Patent GB-757997-A (GB)
Title: Improvements in and relating to apparatus for delaying electric signals
IPC: H03H17/00
Filed: 1953-04-20
Abstract: 757,997. Signal delaying systems. PHILCO CORPORATION. April 20, 1953 [April 21, 1952], No. 10755/63. Class 40 (4). [Also in Group XL (c)] A signal is passed repeatedly through a backcoupled delay device, the signal having imparted to it a characteristic from which may be determined how many times the signal has passed through the delay device, means responsive to said characteristic being provided to accept the signal after it has passed through the delay device a prescribed number of times. In a first embodiment the video signal (0 to 10 mc/s.) is applied to a mixer 12, Fig. 1, together with the output from an oscillator 14 whose frequency sweeps repeatedly from 1000 to 4000 mc/s. as shown at 40 in Fig. 2. The side-bands after passing through the high-pass filter 16 are applied to the back-coupled delay device 18. The period of sweep T1 of the oscillator 14 is equal to the delay required to be imparted to the signal and is equal to an integral multiple, 5 as shown, of the delay of the device 18. The output of the delay device is fed to a further mixer 22 also fed from the sweep oscillator 14 and its output is fed to a low-pass filter 24 whose output as will be seen consists of the input signal delayed by a period T1. It will be observed that any signal 44 introduced into the delay line during the first fifth of the period T1 lies in the frequency range 1000-1600 mc/s.; a signal introduced during the second fifth lies in the range 1600-2200 mc/s. and so on. A signal having made a prescribed number of 'trips through the delay device is thus seen to differ in frequency from those having made a different number of trips by at least 600 mc/s. At the end of time T1 the signal 44 has made five trips through the delay device and the sweep oscillator is about to commence another sweep so that the signal 44 combines with a signal of 1000 to 1600 mc/s. in the mixer to produce the original signal of 0 to 10 mc/s. Other signals in the delay device produce signals of 600 mc/s. or more in the mixer and these are rejected by the low-pass filter 24. The attenuation of the signals around the closed loop is arranged so that any signals passing through the delay device ten or more times have negligible amplitude. Exact relationship is necessary between the sweep time T1 of the oscillator 14 and the desired multiple of the delay of the delay device 18. This may be achieved by the use of a delay line having a very accurately controlled delay time T1 but having a pass-band characteristic inferior to that of the delay line 18. Alternatively the output of the delay device 18 may be connected to a narrow band-pass filter which will produce a series of pulses spaced by the delay time of the device 18. These may be subjected to suitable frequency division and the resulting pulses used to synchronize the sweep frequency oscillator 14. Alternatively since any discrepancy in the delays will produce a frequency shift in the output, a pilot signal may be fed in and the variation in output frequency used to operate a servo mechanism controlling the sweep frequency oscillator 14. In a modification, Fig. 1A, which is immune from these minor frequency deviations, the sweep frequency supplied to the mixer is changed by 100 mc/s., being obtained by mixing the output from a 100 mc/s. oscillator 29 with the output from the sweep oscillator 14. The output from the mixer 22 consists of a carrier of 100 mc/s. modulated by the desired signal, and this is accepted by the band pass filter 30 and passed to a detector 31. In a second embodiment, in which the signal is carried by pulses, each trip through the delay device results in a slight displacement of the pulse, the signal being extracted when the total displacement reaches a prescribed amount. As shown, Fig. 3, the input signal A, Fig. 4, is sampled at times t1, t2, t3 . . . and converted in the coder 82, fed with pulses G, into a pulse code B comprising pulses a, b, c, d, any of which may be present or absent. Each set of four pulses is succeeded by a blank pulse position. The signals are then passed to a delay line 84, back-coupled through a flip-flop 88, the signals being required to pass through the delay line three times before being extracted. Accordingly it is arranged that the pulse interval T1p is at least three times the pulse width Tp. Furthermore, if Ts is the interval between codes is is arranged that delay line introduces a delay 1 TL equal to 2Ts + -T1p. Fig. 4 illustrates 3 at C, D and E the pulse positions after one, two and three trips respectively through the delay line. It is seen that the pulses E are once again coincident with the pulses G. The pulses G are therefore applied to a gate circuit 90 which is fed with pulses from the delay line, to pass only the pulses E to the decoder 92 from which the required output is derived. The pulses G are also fed via an inverter 87 to a suppression gate 86 in the circulating path, to remove the pulses E from the delay line. In a modification the coder 82 is replaced by a device for amplitude modulating the pulses, the flipflop 88 is replaced by a linear amplifier, and the decoder 92 is replaced by a pulse stretcher. For maintainng an accurate relationship between the period of the pulse generator and the delay of the delay line a control flip-flop is backcoupled via the delay line, the flop-flop producing and being responsive only to high-amplitude pulses. The frequency of these pulses and also that of the pulse generator 83 are divided down to a common submultiple and the frequency of the pulse generator 83 is controlled by a detector responsive to any difference in these submultiple frequencies. Specification 661,459, [Group XL (c)], is referred to.