Journal of the Society of Motion Picture and Television Engineers (1950-1954)

ewes represents the octave loss at each fluency since the speed change was a or of 2. The total accumulated of a given frequency represents the si i of losses for all octaves down to the pnt where the curves coincide. For mple, the accumulated loss at 15 kc lals the sum of losses at wavelengths 1 1, 4. 8, etc., mils. This accumulated thus obtained at all frequencies is si wn in the lower curve. his method of measuring frequency has the advantage of using tape d|sctly but has the disadvantage of umulative error as well as somewhat u steady readings at shorter wavek|gths. variation of this method could be ud in which the speed is made conti lously variable over a wide range. Kwever, this is not available on mac nes in the field whereas most mac nes have two speeds. There are various electrical methods 0 approximating these frequency losses. P;ure 6 shows the circuits for three othem. One method is to feed a consnt current source through the playb:k head and measure the output of t system. The voltage across the 1 id approximates the condition of innced voltage produced by a flat surf;e induction of the tape but with no we length losses. However, the flux j ih is not the same as it would be from t >e since all of the flux passes across the f nt gap where different frequency losses my be found. Also care must be taken t prevent resonance of the head within t: frequency range of the measurer-nts since this would cause incorrect ciluation of frequency losses. Another method which is perhaps re accurate (in that the flux path in ; head is the same as it would be from a :orded tape) is the use of an electrongnet placed across the head gap. In t.s method we have in effect a stationary ignet of fixed wavelength whose strength in frequency. This method to be accurate requires evaluation of frequency losses in the electromagnet itself. The last method also makes use of a stationary magnet of fixed wavelength and variable frequency across the gap. In this case, a conductor is placed parallel to the gap; a constant current passing through the conductor produces a constant induction. The conductor used was a brass strip of greater width than the length of the gap. The four curves in Fig. 7 show the frequency losses of the system obtained with all four methods. The A curve shows the losses measured by feeding constant current through the head, the B curve shows those measured by the electromagnet method, and the C curve shows those measured by the conductor method. The B curve was corrected for electromagnet losses. The D curve shows the losses measured by the method which uses tape at variable speed. This curve agrees very closely with the A curve. In fact, the curves all correlate within ±1 db. The D curve was used to calibrate the system for the curves to be shown later. The reproduce system has now been calibrated and the wavelength losses and frequency losses have been measured separately. The next steps can now be taken; measuring the output of the unknown tape and computing the surface induction by correcting the output for those losses. In this case, the tape used in the experiment was a recording having the standard characteristic for the RCA RT-11B tape recorder at 15 in./sec and 7.5 in./sec. Figure 8 shows the resulting surface induction at the two speeds. The lower curves at each speed show the output of the system which has been graphically corrected by 20 db per decade. The dotted curves show the outputs corrected for frequency losses and the upper curves are corrected for frequency and wavelength losses, giving the resultant surface induction. It should be noted that an additional J. D. Bick: Measuring Surface Induction 521