Patent GB-844561-A (GB)
Title: Improvements in or relating to frequency detectors
IPC: G10L25/00
Filed: 1958-10-10
Abstract: 844,561. Frequency discriminators. PHILIPS ELECTRICAL INDUSTRIES Ltd. Oct. 10, 1958 [Oct. 14, 1957], No. 32423/58. Class 40(5). [Also in Group XXXVI] In a frequency detector for the detection of the variation of a characteristic speech frequency situated within a speech sub-band, the speech sub-band, which is selected by means of a filter 4, is supplied to two channels 6, 7 connected in parallel, the channel 6 comprising a differentiating network 8 and a rectifier 9 and the second a rectifier 11, the output voltages from the channel being fed to a ratio meter 13 the output voltage of which is the ratio of the amplitudes of the channel output voltages and a measure of the frequency. Low-pass filters 10 and 12 are included in branches 6, 7 respectively, and, if the input voltage of rectifier 11 is a (t) at a certain instant that to rectifier 9 is #a(t), so that the ratio of the output voltages of filters 10 and 12 is proportional to awdt/adt and therefore to the frequency. In the embodiment shown, the output direct voltages of filters 10, 12 are converted into alternating voltages of frequencies fi f 2 , corresponding in amplitude to the direct voltages, by means of amplitude modulators 15, 16, carrier wave oscillators 17, 18 of frequencies f 1  f 2  and output filters 19, 20, the outputs of the oscillators and filters being connected through attenuators 21, 22 to adjust the strength ratio of the carrierwave oscillations. A ratio meter for use in the detector is shown in Fig. 4. The alternating voltage derived from channel 6 of frequency f 1  and amplitude-modulated by the rectified differentiated input signal is applied to the grid of a pentode 25 through a resistor 23, the alternating voltage derived from channel 7 of frequency f 2  and amplitude modulated by the rectified input signal being applied to the grid through a resistor 26. Slope control is provided for the pentode 25 and the amplitude of oscillations at frequency f 2  at the output of pentode 25 is therefore kept constant, so that the slope of the pentode 25 varies inversely as the amplitude of the input voltage at frequency f 2 . The voltage at frequency fi is selected by a filter 30 and rectified, the output at 38, 39 being a direct voltage proportional to the amplitude ratio between the output voltages of channels 6 and 7. In the embodiment of Fig. 5, the ratio is determined by amplitude limitation instead of slope control. An amplitude limiter comprising rectifiers 45, 46 has an output consisting of a voltage of frequency f 2  modulated by the voltage of frequency f 1  the frequency deviation of said f.m. voltage being proportional to f 1 : f 2 , the amplitude of f 1  < amplitude of f 2 . The limiter output is fed to an amplifier 51 and a filter 52 tuned to fi, the voltage from this filter being then rectified and giving a direct voltage output which varies as the amplitude ratio of the input voltages of frequencies fi and f 2 . Alternatively the ratio meters may be replaced by one in which the output voltages of channels 6, 7 may be converted into voltages of equal frequencies, the output alternating voltage of channel 6 being subsequently shifted 90 degrees in phase and added to the alternating voltage of channel 7 so that the phase of the sum voltage obtained varies in accordance with the desired amplitude ratio.