This document provides a patent specification from 1957 for improvements relating to the inversion of television signals. Specifically, it describes a circuit using a variable-mu pentode that can derive a positive signal from a negative motion picture film, or vice versa. The circuit takes advantage of the non-linear characteristics of a variable-mu pentode to approximate the negative power law required to correctly invert the signals in a way that produces acceptable pictures without compression artifacts.

1. * GB780069 (A)
Description: GB780069 (A) ? 1957-07-31
Improvements in and relating to the inversion of television signals
Description of GB780069 (A)
PATENT SPECIFICATION
Date of filing Complete Specification March 23, 1955.
Application Date March 24, 1954.
780,069 No. 8650154.
Complete Specification PublishedJuly 31, 1957.
Index at acceptance.:-Classes 40(3), F2F2; and40(6), N(1B: 2M: 3M).
International Classification:-HO3f. H04n.
COMPLETE SPECIFICATION
Improvements in and relating to the Inversion of Television Signals
We, JOHN LLEWELYN BLISS, of Derwent House, Barnet Lane, London, N.20,
and PETER RAINGER, of 1, Ormesby Way, Kenton, Harrow, Middlesex, both
British Subjects, do hereby declare the invention, for which we pray
that a patent may be granted to us, and the method by which it is to
be performed, to be particuarly described in and by the following
statement: -
The present invention relates to the inversion of television signals.
In deriving television pictures from motion picture film it is usual
to employ a standard positive film such as is used for optical
projections. Considerable economy of time and money can be achieved if
a negative film is used, and the resulting signal, hereinafter
referred to as a negative signal, is inverted, that is to say
transformed into a positive signal, by electrical means.
The simplest means of effecting such a transformation is by a simple
phase reversal, using a transformer or an additional amplifier stage.
The resulting pictures are, however, most unsatisfactory, having
extreme compression of the white end of the tone range. This
distortion is most objectionable except in cases where the range of
transmission of the negative is very small, as, for example, when the
original subject is dull and lacking in contrast.
This compression arises from the intrinsic nonlinearity of the

2. photographic process. In normal photography the non-linearity of the
negative is compensated by a similar nonlinearity in the positive
printing process.
Thus over a range of subject brightnesses the density D obtained by
exposure to a subject brightness B is of the form D=ylog[,, B/Ba,
where y is a constant determined by the particular photographic
emulsion employed and the processing conditions and B. is another
constant. With normal cinematograph technique y is about 0.6 to 0.7.
When a negative film is scanned the resulting signal is proportional
to the transmission r of the film. Since the density D is equal to
[Pric 3s. 6d.1 -log,0T, it follows that T is proportional to B 'y.
In order to produce an acceptable picture it is necessary to derive a
signal S such that S is 50 proportional to By,, where y7 has a value
in the neighbourhood of 0.4. Assuming y=0,6 it is, therefore,
necessary to introduce into the signal path a device yielding an
output proportional to B04 when the input is proportional sS to B-6',
or, in other words, such that the output voltage is proportional to
the negative v power of the input voltage.
Circuits designed to achieve this result have been proposed but those
so far known have 60 been complicated, and it is the object of the
present invention to provide a relatively simple circuit whereby a
close approximation to the required result can be obtained.
The invention is based upon the realisation 65 that a variable-mu
pentode can be arranged to provide a relation between grid voltage and
anode current suitable for effecting the required correction.
The non-linear characteristics of a variablemu pentode are well known
and it has been proposed to use circuits including a variablemu
pentode to correct or modify a characteristic curve. Thus a circuit
has been proposed for use in television apparatus, including 75 a
variable-mu pentode operating in parallel with a triode to correct for
the characteristics of a cathode ray tube. However the characteristics
of such a circuit differ from those required for the solution of the
particular 80 problem with which the present invention is concerned.
Another circuit has been proposed for use in sound film apparatus to
correct for the characteristics of a sound track on a film, The problm
in this instance is different from 8S the present problem since a
sound waveform is very different from a telvision waveform, excursions
being made either side of the datum level corresponding to zero
signal, and the conditions under which the variable-mu pentode was
operated would not be suitable to deal with the present problem.
78,069 According to the present invention there is provided apparatus
for deriving a positive signal from a negative motion picture film or
vice versa comprising means for scanning a negative or positive
picture film and deriving negative or positive picture signals,

