Projection engineering (Jan 1932-Mar 1933)

Page 12 describe briefly the external appearance of the various types of condenser microphones in use at the present time and the general details of their mechanical construction. The Associated Equipment For reasons that have already been mentioned and that will be explained more fully in the second article, it is necessary that the head of the condenser microphone be located close to its amplifier. The nearer they are together the better, and at the very most they should not be separated by a distance greater than twenty-five feet. The types of condenser microphones differ mainly in the number of stages of audio amplification employed in the amplifier. There are microphones one-, two-, and three-stage CTAs. The output level of a condenser microphone with a twostage CTA is usually eighteen to twenty db. higher than the output level of a single-stage microphone, but otherwise there is little difference in them. All else being equal, a two-stage or threestage microphone is undoubtedly the best to use because the more amplification in the condenser microphone the higher the electrical level of the speech current passing through the microphone cable. Then if there are any electrical disturbances picked up by the cable, the ratio of speech current to noise will be high, and since less amplification will be needed in the main line amplifier to obtain the necessary electrical level, the disturbance will not be as much amplified as it would if a single-stage microphone were employed. In all three types of microphones, the head is usually held in a metal housing that is attached directly to the cylindrical or cubical heavy metal case which contains the amplifier. If the microphone is of the suspension type, this amplifier case is equipped with a bail on the end farthest from the head for the purpose of attaching it to a microphone boom or other support. The housing in which the condenser transmitter is supported is pivoted so that it can be tipped to change the angle which it forms with the amplifier case, and a short length of shielded conductor connects it with the terminal plate on the amplifier. In the older type of condenser microphone, a length of shielded cable ending in a PROJECTION ENGINEERING special locking plug was attached to the microphone, but in the newer microphones the special plug is built right into the amplifier case on the end to which the bail is fastened. A similar but longer cable having a braided metal shield over the conductors and an outside covering of rubber, equipped with plugs on both ends, and known as a microphone cable, connects the microphone to a junction box built into the wall of the sound stage. This junction box is usually constructed to accommodate the cables from six microphones, and it is equipped with fuses in the filament and plate battery leads to the microphone amplifiers. The plate batteries are placed in a metal B battery case near the junction box and connected directly to it. The suspension type of condenser microphone that has just been described is employed mostly for motion picture Sound recording work and is usually supported on a microphone boom, while the other two forms of the microphone, the desk and floor types, are used principally in radio broadcast and public address work. The desk-type condenser microphone usually has the amplifier built in a square box with the transmitter housing supported solidly on top of it in a special frame or set into the face of the box. In the floor type of microphone the condenser head is fastened to the top of a slender vertical pillar. The height of this stand is adjustable from four to six feet, permitting the transmitter to be raised or lowered to accommodate the height of the speaker. The amplifier is contained in a box in the base of the stand. These various types of condenser microphones all operate on the same principle and are identical in internal construction — the only variation in them being in the mechanical features. Fig. 2 illustrates the input circuit wiring of a carbon microphone. It will be understood that as the diaphragm moves in either direction the current through one side of the transformer will increase while current through the opposition side will decrease. This effect contributes to the reduction of microphone hiss caused by the normal current flowing through the carbon granules; also reduces distortion due to even harmonics. The battery current flowing through the microphone transformer Fig. 7. Condenser microphone complete. Courtesy Jenkins & Adair, Inc. Fig. 6. The floor-type of. condenser microphone. Illustrating mounting pillar construction. creates opposing magnetic fields and is therefore balanced. The microphone shown in Fig. 7 is of the condenser type, widely used in sound-picture work. The mounting is practical as the suspension allows the microphone to be adjusted outside the focus of the cameras without difficulty. SCREENS FOR TELEVISION THE screen upon which the projected television image is thrown is second only in importance to the scanning system used, according to William Hoyt Peck, inventor, whose new television receiver is now being demonstrated. Peck tested numerous types of screens during his work with color motion pictures for many years. For projecting pictures from behind the screen, he found waxed cloth superior to the various glass screens which had formerly been used. He therefore used that type of screen for his television projector during early experiments. Later, however, he determined to try glass with a specially ground back, and found this to give about twice as brilliant an image, though it had not worked nearly so well for motion pictures. He further found that the image was much clearer when projected against the shiny surface of the glass, the ground side being toward the audience. The system which Mr. Peck is demonstrating utilizes a novel method of scanning, in which fully corrected reflecting lenses transmit all the light from the crater tube to the screen without distortion.