GB801351A - Improvements in or relating to condenser microphone arrangements - Google Patents

Abstract

801,351. Circuits for electrostatic instruments; condenser microphones. STANDARD TELEPHONES & CABLES, Ltd. Jan. 14, 1955 [Oct. 16, 1953], No. 28606/53. Class 40 (4). In an arrangement for coupling a condenser microphone to an amplifier, of the type in which an oscillator is connected to a discriminator, the output of which feeds the amplifier, the microphone being effectively included in the frequency determining elements of the oscillator, or in those of the discriminator, the oscillator is a crystal triode oscillator. A preferred form of condenser microphone, which forms the subject-matter of the divided Specification 801,352 is also described. Condenser microphone in tuned circuit of oscillator-Fig. 1. The microphone 1 is connected by an impedance step-down transformer 2, coaxial cable 3 and impedance step-up transformer 4 so as to be placed effectively in series with the inductance 6 in the emitter circuit of crystal triode 5. The base resistor 10 carries the emitter and collector currents, and these, being in phase, provide regenerative feedback, so that the circuit oscillates at a frequency determined by the microphone capacitance. Consequently, the output oscillations from the collector of transistor 5 are frequency modulated by the sound waveform received by the microphone. Negative feedback is applied over a feedback loop which includes the microphone, R.F. choke 31, and D.C. blocking condenser 32. The output of the oscillator may be transmitted over a radio link to a frequency modulation receiver or, as shown, may be passed directly to a discriminator 11-17 tuned to the mean frequency of the oscillator. The output from the discriminator is an audio-frequency signal corresponding to that received by the microphone. If the microphone is to be used at a distance from the amplifier, the oscillator and discriminator should be as near to the microphone as possible to reduce the length of cable carrying current at oscillator frequency. Power for the oscillator and discriminator may then be supplied from the amplifier over the screen and one conductor of the screened twin cable used to carry the audio-frequency current, the arrangement being as shown in Fig. 2. Condenser microphone in tuned circuit of discriminator-Fig. 3. The oscillator is stabilized by a piezo-electric crystal 33 and the transformer 4 is connected into the discriminator circuit. The only frequency in the cable 3 will be the oscillator frequency and interference between the cables 3 of different microphones may be eliminated by tuning the oscillators to appropriate widely - spaced frequencies. A balanced bridge form of discriminator is used, the reflected capacity of the microphone being balanced by the condenser 34. Specifications 569,260, 587,990 and 594,077 also are referred to.