3. respectively, therefrom and a circuit for inverting the picture
signals comprising a variable-mu pentode having its control grid
biased approximately to cathode potential, or slightly positive
relatively to cathode potential, the picture signals being applied to
drive the control grid negatively, and the circuit being so
constituted that inversion of the signals takes place between the
input and output of the circuit.
The process when a positive picture film is scanned may be used in
recording by applying the negative signals to produce a negative
picture, and photographing this negative picture to produce, directly,
a positive picture fihlm, thus avoiding the need for a printing
process when a positive picture film is required.
In film scanners of the "flying spot" type it is usual to suppress the
scanning beam at the end of each line and frame in order to eliminate
spurious signals which would otherwise occur during the retrace or
flyback of the scanning spot. During these suppression intervals the
output is zero, corresponding to true black when positive film is
scanned, but corresponding to an intense white, exceeding any white
intensity in tlie picture, when negative film is scanned. This white
signal has to be removed by means of suitable blanking pulses.
In "' flying spot" film scanners the signal is usually clamped prior
to the insertion of blanking pulses, the clamp being driven during the
flyback intervals, when, as has been stated already, the signal level
corresponds to absolute black when positive film is scanned. When
negative film is used the large white signal generated must be
replaced by a signal at or near black level in order that the clamp
circuits in the blanking pulse mixer can operate correctly.
According to a feature of the invention the required signal is
introduced by cutting off the anode current of the variable-mu pentode
by means of a negative pulse applied to its suppressor grid. The pulse
timing and duration must be such that it embraces the clamp pulse
interval. It has been found possible to use standard synchronising
pulses for this purpose. The resulting signal lies very near true
black level as picture blacks reduce the anode current to a very low
value.
The invention will be described by way of example with reference to
the accompanying drawings in which Fig. 1 is a circuit diagram of a
preferred embodiment, and Fig. 2 contains idealised waveforms, not to
scale, at various points in the circuit of Fig. 1 in response to an
input of sawtooth waveform, A signal to be inverted, of the waveform
(a) in Fig. 2, is applied, with black positive, through a terminal 10
to the control grid of a pentode V,. The amplified and inverted signal
at the anode of the valve V1 (Fig. 2(b)) is applied to the control
grid of a variable-mu pentode V. which is arranged by the flow of 7C

4. grid current and the action of a capacitor 13 and resistor 14 to
restore the D.C. component in such a way that full picture white
corresponds to earth potential. If preferred a conventional clamp
circuit may be employed for 25 this purpose. The curvature of the grid
voltage/anode current characteristic of the pentode V. approximates
closely to the required negative power law, and inverted and corrected
signals may be obtained at a terminal 15 from 80 the anode of a phase
reversing output stage V3.
However, the approximation to the desired law is improved, in the
example illustrated, by applying to the screen grid of the pentode V.
8S a waveform as shown in Fig. 2(c) which is that of Fig. 2(b)
inverted. This waveform is obtained by applying the signal from the
anode of valve V, to the control grid of a pentode V,, the valve V.
being coupled to the screen grid 90 of the pentode V through a
cathode-follower triode Vi. The signal has its D.C. component restored
by grid current in the pentode V4 and by the action of a capacitor 16
and resistor 17.
The effect of D.C. restoration may, in this case 95 also, be obtained
by means of a clamp circuit.
Negative-going pulses as shown in Fig. 2(d), which may conveniently be
mixed line and frame synchronising pulses, are applied at a terminal
18 to the suppressor grid of the pentode V_. Each pulse cuts off the
anode current in pentode V. during an interval within the line
suppression period and the waveform at terminal 15 is thus as shown in
Fig. 2(e). The output voltage during each pulse is near pie- 10o ture
black and may be used subsequently in the apparatus chain as a black
reference voltage. In this way the signal may be dealt with in the
same way as if it had been derived by scanning a positive picture. A
diode V, pre- 11 vents the suppressor grid of the pentode V2 from
being driven above earth potential.
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