International projectionist (Jan-Dec 1957)

The present optical vs. magnetic reproduction controversy invokes still another correspondent to voice his opinion. A Defense of Magnetic Reproduction YOUR ARTICLE "Is Magnetic Reproduction Worthwhile," with Mr. Robert Mitchell's reply in the January issue of your valuable journal has interested me very much. I do not wish to challenge the general conclusions at which he arrives, since the subject under discussion is highly controversial, and there are many arguments on both sides of the question. Let me deal first with some of the statements relative to optical tracks. The author states, for example, that "high frequency noise and distortion . . . seems to be . largely dependent upon the method of recording used, the variable density method suffering the most." The matter of relative noise levels of variable density and variable area tracks is debatable, and I agree that on a new, carefully processed print, noise from a variable area track is considerably (6 db) lower than on a comparable variable density print. With repeated runnings, the variable area track becomes successively noisier due to accumulation of dirt and scratches on the clear areas of the variable area track. The variable density track with an average density of 0.5 — 0.7 is not nearly so susceptible to this type of deterioration. The second statement that variable density suffers most from high frequency distortion is completely at variance with both theory and practice. In modern variable density and variable area recording systems, and with good laboratory control, excellent sound tracks can be produced with either system. The variable area track has the advantage of a somewhat higher volume level and a slight edge in signalto-noise ratio. Harmonic Distortion However, in the reproducing system the variable area track is subjected to severe harmonic distortion from two soundhead defects, whereas the variable density system is unaffected. I Tefer to the effects of non-uniform scanning slit illumination and of azimuth deviation. By DR. JOHN G. FRAYNE Engineering Manager, Westrex Corp., Hollywood It is shown in numerous articles in technical literature that uneven illumination and/or azimuthal error will introduce harmonic distortion in variable area tracks, the effects being greatest at the high frequencies. On the other hand, the only effect of Referee's Note At this point, since there have been so many adherents to both sides of the optical-magnetic debate in the pages of this magazine, IP feels that the most impartial introduction to this article is to quote Dr. Frayne: "... I have greatly enjoyed reading over the years the various articles by Mr. Mitchell in your valuable journal, and I trust that these comments will not be considered as a criticism of Mr. Mitchell, but rather an honest attempt to clear the air on the important subject of magnetic recording, and put the public discussion of this very important field on a solid foundation." azimuthal error on variable density is attenuation of output at the high frequencies. Both of these effects on variable area are reduced by reducing the slope of the wave-form; hence, the recent adoption of two double bilateral tracks which are preferable to a single bilateral of amplitude equal to the two component tracks. Now we come to the author's comments on CinemaScope magnetic track. He states that the latter is "extremely irregular when uncompensated by the recording amplifier." He then shows a curve (B of Fig. 3, IP, Jan. 57, p. 28) which is claimed to be the recording characteristic of the magnetic stripe. In actual fact, the recording characteristic for CinemaScope is essentially flat except for a boost of about 7 db at 50 cycles and about 2 to 4 db at 8000 cycles, and neither of these are dictated by the recording process. The pre-emphasis at the low end is compensated by the complementary post-emphasis in the pre-amplifier in the reproducer, and its only purpose is to reduce 60 or 50 cycle hum on reproduction. In studio production recording, a flat low end is generally used in recording. The slight amount of pre-emphasis at the high frequency end is put there to offset magnetic gap scanning losses in reproduction and to insure an overall flat response to 8 k.c. CinemaScope Characteristics A typical CinemaScope recording characteristic is shown in Fig. 1. A typical CinemaScope reproducing curve is shown in the same figure. The FIGURE 1 II 1 1 1 1 1 1 1 1 1 1 CINEMASCOPE CHARACTERISTICS i iii + 20 m a Z +15 UJ Z +10 o Q. UJ +5 UJ > 0 tt u 5 or ° ARECORDING PREEQUALIZATION B-REPRODUCING CHARACTERISTIC — B A -— ^ -10 2 0 IC 0 i X)0 10, 000 FREQUENCY IN CYCLES PER SECOND 22 INTERNATIONAL PROJECTIONIST MARCH, 1957