Patent GB-1479594-A (GB)
Title: Ultrasonic image pick-up tube arrangements
Inventor: IAN MACDONALD GREEN
IPC: H04N3/10
Filed: 1974-09-02
Abstract: 1479594 Ultrasonic image pick-up arrangemerit EMI Ltd 2 Sept 1974 [1 Sept 1973] 41246/73 Heading H4F In an ultrasonic image pick-up tube arrangement spurious signals, termed "whiteout" (caused, e.g. by mechanical vibration of the tube target by other than image radiation), in the tube output signals are sensed during periods when the tube scanning beam is blanked and subtracted from the total output signal at other times. As shown in Fig. 1, the signal output of an ultrasonic pick-up tube 1 is supplied by a head amplifier 2, passing signals of the frequency of the ultrasonic radiation irradiating the object and an A.G.C. circuit 3, to an adder 4 the output of which is supplied to a blanking circuit 6. A blanking signal generator 6 produces a special blanking waveform 2(b), Fig. 2, (compare with the blanking waveform 2(a) supplied to the tube beam) in which the blanking action is suspended for a small interval (for an integral number of cycles of the ultrasonic frequency, e.g. 4 MHz). The output of the blanking circuit 6 is fed to synchronous demodulators 8, 9 which have as respective second inputs the in phase ultrasonic frequency α and the quadrature phase ultrasonic frequency # from source 7. The demodulated outputs are fed to respective pairs of sample and hold circuits 10, 11 and 12, 13. Circuits 10 and 12 are supplied with sampling pulses 2(c) occurring during the suspended blanking interval so that a sample of spurious signals present at this time is taken and circuits 11 and 13 are supplied with sampling pulses 2(d) so that a sample of signals occurring during the normal blanking portion of waveform 2(b), by definition zero, is taken as a reference. The difference between the outputs of each pair of the samples and hold circuits is integrated in respective integrators 14, 15 to provide outputs which, via respective voltage controlled amplifiers 16, 17 control the amplitude of the α and # components which are supplied to adder 4 to cancel the whiteout (spurious) signals and give a corrected signal output S. Other spurious signals, e.g. spikes due to the line fly-back are minimized by blanking the signal input to the A.G.C. circuit 3 by means of a blanking waveform of the form shown in 2(b) so that if the spikes are of greater amplitude than the largest signal the A.G.C. does not act on them and the spurious signals are then "interpreted" as "whiteout" signals and eliminated as described above. This type of spurious signal may also be minimized by making the ultrasonic frequency an odd multiple of half line frequency so that the spurious signal from the synchronous demodulators reverses in sign from line to line and may be filtered out. The Specification includes a description of the manner of operating the arrangement for optimum results by increasing the number of samples which are taken per line scan, by compromising between the rate of correction and the signal-to-noise ratio and the manner of operation when the frequency of the whiteout signals changes from the frequency of the ultrasonic transmitter due to the Doppler effect when there is relative movement between the object (secne) and the pick-up tube and associated ultrasonic transmitting source.