International projectionist (Jan 1959-Dec 1960)

This diagram illustrates the construction of a magnetic head. acteristic which exacts a minimum penalty for slightly incorrect record level adjustments. Audio recordings can be stored indefinitely or replayed thousands of times with no deterioration of signal. And tape still is the only practical means of producing professional quality stereophonic sound where more than two channels are involved. BASIC COMPONENTS OF A MAGNETIC RECORDER A quick review of the components of a magnetic recording system will aid in our understanding of the record and reproduce processes. It will point out some of the principles involved, and the precise assembly techniques necessary for professional quality equipment. Magnetic tape: Modern magnetic recording tape consists of myriads of minute iron oxide particles suspended in a synthetic resin binder and deposited on a plastic backing. The iron oxide material is of course the actual magnetization medium, and it is very important that it can be distributed uniformly on the face of the tape. (Other characteristics necessary for television tape will be described when we discuss the video application.) Magnetic heads: No assembly in a magnetic recording system is more important than the heads, which convert electrical current to a magnetizing force during the recording operation, then reconvert that magnetism to an electrical current during the reproduce mode. As we shall see later, the heads are the primary limiting factors in determining the frequency response of a system. Professional quality equipment employs three separate heads — erase, record, and reproduce — each specially designed to perform its specific function. The Recording Head The operation of the record head is essentially the same as that of an elec tromagnet. If we insert a core of permeable material within a coil of wire, then run a direct current through that wire, we can set up an intense magnetic field that will attract any nearby material which is capable of being magnetized. If instead of the direct current we used an alternating current, we would first attract then repel that material (at a rate controlled by the frequency of our a-c) until it assumed a position that was neutral in respect to the alternating field. In a magnetic recording head the core is shaped like an incomplete ring — the discontinuity forms the head "gap" — with a coil of wire wrapped around it. When the signal to be recorded is converted to an electric current and passed through the coil, a strong magnetic field is created across the gap. If we now pass our magnetic tape across the gap, the metallic particles in the tape will assume a magnetic pattern which is a function of the instantaneous magnitude and polarity of the original signal. The Reproduction Head Although the reproduce head is constructed almost identically the same as the record head, its operation is fundamentally different. This head functions more like an electric generator. When we move a conductor through a magnetic field, as we do in a generator, we induce in that conductor a voltage whose amplitude is a function of the magnitude and direction of the magnetic flux and the velocity with which the conductor cuts through the field. We can, of course, achieve the same results by passing the magnetic field across a stationary conductor, as the only requisite is that the conductor must cut the lines of force. (Note here that, assuming a constant field, the amplitude of the induced voltage is dependent upon the speed with which the conductor cuts the lines of force; this is an important factor in the low-frequency response of a magnetic recording system.) Similarly, when we move the recorded tape past the gap in a reproduce head the magnetic flux on the moving tape will induce a voltage in the head coil, just as happened when we moved the magnetic field across the conductor in our generator. This induced voltage will be proportional to the number of turns of wire on the head coil, the permeability of the core material, and the time rate of change of the magnetic flux. Assuming a constant tape speed across the head, the last factor means that the output of a given reproduce head will increase directly with frequency (as frequency rises the recorded wavelength of the signal on the tape is shorter, resulting in a greater rate of change of flux across the head gap for a given tape speed). The Tape Transport The function of the tape transport is to move the tape accurately across the heads at a precisely constant rate of speed. We can consider that all tape transports consist basically of three major divisions — first a tape supply system, then a tape drive system, and finally a tape takeup system. These divisions can be likened to two reservoirs with a pumping station between them that removes material from one reservoir and adds it to the other. Most professional quality equipment employs three motors, one each for the supply system, drive system, and takeup system; however if weight or volume is important (such as in portable machines) high quality results can be obtained by using one motor to drive the tape and employing mechanical coupling to the supply and takeup turntables. Usually the tape supply and tape takeup systems can be considered as identical assemblies, with the only probable differences being in the brake band configuration and the connection to the Shown here is a professional quality tape transport mechanism which is used extensively by recording studios and radio stations. INTERNATIONAL PROJECTIONIST APRIL 1959