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

of the television recording would be identical to that of the original and represents an idealized case. Unfortunately the tone reproduction of the original television camera varies according to the camera tube employed and in most cases is far from perfect. The B.B.G. has in use six types of camera. If correction is applied it should be done in two steps, the first one in the camera chain to obtain better tonal characteristics for direct transmission, and the second step in the recording process to compensate for its deficiencies. At the present time, owing to signal/ noise ratio, absence of black reference level and other considerations, it has not been found possible to correct each type of camera to give a standard transfer characteristic. (6.1) Transfer Characteristics of Recording Process Six transfer characteristics are involved as follows: (a) Amplitude linearity of the electrical equipment associated with the display unit. (b) Transfer characteristic of the cathode-ray tube used for recording. (c) Optics of the recording camera. (d) Overall characteristic of the negative stock and its development. (e) Grading of the print. (f) Type of printing stock and its development. (a) The amplitude linearity of electrical equipment can be made very good indeed. The B.B.C. specification for the Lime Grove 35mm installation specified a limit of ±2% up to 100% modulation and an additional limit of ±5% up to 150% modulation. The contrast-correction amplifier provided, however, considerably modifies this response (see Sec. 6.3). (b) The transfer characteristic of a recording picture monitor may be taken by applying a special test waveform as described in Sec. 5.1, or by applying a number of waveforms in turn, each giv ing a uniform screen brightness of different value. (c) The lens system of a recording camera may give rise to multiple reflections from the lens surfaces themselves and from the lens barrel. These produce a naze over the picture which affects the slope of the transfer characteristic, particularly in the dark regions. (d) The normal density/log-brightness characteristic of the negative stock is somewhat modified by the short exposure period obtained in television recording. Even with a long-afterglow phosphor this is short by photographic standards, and the law of reciprocity breaks down. The effect is to sharpen the toe of the curve, which is an advantage since the straight portion is slightly increased. The development control gamma of the negative must be such that the overall mean contrast gradient of the recording process is approximately unity. (e) The grading of the print follows normal motion picture practice, less frequent shot-to-shot grading being required, since this is controlled by the television camera operators. (f) A standard motion picture technique is used in order to simplify the speed and ease of obtaining copies. The electrical contrast correction in conjunction with the negative development provides a negative of standard characteristics. (6.2) High-Gamma Recording The contrast range which can be accommodated in the recording cathoderay tube is limited by the various reflections which occur (see Sec. 5.4). If the contrast range is reduced by lowering the contrast gradient electrically, then the effect of these reflections is minimized. In the Lime Grove recording installation the contrast-correction amplifier reduces the electrical contrast gradient to approximately 0.4 over the mid-tone range, and allows an increase of this figure to about 0.8 in the highlights and shadows. The negative control gamma 382 April 1953 Journal of the SMPTE Vol. 60