The Story of the Record

Radio Record, Volume III, Issue 48, 13 June 1930, Page 3

The Story of the Record

How Electricity Aids in Bringing the Artist into the Home

ing hard record, when placed on r turntable, can produce in life-like realism the performance of an artist of world fame. The story is an interesting one and entails many processes. Following is an account of the whole process written by one who is in the nerve centre of the gramophone company, Hayes, Middlesex, England, where the H.M.V. records are produced. In this article (adapted from an original in "Wireless World’) Mr. BH. M. Payne (of the research staff) tells in a way that abounds in interest and simplicity how the record goes through its processes. from the studio to the finished record, We shall consider here the making of a record and follow it through its five stages, viz.:-(1) The recording*studio; (2) the recording system; (8) the original-wax.dise with its turntable; (4). the copying process; and (5) the commercial manufacture of records. The Recording Studio. [THIS should be considered rather as the place where the original sounds are made, for it must be remembered that a large proportion of the presentday recording is done in public halls or at open-air events. In general, chamber music, vocal, instrumental and dance recordings are made in the studios, whilst orchestral and grand organs have to be recorded in large halls or churches, which are often specially damped. Speeches at public functions must, of course, be recorded on the spot, and no special arrangements for "yfamping can be made. Damping in recording is just as important as in broadcasting. When the sounds have to be conveyed to the listener through a single channel and not as if he were at the actual performance, where there are many channels, special precautions have to be taken to give the right perspective. The music track of the record consists of a spiral groove of practically eonstant depth and cross section, having a spacing between the grooves of about 1/100th of an inch. Upon these grooves are the lateral cuts made by the recording stylus actuated by the sounds being recorded. ‘They represent the wave-form of these sounds. Assuming an amplitude or space taken by a wave created by the note of 586 eyeles per second, the amplitude of a note of 27 cycles-bottom "A" on the piano-would have to be 1-16 of an asf This condition is impracticable, Ei have all wondered how the shino as Such a large amplitude would cause thek.needle groove to trespass on the space allotted to the adjacent six music spiral grooves. It has been found most advantageous to carry the constant velocity system down to about 200 cycles per second only, and below this frequency the characteristic response falls away at almost constant amplitude so that there is equal risk of a wave breaking through from groove to groove on all notes below 200 cycles per second. This unavoidable defect in dise records is corrected in the best types of electrical reproducer by designing the pick-up so that its. voltage gutput characteristic curve is substan"tially the reverse of the recording curve as to boost up the bass notes and ve them their proper relationship with the treble. Fig. 1 shows the general fecording scheme, including a monitoring device. (a) The microphone used in the making of the highest quality records is

of the condenser type, which, in conjunction with its local amplifier, gives a substantial flat voltage output curve when plotted against frequency. The output from the condenser microphone is insufficient to operate the electromechanical recording stylus point without the aid of amplifiers, consequently the microphone and its associated amplifiers have to be designed so that the current delivered to the recorder movement is proportional to the original sound pressure upon the microphone diaphragm. (b) The main amplifier has four stages of amplification choke-coupled, the last stage, however, being of the "push-pull" type. The calibration curve of the recording amplifier shows a practically straight-line response at all frequencies from 50 to 5000 cycles per second. (ce) The electro-mechanical recording movement consists of a balanced armature electro-mechanical system. The armature axis is mounted on knifeedges and terminates in a rubber rod about 8in. long. The cutting stylus is attached to the end of a light bar in a vertical plane perpendicular to the armature axis. The proportioning of the various parts of this recorder is very carefully arranged so that their masses and elasticities form the elements of a mechanical equilavent of an electrical filter system, the terminating resistance of which ‘is represented

by the rubber rod along which the mechanical vibrations travel torsionally, and are thus completely dissipated. Two of the main problems of the recording expert are to obtain proper balance of the instruments or artistes in order to ensure life-like reproduction, also to gauge the loudness and probable wearing qualities of the recording. He is considerably helped in these respects by the use of a monitoring loudspeaker, with its associated amplifier, which is bridged across the recorder movement and whose input impendance is high, compared with that of the recorder impedance, go as not to

affect the overall recording characteristic. The Original Wax Dise. HE "wax" disc on which the original sound trace is cut is composed of insoluble metallic soaps, compounded with various agents to produce a fine and homogeneous texture, the whole being very highly refined, made into cakes, and then shaved on a facing machine to a mirror-like surface. The "wax" discs are about lin. thick, and have a diameter slightly larger than that of the finished record. © The wax is. placed on the heavy turntable of the recording machine. This machine is really a special type of lathe arranged so that the soft wax disc rotates in a horizontal plane, whilst the cutting stylus of the recorder movement cuts the trace of the sound-waves as it is fed toward the centre of the disc. Special precautions have been taken to ensure the even running of the recording turntable, as any slight waver will be the cause of imperfect reproduction. The rotating speed of modern records is precise ely 78 revolutions ‘ per minute, and is checked by means of a tuning-fork-controlled stroboscopic device, so that all sides of a complete opera on several discs will be in perfect tune. The shape of a cross-section of a record groove in common use is approximately a sector of a circle, being about 0.006in. wide at the top and 0.0025in,

deep. The point of the recording stylus is usually of sapphire, which retains its shape even after cutting many miles of wax (the length of track on both sides of a 12in. record is often well over one-third of a mile). The cutting stylus is ground to form a very sharp and clean-cutting edge; the "wax" ribbon which is chiselled off by the sapphire point is quickly removed, by air suction, so as to presérve the clean face of the wax disc, and the depth of cut may be kept constant by an "advance ball" which glides lightly on-the wax in front of the cutting point.

It will not be out of place here to mention some further difficulties of the recording expert. The wax has to be cut in a small room at the temperature of, a hot summer’s. day, and, in many cases, several waxes of the same musical item have to be made. The artis~ tie temperament. is very easily ruffled, and, since no blemish whatever is ale lowed in the finished record, the ree cording expert often has a very nervee straining job. The actual ‘recording machine, amplifiers, and monitoring device operate in an ante-room to the recording studio, and, as it is usually advantageous for the artistes to hear a "play-back" of the record which they have just made, a yery delicately-mounted electrical. pick-up is arranged to track over the newly-cut wax, and feeds into a loudspeaker in the studio itself. This "play-back" is only employed upon the trial waxes, as the soft wax may be damaged by the pick-up needle. Some readers may wonder 1 w such difficult subjects as the speech of H.M. the King at the opening of the Naval Conference, the Aldershot Tattoo, or even the nightingale in a Surrey garden can be recorded. This is, done by means of a mobile recording van, which is a replica of the recording ante-room and is complete with its own power supply ‘and monitoring arrangements. In the case of outdoor recording, several microphones often have to be run out from the recording van, and, in such cases as the nightingale, extreme patience had to be exercised during an all-night vigil. ~ In-our next issue Payne will describe the remaining process-copying and commercial manufacture,