| 摘要 |
1,168,212. Automatic gain control. H. KERSTEN. 15 March, 1968 [18 March, 1967], No. 12790/68. Heading H3T. In an automatic gain control amplifier for recording and/or reproducing sound, and more particularly a limiter or compressor, having a recovery time constant which has, at the start of the recovery an average value of about 0.5 to 1.0 seconds, irrespective of the control deviation, this recovery constant is switched, after the lapse of a delay time equal to at least one half, but preferably to a whole cycle of the lowest audio frequency to be transmitted automatically to a lower value of about 0À1 seconds. Transformer 1, Fig. 1, feeds signal to two centre-tapped secondary windings one of which supplies voltage E s1 to a full-wave peak rectifier comprising diodes 2, 3 and transistor 6, the emitter circuit of which includes a long time-constant circuit comprising resistor 9 and capacitor 8, across which a positive control voltage is produced. The second transformer secondary supplies voltage E s2 to a full-wave peak rectifier comprising diodes 4, 5 and transistor 7, the emitter circuit of which includes a short time-constant circuit comprising resistor 11 and capacitor 10 across which a negative control voltage is produced. The voltage across capacitor 8 is applied to the control member 12 of the automatic gain control amplifier, while the voltages across capacitors 8, 10 are combined in a resistive adding network 20, 21 and applied to the base of a transistor 22. Transistor 22 is normally biased non-conductive by an emitter bias network comprising resistor 25, diode 24 and capacitor 26. The outputs across capacitors 8, 10 from the two rectifying circuits, or alternatively the values of the resistors 20, 21 are so proportioned that with automatic gain control in action the voltage from capacitor 10 preponderates at the base of transistor 22, etaining it cut off. When the signal ceases the voltage across capacitor 10 falls more rapidly than that across capacitor 8, so that after a predetermined time interval the voltage from capacitor 8 preponderates at the base of transistor 22, biasing it positively. Transistor 22 then becomes conductive and shunts resistor 23 in parallel with resistor 9, giving the gain control characteristic beyond this point a short time-constant. The resistors 9, 11 may be omitted, discharge of the capacitors 8, 10 taking place through the resistors 20, 21 alone. A slow-access storage member, comprising series-connected capacitance and resistance may be connected in shunt with the capacitor 8 (Fig. 4a, not shown), it may be isolated from that capacitor by means of an interposed amplifier, being arranged to discharge through a shunting resistor (32, Fig. 4b, not shown). In order that the whole of the voltage across this additional capacitor 29, Fig. 4c, may be available across the resistor 32 during discharge, the resistor 28 may be shunted by a diode 33 poled so as to be non-conductive during the charging process. |
