Intermodulation Distortion of low Frequencies in Sound Film Recording

Bibliographic Reference(s)

• 1 Leshing M. Ingman T. Pier K. : “Reduction of Development Sprocket Hole Modulation,” J. Soc. Mot. Pict. Eng. , XXXVI , 5 (May, 1941 ), p. 475 . EXTERNAL
• 2 Albin F. G. : “A Dynamic Check on the Processing of Film for Sound Records,” J. Soc. Mot. Pict. Eng. , XXV , 2 (Aug., 1935 ), p. 161 . EXTERNAL
• 3 The reference to 80, 100, or any percentage modulation of the light modulator has little significance. Reference to per cent modulation of variable-density film has no significance at all. If a modulator such as a light valve, deflected penumbra, or glow lamp, has fixed average intensity, the modulation may be then stated definitely in terms of percentages of complete or 100 per cent modulation. In this case, 100 per cent modulation is that level at which the amplitude of the wave is equal to the average or mean value of light intensity in which case the excursion of light intensity extends from zero to double the mean. EXTERNAL
• The use of bias (for ground noise reduction) and reverse bias (for level range extension) has complicated this concept. However, for a reference point, some arbitrarily chosen mean value of light intensity is established, and the percentage modulation is referred to this. The use of this mean value and 100 per cent modulation thereof imposes upon the film an excursion of exposure which the film may accommodate with reasonably low distortion. This is the customary procedure in studio recording practice. The percentage modulation values referred to herein are so reckoned. EXTERNAL
• 4 Hansen E. H. Faulkner C. W. : “Mechanical Reversed-Bias Light-Valve Recording,” J. Soc. Mot. Pict. Eng. , XXVI , 2 (Feb., 1936 ), p. 117 . EXTERNAL
• 5 The over-all effective gamma determined in this manner is the gamma of a simple curve, T p = KE n γ having only a single value of gamma, which curve includes the 2 points used for the computation of gamma. Generally, film characteristics as used in practice involve not only the straight portion of the H and D curve, but the toe and shoulder as well. The positive film H and D toe region is extensively used in variable-density sound recording. The curve involving both the straight line and toe portions requires multiple values of gamma for expression. To express such a curve in terms of gamma, and to take delta-db data to compute gamma, requires that the curve be scanned in short increments. The gamma for each increment as defined by 2 exposure values should be calculated by use of Formula 2. In practice it has been learned that the over-all effective gamma should not exceed unity to any appreciable extent in any portion of the film characteristic curve. Gamma in excess of unity is equivalent to expansion, and is particularly deterimental to speech intelligibility. Compression in the same degree is acceptable. Thus, if low and medium level recordings, made with ground noise reduction, are confined to the straight portion of the H and D curve (which portion has an over-all effective gamma of unity or only slightly higher), the higher level recordings will extend into the region of the toe of the positive H and D curve where the gamma is lower and some compression is encountered. This is the preferred condition of variable-density recording technique. For routine checking, 3 delta-db control points are required; the first at bias value of exposure, the second at unbias, and the third at reverse bias. The gamma as computed from the first two should lie between unity and 5 per cent greater. The gamma as computed from the second and third should lie between unity and 10 per cent less than unity. The ability to check these 2 ranges separately is a unique advantage enjoyed by the delta-db test over the intermodulation test. For this paper, the term “Effective Over-all Gamma” has been adopted to represent the exponent of the over-all response curve, which is plotted with the current in the modulator as the abscissa and the photocell output current as ordinates. Effective over-all gamma thus includes all factors in the system which affect contrast or linearity, and is the figure which must be reduced to unity for linear performance of the system. The use of the term gamma in this manner has aroused some criticism, but the criticism is based upon academic rather than practical considerations. EXTERNAL