International projectionist (Jan-Dec 1957)

iii i i i i i i i M i i ■ i MAGNETIC REPRODUCING CHARACTERISTIC i i i i F LAT REP HO 3U CI N I 1 SYSTEM + 5 o 0 Z z o 9; 10 LJ / > < 20 LxJ cr ' 100 1000 FREQUENCY IN CYCLES PER SECOND 10,000 20,000 FIGURE 2 peculiar shape of this curve is not, as Mr. Mitchell says, to compensate for the magnetic recording characteristic but for the magnetic reproducing characteristic. This is best explained by reference to Fig. 2. Here is a playback over a flat reproducing system of a magnetic frequency film recorded flat into the magnetic head. Ignoring the "bumps" at the external low end, the curve rises with a slope of 6 db per octave from about 60 cycles to 2 k.c. It then flattens out and finally drops as shown in the figure. The 6 db per octave rise is inherent in the method of scanning magnetic tracks. The emf (voltage) generated in the coils surrounding the head core is proportional to the rate of cutting of the magnetic flux recorded on the film, not to the flux itself. Now since the elementary magnets in the track become shorter inversely with the frequency the rate of cutting the lines of flux increases directly with the frequency. Thus, a 2 k.c. tone will produce twice as much voltage as a 1 k.c. tone, or as engineers say: "It increases 6 db per octave." Now why does the reproduce characteristic flatten out and finally decline? First, there are the well-known scanning losses exactly analogous to those met in optical scanning, except the magnetic slit (gap) is not so well defined. Then there is self-demagnetization within the tape which increases progressively as the elementary magnets become shorter and shorter with increasing frequency. The bumps at the low end are caused by the presence of shields and/or coil forms which set up alternate magnetic flux paths which do not thread the pick-up coils. Reproducing Equation The reproducing characteristic described above is elegantly described by the equation:* 12 3 e = K -t/x sin (vrf/fo) vrf/fo where K = a constant t = the demagnetization constant of the medium A = the physical wavelength of the recorded signal f = frequency of the signal f0 = cut-off frequency region 1 of the above equation accounts for the 6 db/octave slope 2 accounts for the demagnetization losses 3 accounts for scanning losses As I stated in the beginning, I do not wish to argue with Mr. Mitchell's conclusions. However, I do wish to emphasize that if these conclusions were arrived at on the basis of false presumptions of the physical process, then they should be suspected as being colored by the injection of incorrect hypothesis. If they are based on observations over an extended period of time under a wide variety of operating conditions, the conclusions may be valid regardless of basic misunderstandings of the magnetic recording method. Simplex Joins GPL Simplex, formerly International Projector Corporation, is now a subsidiary of General Precision Laboratory, removed from its status as subsidiary of General Precision Equipment Corporation. John L. Alden will continue as president and chief executive officer. The move, according to Hermann G. Place, president of GPE, was made "to bring about the dual advantage of placing greater manufacturing facilities under direct GPL control and of making GPL's extensive research facilities more readily available for work with Simplex's many projects in improving motion picture theatre projection equipment." Simplex also will handle current and future orders for airborne Doppler navigation equipment. GPL has recently received an Air Force contract for an additional $17,000,000 worth of Doppler equipment. RCA Nets $40,031,000 RCA has announced a net profit of $40,031,000 for last year, equal to $2.65 per common share. Also noted was that the corporation did the largest volume of business in its history in 1956. Sales amounted to $1,127,744,000, an increase of 7% over 1955. * Elements of Sound Recording, by Frayne & Wolfe, John Wiley & Sons, 1949. BE SURE YOUR NEW PROJECTION ARC LAMPS provide spot focusing. The entire burner assembly should be movable so that the position of the arc can be shifted to "feel" for the best screen light without disturbing the relative carbon positions or the equilibrium of the arc. ^ This is Another EXCLUSIVE FEATURE of the New Strong U-H-l PROJECTION ARC LAMP INTERNATIONAL PROJECTIONIST • MARCH 1957 23