GB1473831A - - Google Patents

Abstract

1473831 Companders DOLBY LABORATORIES Inc 13 May 1974 [17 May 1973 5 Sept 1973] 23638/73 and 41673/73 Heading H4R A dynamic range modifying circuit comprises, in series across the input terminals, a dynamic range linear impedance Z 1 and a variable impedance Z s including a negative resistance component R L and being controlled 19 as a function of a signal in the circuit. As shown, an input voltage V 1 is converted to an input current i 1 by, e.g. a high impedance source, and the output V 2 is a voltage. The circuit constitutes a 10 dB compressor for frequencies above 1À5 kHz and the corresponding expander is similar except that the input is voltage V 4 and the output a current extracted at 17. For low frequency, low level input signals, the capacitive reactance is high and the - 2-16 K resistance R L partially cancels the 3À16 K resistance Z 1 to provide a 1 K resistance resulting in an unmodified dynamic range. At high frequencies and low levels the low capacitive reactance shunts R L and the 3À16 K resistance results in a compression factor of 3À16 : 1, i.e. 10 dB. The control circuit 19 responds to a high level signal at a particular frequency to vary the capacitance and thereby exclude that frequency from compressor action. The expander operates in a similar manner to present different impedances to the input according to signal frequency and level in order to restore the signal to its original form. The circuit constitutes a series version of Type 3 device of Specification 1,432,763. A similar circuit, employing an inductor instead of a capacitor as the variable reactance, consitutes a series version of the Type 2 device of Specification 1,253,031. Other circuits are also described in connection with Figs. 4 and 7 (not shown) in which positive resistances replace the negative resistances R L of the above circuits in order to provide series versions of the Types 1 and 4 devices of Specifications 1,120,541 and 1,432,764 respectively, The Specification also describes circuits by which the variable impedance Z s , with positive or negative resistance components, may be realized. These circuits include a series path such as 1-2, Fig. 9, whose current i is detected and used to derive a voltage V s which is introduced into the path 1-2 such as to simulate the characteristics of the impedance Z s according to the Type of circuit to which this sub-circuit is applied. The circuit shown in Fig. 9 includes a fixed high pass filter C 1 R 1 and a variable high pass filter R 2 C 2 R v and is the subject of Divisional Specification 1,473,832. Filter circuits in the loop may include limiters to prevent overshoot of the introduced voltage above one tenth of the maximum input signal level as in Divisional Specification 1,473,833, and may be arranged in parallel paths as in Divisional Specification 1,473,834.