In the days of the phonograph with its cylindrical wax records, it was fairly commonplace to make one’s own records. Unlike the modern gramophone the old phonograph was fitted with a lead screw for keeping the sound box, or, as it was then more commonly called, the reproducer, tracking correctly in the spiral groove of the soft cylinder. All that was necessary for recording was the substitution of a sharply pointed needle in place of the ball-ended reproducing needle, so that the diaphragm when thrown into vibration made a varying depth of cut. Cylindrical wax records reproduce sound by change in the depth of the groove, thus differing from the modern method of recording, in which the groove is in the form of a wavy line of uniform depth. Attempts at home record-making were discontinued with the coming of the flat disc and its transverse recording, and with the development of valve amplifiers and electrical reproducers, the production of equipments for home recording is again receiving attention. This time, however, home recording will establish itself. While almost every radio set can now be used for the electrical reproduction of records, the next adjunct likely to become standard is the provision of a recording cutter. This, developing side by side with the home cinematograph, is leading in the direction of the radio set with its valve amplifier becoming a complete home entertainer having many functions.
Several home record-making equipments have recently appeared on the market. While there is undoubtedly a need for this class of apparatus primarily for making records for broadcast transmissions, the development is likely to receive a setback unless the initial results are reasonably satisfactory. Tests reveal that home recording is now possible, resulting in a satisfactory degree of perfection.
Powerful Motor Drive Required.
The following is a description of the home recorder developed by Messrs. S. G. Brown. Ltd., and which was originally demonstrated by Mr. Sidney Brown at the Institution of Electrical Engineers, as this serves to illustrate the main problems of design and the results obtained. Those who have experimented with recording quickly appreciate that the first difficulty is that of obtaining sufficient power to rotate the record under the recording cutter. As the recorder is much heavier than a soundbox a powerful motor drive is required and in addition the load on the motor is made greater by the sharp cutting point of the stylus, and the fact that its hold on the record is greatly increased as it vibrates. In the Brown equipment we find a very large type of double spring motor, normally capable of playing through a number of records. By the substitution of new pinions the rate of rotating the turn-table is unaltered, but the motor is discharged in the course of making a single one-side recording. In this way considerable power is rendered available. As resistance to rotation increases with the diameter of the record, its size has been reduced to 6in. across, giving normally about one to one and a half minutes’ playing.
A particularly robust form of construction is employed for the arm which carries the recorder. A heavy casting mounted in centres provides for the traverse across the record. This is guided by a spring-loaded point travelling in a buttress spiral thread cut in the under-face of the turn-table. A hinged joint suspended from its centres allows the recorder to be lifted from the surface of theturntable, and in so doing the guide pin is released from the thread on the underside. A trigger action lowers the recorder on to the record, at the same time engaging the pin on the underside as well as closing a contact which, brought out to terminals, provides for lighting an indicating lamp showing that the recorder is in action. A meter is also thrown into circuit.
The Recorder Tested.
In action the recorder operates the reverse way to a gramophone pick-up. Where, in the latter case, the vibrating armature generates a current, we have now a varying current actuating an armature. In design, therefore, the mechanism of a recorder resembles a loud speaker movement arranged to vibrate a pivoted cutter instead of a diaphragm. In this instance the design closely resembles the Brown “V” action loud speaker unit, arranged to impart movement to a sharply pointed diamond standing almost perpendicular to the face of the record. This point is adjustable and can be readily replaced, although owing to its hardness it shows no sign of wear.
Many materials may be used for recording upon, and that most generally adopted is aluminium. Soft materials like celluloid and bakelite are fairly suitable, but metal discs probably give better results. A mild steel disc, in fact, serves quite well, giving results superior to those obtained with bakelite. Examination of the spiral under the microscope reveals that the groove is made by turning up a burr on the metal as apart from actually paring the surface away. Bakelite, being brittle, is actually cut, the surface being removed as powder, and is less satisfactory. When reproducing from the surface of the aluminium record, however, a pointed steel needle sinks into the bottom of the groove, deepening the cut in the soft metal surface. It is better, therefore, to use either a needle with a rounded point or, preferably, a fibre needle. The Brown equipment examined was fitted with a recording microphone, which, being of a heavy-duty type, gave ample output. It should be noted, therefore, that with this equipment record making is effected without the aid of a valve amplifier. To operate the microphone circuit an 8-volt battery is required, and when speaking or singing close to the mouth-piece of the microphone a record is produced which plays with almost the average loudness of the ordinary gramophone record.
The quality of reproduction, while not being quite up to the standard of the ordinary record, is, nevertheless, pleasing, and particularly so when applied to the recording of broadcast transmissions. The range of frequencies covered is the same as that customarily handled by pick-up or loud speaker and its associated amplifier.
It is thought that the ultimate application of the home recorder will be that of making records from broadcast items. In this way records are obtained quite cheaply which, when played, give results comparable with the purchased record. The future, no doubt, will witness considerable development in the application of the home recorder to the broadcast receiver.