Patent GB-1045437-A (GB)
Abstract: 1,045,437. Transmission systems. RADIO CORPORATION OF AMERICA. March 26, 1964 [April 8, 1963], No. 12883/64. Headings H4J and H4R. [Also in Division G1] Apparatus for translating high level audiofrequency signals for reproduction at low levels comprises a circuit for automatically providing relatively more boost for low frequency components as compared to other frequency components of the high level audiofrequency signals when the low frequency components are of low level and for automatically providing progressively less boost for the low frequency components as their amplitude increases, the circuit also automatically providing progressively more boost for mid-range components of the high level audio-frequency signals as their amplitude increases from a predetermined level. The circuit preferably also attenuates the high frequencies at the loudest volume levels of programme material. Sound from an orchestra 5 (Fig. 1) is picked up by two stereophonically related microphones 6, 11 which feed identical frequency equalization networks 14, 40. Network 14 includes potentiometers 15, 16, 17. As the slider of potentiometer 15 is moved to the right the amount of low frequency boost is reduced. As the slider of potentiometer 16 is moved to the right the amount of mid-range frequency boost is increased, and as the slider of potentiometer 17 is moved to the right the amount of high frequency attenuation is increased. Network 14 is first adjusted manually such that the maximum level of the original signal may be reproduced without tonal distortion at the maximum level at which the programme is to be reproduced. Three signal-responsive variable resistors 18, 19, 20 are provided. Resistor 18 functions as a compressor amplifier for low frequencies and comprises a cadmium sulphide photo-cell whose resistance decreases as the light impinging thereon increases and a transistor amplifier. The transistor amplifier is terminated by a load resistor. A feedback circuit from the amplifier output is coupled to drive a light source such that the intensity of the light is a function of signal amplitude. The light is directed on to the cadmium sulphide cell to modify the signal applied to the transistor amplifier. Thus, as signal amplitude increases, the resistance of resistor 18 automatically decreases to decrease the bass boost. Signal responsive variable resistors 19, 20 act in a similar manner as expander and compressor amplifiers for mid and high frequencies respectively, but two, rather than one, light responsive cells are used. Resistor 19 has a threshold level at some medium volume of programme material at which its resistance starts to decrease. Resistor 20 also has a threshold level whereby it only becomes operative at higher volume levels. The signals from the two channels may activate loud-speakers 34, 35 directly, or indirectly through gramophone records or magnetic tapes. In a second embodiment (Fig. 5, not shown) an amplifier 82 is coupled between input terminals 80 and output terminals 84. A frequency responsive negative feedback network includes signal level responsive resistors 88, 89, 90. Resistor 88 controls the low frequency feed back characteristic, resistor 89 controls the mid-range frequency feedback characteristic above a threshold and resistor 90 controls the high frequency feedback characteristic. Resistors 88, 89, 90 may be of the same general type as those of Fig. 1.