Projection engineering (Sept 1929-Nov 1930)

Page 10 Projection Engineering, Septetnber, 1929 brought into play, one to obtain long shots of the full orchestra in action and one or more to obtain closeups of various artists during the rendition of solo parts. These cameras must be installed in soundproof booths in order that the clicking of their mechanism may not be picked up by the microphones and reproduced audibly during pianissimo passages. The acoustical director has two major problems to consider. First, he must see that the arrangement of the artists and the acoustical nature of the studio is such that no troublesome echoes or reverberations exist, while, on the other hand, the studio should not be dead enough to produce a muffled record lacking in the desired brilliancy. A greater amount of reverberation is desired to give a pleasing brilliancy to an orchestral number that can be tolerated for the best articulation of solo singing or speech. Where both solo and orchestral music occur in the same program a compromise is usually attained. Microphone Placement and "Mixing' The director's second concern is the proper distribution of a battery of microphones, similar in form to the broadcast studio microphones, at proper points around the orchestra. These microphones, sometimes as many as six in number, are connected to an instrument in the recording booth appropriately known as a mixer. By means of this device the amount of electrical energy taken from each individual microphone may be varied at will in aggregating the total response which is fed to the recording mechanism. The placing of the microphones and the proper manipulation of the mixer controls to give the most realistic record con Above: A "variable area" film track, employed by RCA Photophone. Below: A "variable density" film track, employed by Electrical Research Products, Inc. stitute an art in which proficiency is attained only by experience. Once mixed, the energy level of the ensemble can be raised or lowered, but the relative importance of the several instruments cannot readily be varied. The composite electrical impulses are then amplified to a sufficient degree and fed into the recording mechanism. This is essentially a cutting stylus which is made to vibrate in strict accordance with the energizing current. In the "lateral" type of recording, this vibration is along a radius of the disc record so that the stylus cuts a spiral groove on the disc of constant depth (about 0.0025 inch) but of varying width. In the "hill and dale" type of recording, the vibration of the stylus is perpendicular to the surface of the disc A.L0W LOOP SO ~>A~ Fi^.i w\!E<, A^ROMUATELV IN ^'■^ W-* SOE OF HEAD WHEN INTERMITTENT HAS ;'-T.' CEASEO 1 SUPPORTING PIUS -) ( HEAD SPROCKE /-n\ lamp socket Details of ERPI film pick-up attachment on Simplex Projector. The light from the exciting lamp is brought to a focus on the film as a line 0.001 by 0.0S0 inch, thus covering the record with an allowance of 0.010 inch for variation of position. giving a spiral groove of constant width but of varying depth. The lateral method is used almost exclusively in sound picture records. The original wax is a disc of metallic soap from 13 inches to 17 inches in diameter and about 1 inch thick. This is initially given a high polish and is then mounted horizontally on a turntable driven at a uniform rate and synchronized with the film passing through the cameras. This synchronization is accomplished by electrical means of a highly technical nature. Stylus Travel Contrary to practice with ordinary phonograph records, the stylus is made to travel outward from the center of the disc at such a rate that it cuts a spiral having the pitch of about one hundred turns per inch. The table rotates approximately 30 revolutions per minute, which is less than one-half the speed of the ordinary phonograph. The larger records have a playing time of from eleven to twelve minutes and the stylus cuts its way over the wax at speeds -varying from 70 to 140 feet per minute. The Play-Back After the wax has been cut it is, of course, desirable to be able to play it at once in order to detect any flaws. For this purpose a special reproducer known as the play-back is employed. This is made extremely light so as to produce no appreciable wear on the relatively soft wax record. If the wax is declared satisfactory, it is then dusted with a fine conducting powder and electroplated, giving thus a negative copy of the wax, which is called the "master." By successive electroplating steps duplicates of the "master," known as "stampers," are obtained, from which large quantities of playing records may be pressed. By taking proper precautions the acoustical fidelity of the record is in no way injured during these processes. The Projection Let us now experience a quick transportation from the producer's studio to our favorite motion picture theatre. In the projecting booth a horizontal turntable is mounted beside each projection machine. This table is rotated by the same motor which drives the film through the projector so that, if the operator sets the needle of the reproducer at a marked point on the record coincident with threading a marked portion of the film over the driving sprocket, then perfect synchronism between sound and picture will be maintained. The reproducer is an electromagnetic pickup device driven by the needle as it vibrates across the irregularities in the spiral groove on the record. This device is quite