc cutting lathe. Strobę markings are ma* c': *ed into the table‘s periphery. The turn-łable is parłly hollow and has a number of smali holes drilled into its top face on which the disc rests. The spindle at the center of the table is also hollow. When a master is being cut, a sucłion linę is slipped over the spindle and the resulting vacuum holds the disc firmly and prevenłs slip ping or warping. The entire cutting lathe costs around $12.000.

In łaking the phołograph, a number of col-ored lights were arranged around the disc in such a way as to show the light pałterns formed by the signals in the grooves. These pałterns are used by the recording engineer to measure the recorded velocity of the signal that has been cut.

The outer band on the disc, with only one of the two channels being cut. is a frequency-response run. The standard RIAA equalizałion was used. First, a l-kc. reference tonę was cut and this was followed by narrow bands of łones beginning at 15 kc- and going all the way down in step to 30 cps. The latest stereo cutters are able to cut tf»i« frequeocy ranne within 2 db without any difficulty. Some cutters may even go down to 20 cps within 2 db. This is not to say that it is impossible to rut even lower frequencies or frequencies as high as 20 kc. or perhaps 25 kc. o- thn disc. but such signals will be substanłially reduced in most recordings are madę so that the poorest as well as the best cartridges and arms can track them. Compro-mises must. of necessity, be madę and sińce record companics look to the mass market for their outlets most records today are not intended to com-bine, in a single pressing. the maximum possible degree of quality obtainable.

There are. of coui^se, exceptions to this and some recording companies do produce pressings that are specifically designed to be used solely with the best hi-fi sound reproduction systems. In some cases manufacturers indicate on the album that only top-quality arms and cartridges should be used in play-ing the record. Obviously a greater degree of skill in recording and in cutting the master is employed along with the most advanced design of the asso-ciated electronic equipment.

Microphone Techn»que

One of the technical factors which

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level. if they appear at all, and not within the 2 db. limit.

The second band consists of some stereo program materiał. Bołh channels were in op-erałion for this part of the disc. Separałion bełween the two channels on the order of 18 to 22 db is common, although figures as high as 35 db have been measured under the very best of conditions with some cutters.

The inner band on the disc illusłrałes the dynamie rangę possible. Here again, in order to show a morę easily phołographed light pał-tern. only one of the stereo channels was in use. A constant tonę of 1000 cps was recorded here. first at the maiimum usual level. Then the level was gradually reduced until the signal disappeared into the noise on the disc. The dynamie rangę obtainable is on the order of 50 db in the case of most stereo dises. Of course, it is possible to get even greater dynamie rangę of perhaps 60 or even 65 db if morę disłorłion is allownble or if a restricted frequency rangę is used. Further, remember that the usual stereo disc master is madę from a master stereo tape recording, and the limi-łałions of the tape system must be taken into account. If a live performance were to be cut directly on the disc. the limitation of the tape recording system, minor as it is with Professional equipment. would not be a factor.

It appears then that the gap bełween the auality of the very finesł monophonic disc and the very fmest słereophonic disc available today. once quite wide. has become increasinqly narrowed.    “

(Photo by Bob Loeb)

contributed to our success in the stereo disc field was the adoption of the mid side for MS) microphone »erhnique. This technique employs a Telefunken stereo microphone which consists of a self-contained two-microphone unit with crossed cardioid and figure-8 pat-terns on one axis. This stereo microphone can be placed in a central posi-tion rather than is the case with the morę generally used technique involv-ing two widely separated nr.crophones.

One of the phenomena created by the MS technique is the feeling of depth experienced by the listener. This is in addition to the breadth and di-rectionality illusions which are the basis of all stereo sound. This recording technique results in musie that gives the listener the sensation that by approaching his speakers he can walk into the center of the orchestra. In addition. stereo produced by the MS method doesn*t confine the listener to (Continued on IHige 145)

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