There are two types of discs, each having certain advantages and disadvantages, in general use today for instantaneous recording. It will be worth while to consider these in some detail, since the type of disc used has an important effect upon fidelity.
Cellulose-Coated DiscsThis is the best grade of disc made today for instantaneous recording purposes, and consists of a heavy aluminum disc which is coated on each side with a special cellulose compound (cellulose nitrate is often used). This coating is soft enough to be cut by a steel or sapphire cutter needle and yet durable enough so it can be played with a steel reproducing needle immediately after cutting. The cellulose compound is carefully prepared and filtered to remove impurities which might dull the cutting needle and produce surface noise, and is applied in such a way as to produce a mirror-smooth surface of uniform consistency.
The use of an aluminum base for the cellulose makes the disc unbreakable and non-inflammable. The thread cut from the disc during recording is highly inflammable, however, and should be placed in a close metal container filled with water immediately after a recording is made.
The highest quality cellulose-coated discs are available in various diameters from 6 inches to 16 inches, with the larger discs naturally being considerably higher in price. Low-priced cellulose-coated discs in diameters from 6 to 12 inches are also available, these being intended for test recordings, and for uses where high-fidelity and permanence are not essential. The aluminum base on these discs is considerably thinner than on the standard coated discs, making the records flexible and less durable.
Either sapphire needles, hand-polished steel needles or diamond needles are required for cutting cellulose-coated discs. It is highly essential that the needle be sharp, properly ground and mounted at the correct cutting angle. The average cutting life of a sapphire needle is from 10 to 15 hours, after which it should be sent back to the factory for re-sharpening. These needles may be re-sharpened about eight times, according to one manufacturer. Hand-polished steel needles of high quality give results practically equal to those of sapphire needles for one or two 16 inch recordings, but surface noise increases rapidly with additional recordings as the needle becomes dull. The choice of needle will depend upon the nature of the particular job at hand.
Cellulose-coated records can be used at either speed, but will have slightly more surface noise at 33 1/3 rpm. Pressure on the cutting needle is quite critical, and must be carefully adjusted to give a groove depth of .003 inch, this will make the thread which is cut from the groove about the thickness of a human hair. On some recorders a counter-balance spring or weight is provided for adjusting the depth of cut, placing a weight of from one to three ounces on the needle, while others have in addition an advance-ball, which is a piece of sapphire about the diameter of the lead in a pencil, with a rounded and highly polished end. This advance-ball rides on the cellulose surface of the disc, about 3/16 of an inch ahead of the cutting needle, and is so mounted as to maintain automatically a constant depth of groove throughout the recording regardless of variations in the density of the wax.
Surface noise on a properly cut cellulose disc is extremely low for about six playings of the disc and as many as 100 playings are possible before surface noise makes the recording unsatisfactory for further use. Record-preserving fluids are available for application immediately after cutting; these tend to harden and lubricate, the grooves, reducing the wear which is caused by the reproducing needle. The use of a fluid such as this is recommended when discs are to be preserved for any period of time and are to be played back many times.
Aluminum DiscsAluminum disks, with their surfaces polished to mirror brightness, are considerably lower in cost than coated discs but have a higher surface noise, and do not give as true reproduction. Diamond-pointed cutting needles are required for aluminum, with one needle having an average cutting life of several hundred hours. Considerably heavier needle pressure is required, from 15 to 18 ounces, and consequently recorders designed both for coated and aluminum discs must have some means of varying the needle pressure. One manufacturer supplies a special weight which is attached when cutting on aluminum.
Experience will enable you to determine when the fidelity available from a modern aluminum disc will be satisfactory for a particular job. Another point to bear in mind is that aluminum discs are more durable and have a longer life in any climate than coated discs; for this reason aluminum might be preferable when recording voices of children for preservation over a period of years. Non-metallic needles, such as those made of thorn, cactus, bamboo or fiber must be used when playing aluminum discs. Incidentally, pre-grooved aluminum discs are intended only for the lower-priced home recording systems; you will always use blank discs, for professional quality recorders all have means for properly spacing the grooves.
The foregoing information, together with the analysis of the market for recording in your locality should enable you to choose intelligently a suitable sound recording system for your particular business. Various systems which are considered suitable are illustrated in this article, but there are many others on the market which will prove satisfactory. Select your unit carefully, after studying all available literature and information, for upon your choice hinges to a great extent the success of sound recording as a side-line for you. You can, of course, secure literature free on request from the manufacturers of sound recording equipment.
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