In 1952, the distinguished American music/audio writer and critic, Edward
Tatnall Canby, wrote a series of articles for The Saturday Review which was
published in book form as "The Home Book of Recorded Music and Sound
Reproduction", by Prentice - Hall, Inc. New York. As was Canby's usual style (and that of most New York critics of the day), it was pompous, superfluous and mostly talked down to the reader. It covered a lot of ground, and despite containing a number of errors, it still made for interesting reading.
I have adopted Canby's basic framework for this article, along with a lot of re-writing and correction of the obvious mistakes. I have also added much material from my own direct experience as a recording engineer for RCA Victor in the 1960's.
... Graham Newton
To every person who has read the "Technical Information" box on the jacket liner notes of an LP record, the names Scully, Presto, Neumann, Westrex,
Grampian, HAECO, Pultec, Langevin, Fairchild, Altec, Capps, Transco, Audio-Devices, Soundcraft and others may be familiar, almost household names.
These were all manufacturers of equipment and supplies that touched the process of making phonograph records, from the stage of being converted from a "master" to a release pressing intended for home playback.
THE LACQUER CHANNEL
It all starts with the disc recorder or "lathe" as it is often referred
to... a precision piece of equipment designed with a slowly rotating feed
screw mechanism and carriage to uniformly move a cutting head across the
radius of the disc. The accurately shaped cutting stylus, mounted in the
head, cuts a VERY precise spiral groove across a flat lacquer coated
aluminum disc spinning at an exact speed of 33-1/3, 45, or 78.26
revolutions per minute, the standard phonograph operating speeds. Scully, Presto and Neumann are probably the best known manufacturers of disc recording lathes.
There were instances of early recordings (mostly acoustical) that played at speeds as low as 60 rpm, and as high as 90 rpm, but by the time electrical
recording replaced acoustical, standard speeds were adopted.
THE CUTTING HEAD
Installed on the lathe, the cutting head is simply a phonograph pickup in
reverse, that is, feed audio in and get mechanical motion out. Other than
its greater size, the specially shaped cutting stylus, and the feed screw
mechanism which moves the head across the record to make the spiral groove,
the internal workings are very similar. A great deal of literature has
been written about the cutting stylus, its shape and the resulting groove
cut by it, many written by engineers Frank and Isabel Capps, whose company
produced many of the innovations in disc recording styli over the years,
not the least of which was the Capps (ANM) Antinoise Modulation Stylus and
the Cappscoop stylus which provided a further 3 dB reduction in noise for
THE RECORDING STYLUS
Contrary to what you might think, diamond IS NOT a good material for a cutting stylus, but IS excellent as a reproducing stylus. The recording stylus is one of, if not THE most important component of the recording process, and was probably first used by Edison in 1877.
Home cutting styli were often made of a steel alloy called Stellite because
it was inexpensive to manufacture, and although cutting styli have been
made of other materials, most cutting styli for professional use are made
of corundum, commonly known as sapphire.
Because of its lack of grain, crystalline structure and cleavage, sapphire
may be ground to very accurate dimensions and angles, while retaining a
very fine cutting edge. These properties are of prime importance in
manufacturing a recording stylus. Believe it or not, a sapphire recording
stylus will outlast a diamond and produce superior recordings.
A small sapphire rod is ground with a flat face and mounted in an aluminum shank to make it easier to install and remove the stylus from the cutting
head as the need arises. The end of the sapphire rod is ground to a point
with a rounded tip, and extending upwards from the tip, along the flat
edge, are tiny burnishing facets that polish the groove as it is cut,
producing a quieter groove. These facets are critical since, if ground
incorrectly, they will affect not only the signal to noise ratio but also
the high frequency response of the recording. High quality recording
demands that the stylus noise be typically in the range of 57 to 60 dB
below a recorded 1kHz reference level of 7 centimeters.
For microgroove recordings (16-2/3, 33-1/3 and 45 rpm), the recommended cutting stylus tip radius is .00025 inch or less, and for coarse groove
recordings (33-1/3 rpm transcriptions and 78 rpm) the recommended radius is .0015 inch. You can see why a 78 played with an LP stylus sounds so bad! For coarse groove recording, the stylus was often mounted with a slight mechanical bias toward the center of the record. This ensured the thread of cut lacquer material or "chip" would be thrown toward the center of the disc thus avoiding fouling the stylus tip.
In both LP and coarse groove recordings, the included angle (that's the angle formed by the groove walls) is 88 degrees plus or minus 5 degrees. The top of the groove is supposed to measure not less than 4 mils across, for a recording to be played with a 2.5 mil stylus, and in the case of LP's played with a 1 mil stylus, no less than 2 mils across at the top.
A tremendous improvement in cutting "fine groove" records occurred with the introduction of the hot stylus. This is no more than a winding of 6 or 7 turns of "Nichrome" resistance wire around the shank of the stylus, near the tip. Connected to a power supply with adjustable current and metering, is used to bring the cutting stylus to a carefully controlled even temperature. The hot stylus momentarily softens the lacquer surface of the recording disc while the groove is being cut. The lacquer thus offers less resistance to the cutting process, the cut is smoother with less distortion, and the necessity for diameter equalization is virtually eliminated, being reduced to about a 2dB difference between the outside to inside of a disc.