Patent GB-1150625-A (GB)
Title: Electronic Image Transmission System
Inventor: BOERSMA SIPKO LUU
IPC: H04N7/00
Filed: 1966-06-01
Abstract: 1,150,625. Image transmission. S. L. BOERSMA. 1 June, 1966 [3 June, 1965], No. 24478/66. Heading H4F. [Also in Division H1] In a transmission system the picture to be transmitted is imaged on to the left lower quadrant of a vibrating piezo-electric crystal 2 in the form of a thin rectangular plate, Fig. 2, mechanically excited at its corner by means of a transducer 4 fed by a signal V(t), e.g. a noise voltage, in such a way as to make all the crystal modes vibrate at equal amplitudes. The crystal has a conductive coating 1 and a built-in semiconductor PN junction, and close to its surface is a conducting mesh 3. The mechanical stresses in the crystal produce an electric field perpendicular to the surface of the crystal and proportional thereto, and due to the two electrodes this field causes a current to be produced perpendicular to the surface which gives over the PN junction a voltage difference proportional to the mechanical stress. This A.C. voltage is rectified by the PN junction so that the N layer acquires a uniformly distributed positive charge. When the left lower quadrant of the crystal is illuminated by the picture to be transmitted the light intensity at each spot of the PN junction 2 creates a local photo-current that tends to lower the positive charge of the N layer giving rise to an extra A.C. current proportional to the picture intensity. This current is integrated over the whole of the crystal area by electrodes 3 and 1 and due to the orthogonality (see Specification) of the component picture intensity signals the output signal U(t) comprises a plurality of carrier waves each modulated by an amplitude coefficient of one of the component picture intensity signals. U(t) contains all the necessary image information for direct transmission in a video channel and, as shown in Fig. 2, is amplified, 5, and fed to receiver crystal 6, 7, 8 that is identical to the transmitting crystal. The transducer 9, fed by the same noise voltage, excites all the modes of the receiving crystal with equal magnitude so that at the PN junction 7 an A.C. voltage is formed to which is added the amplified video signal U(t) fed to conducting electrode 6. Junction 7 is a rectifier and acts as a detector, its output being time averaged by means of the capacitance of the junction 7 and the capacitance between the N layer and the conducting mesh 8. There is thus formed in the N layer a charge distribution. In the left lower quadrant of the receiver crystal a charge picture is formed that is a true picture of the object to be transmitted and in the right upper quadrant an inverted picture appears with negative polarity, Fig. 1 (not shown). The charge, or rather current, image may be made into a visible picture by letting it modulate a wide electron beam 10 so that a visible image of either the normal positive picture or the inverted negative picture is formed on fluorescent screen 11. The function of elements 1 and 4 are interchangeable, when V(t) is fed into 1 the video output U(t) appears at 4 and when both V(t) and U(t) are fed to 1 and 4 or to 4 and 1 the charge image is formed in the junction plane. In this system there is no definition limiting line pattern and optical unsharpness in the transmitter can be corrected at the receiver by electric filtering in the video channel. When moving pictures are transmitted the carriers (lines) develop side bands. There are gaps in the middle and lower parts of the line spectrum. A sound signal may be added in the gap at the lower end of the spectrum and in the other inter-carrier spaces two extra series of carriers may be accommodated from two extra crystals, e.g. colour transmission may be achieved at no extra bandwidth, only the highest frequencies (the fine detail) remaining uncoloured. More than one transducer may be used to reproduce the pattern of vibration in the receiver. Instead of the piezo-electric semi-conductor crystal a stress-optic bi-refringent crystal may be used placed between polarizers, the transmitted light being collected on a photo-cell. If the amplitudes of the video carriers are transmitted by ordinary amplitude modulation, i.e. without suppressed carrier, then signal V(t) is not necessary for correct demodulation and U(t) may be fed into both the transducer 9 and electrode 6. With this arrangement the system is insensitive to phase distortion in the video channel In a second embodiment, Fig. 3, an auxiliary quantity is expanded instead of the intensity distribution itself. As shown, the picture to be transmitted is the transparency 16 and at least one plate 13 of transparent material in which a set of vibration modes, i.e. standing wave patterns which vary the refractive index of the plate, is sustained by means of a noise source V(t) and transducer 14, is illuminated by a monochromatic plane wave 12 and the light diffracted by the plate illuminates the object 16, the transmitted light which is proportional to the light amplitude of the object, being focused on to a photo-electric cell 18 together with light 19 and 20 from the original light source. The resulting video signal is transmitted via amplifier 21 to a transducer 22 which sets up a set of vibration nodes in a plate 23 identical to plate 13. Plate 23 is illuminated by a plane monochromatic wave whose frequency may be different from the light used in the transmitter and the different diffraction orders are arranged in the image plane of lens 24. The eye squares this light amplitude distribution so that the image intensity is proportional to the square of the amplitude of transmission, i.e. to the intensity of the object. For a two-dimensional image the system includes two vibrating plates, the second plate being excited in the same way as 13 but with a phase difference of 90 degrees at all frequencies. Instead of two plates 13 one birefringent plate may be used having a 90-degree path difference between the ordinary and extraordinary ray. The video signal U(t) comprises a number of carrier waves whose frequencies are characteristic of specific functions of the current of the entire photo-surface 18 and whose modulation represents the amplitude of these functions. A mathematical analysis of the operation of both systems is given in the Specification. In the receiver of Fig. 3 a second positive image is formed inverted with respect to the first. Since the transmitted spectrum is a line spectrum the interline space may be used to accommodate two extra series of carrier frequencies for colour transmission. Thus three vibrating plates 13 of different dimensions may be used, diffracting red, green and blue monochromatic light on to the transparency 16. In the receiver the combiner signal is fed to three plates whose shapes correspond to the transmitter plates, each plate responding only to its own set of vibration modes. The plates are illuminated by red, green and blue monochromatic light and by optical means the three diffracted beams are superimposed to form a single colour picture. In both Figs. 2 and 3 standing wave patterns are produced but for a one-dimensional image a travelling wave may be used.