H - Electricity – 04 – R
Patent
H - Electricity
04
R
179/42, 379/63,
H04R 3/00 (2006.01) H04M 1/60 (2006.01) H04M 1/74 (2006.01) H04M 19/00 (2006.01) H04R 3/04 (2006.01)
Patent
CA 1077632
Abstract of the Disclosure A method and circuitry are disclosed which allow the operation of carbon microphones in telephone sets at low levels of average current. The microphone is connected to a pulse-generating circuit which energizes the microphone with pulses of current that effectively sample the incident acoustic signal at a superaudible rate. The level of each current pulse is chosen so that the microphone performs at the high signal-to-noise ratio associated with high current levels and the duty cycle is chosen to match the low average value of available current. The audio signal is recovered by demodulating the resulting PAM (voltage) output signals using a low pass filter. The level of the recovered audio signals is then amplified to the level required by the telephone transmission line. The sampling approach, using the carbon microphone as the sampling element, extends the utility of the carbon microphone to operation at low average current levels which hitherto were unusable because of low signal-to-noise ratio. This method may also permit a uniform reduction in current levels on all the lines of an associated telephone central office thereby reducing total power consumption and allowing the use of higher resistance, fine gauge transmission loops. - i -
287318
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