Patent GB-1393690-A (GB)
Title: Variable frequency response filter and bandpass filter and dynamic noise filter employing same
IPC: H03G5/16
Filed: 1972-07-24
Abstract: 1393690 Noise limiters R S BURWEN 24 July 1972 [26 July 1971] 34511/72 Heading H4R A variable band-pass filter provides filtering of the high and low frequency portions of the audio range in accordance with the magnitude of the high and low frequency signals. From providing a narrow pass-band within the audio range when no signal is present the filter increases and decreases the high and low frequency cut-off points respectively as the signal content in the high and low frequency portions increases. An integrating forward signal path 60 (Fig. 2), and an integrating feedback path 64 are connected in a closed path. A resistive feedback path 61 is also provided, to produce a mid-frequency reference (at 700 Hz, Fig. 2A). Path 60 includes an integrating amplifier 76 whose gain decreases with increasing frequency. Path 64 includes an integrating amplifier 114 whose gain decreases with decreasing frequency. F.E.T.s 74, 94 and 112, 132 have biasing circuits which control the rate of integration in each path 60 and 64 and reduce unwanted variation in the rates of integration. These biasing circuits respond to high frequency and low frequency control signals applied at lines 92 and 134 respectively. These control signals have a fast attack and slow decay characteristic in the upper and lower frequency portion of the audio range. As the high frequency control signal applied to line 92 increases, indicating more high frequency content, the resistances of F.E.T.s 94 and 74 are caused to decrease. This causes the amplifier 76 to increase its high frequency integration rate and to shift the high frequency cut-off portion (Fig. 2A). Increase in the low frequency control signal applied to line 134 increases the resistances of F.E.T.s 132, 112 with resultant shift of the response curve of integrating amplifier 114. Consequently the low frequency response of the variable band-pass filter is extended by lowering the L.F. cut-off point. A filter network 58 sharpens a corner 63 of the response curve. A second filter network 66 sharpens corner 65 of the response curve. Control signals.-Signals from a low pass filter 150 (Fig. 3), (i.e. those which lie in the range of variation of the low frequency cut-off point), are applied to a limiter 152 to prevent excessive signals affecting subsequent circuitry. Two rectifiers 154, 156 provide fast attack and slow decay. A non-linear filter 158 causes signal filtering and smoothing in RC # filter 160, with augmented response produced by feedback from differential amplifier 162. When the input to filter 158 exceeds the output of amplifier 162 by a percentage determined by resistors 166, 168, the output of amplifier 164 increases, overcomes diode 174 and causes the amplifier 162 to respond more rapidly to the input signal. A maximum signal level for the low frequency signal is established by an amplifier 176 and rectifier 180 through a resistor 182. A high frequency control signal is obtained by substituting a high pass filter for the low pass filter 150. A multi-channel system (Fig. 1, not shown), may have separate high and low pass filters for each channel or the high and low pass filters may be shared by two or more channels.