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Automatic Volume Control Compressor and Expandor Circuits

IP.com Disclosure Number: IPCOM000093448D
Original Publication Date: 1967-Sep-01
Included in the Prior Art Database: 2005-Mar-06
Document File: 4 page(s) / 81K

Publishing Venue

IBM

Related People

Liberman, RA: AUTHOR [+2]

Abstract

These automatic volume control, compressor and expandor circuits are used as part of the baseband hardware of a multiplexer. The automatic volume control AVC circuit controls the average signal power of the multiplexed channels. The compressor and expandor, compandor, circuits reduce the extent of variation of the instantaneous speech power. The ratio of the peak signal power to average signal power is reduced allowing the average signal power of the speech channels to be increased. The compressor reduces the dynamic range of speech. The expandor restores the speech to original form. Each circuit requires variolosser circuitry hereinafter described but eliminates the need for transformers to isolate variolosser DC control current from adjacent circuits.

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Automatic Volume Control Compressor and Expandor Circuits

These automatic volume control, compressor and expandor circuits are used as part of the baseband hardware of a multiplexer. The automatic volume control AVC circuit controls the average signal power of the multiplexed channels. The compressor and expandor, compandor, circuits reduce the extent of variation of the instantaneous speech power. The ratio of the peak signal power to average signal power is reduced allowing the average signal power of the speech channels to be increased. The compressor reduces the dynamic range of speech. The expandor restores the speech to original form. Each circuit requires variolosser circuitry hereinafter described but eliminates the need for transformers to isolate variolosser DC control current from adjacent circuits.

The AVC circuit is in drawing A. The input signal Vin is applied via line 10 to a transistor phase split amplifier. The signal at the emitter of transistor Q4 is a replica of the input. The signal at the collector is very nearly the same as that at the emitter but inverted. The double-ended output of the phase split amplifier provides a balanced input to a diode bridge variolosser. The variolosser acts as a voltage controlled attenuator to maintain a constant voltage at its output. Following the variolosser is a well-balanced differential amplifier and a single- ended amplifier follows the differential amplifier. Since the control current is made up of a DC component and a small AC component, both last named amplifiers have good common mode rejection.

The input signal is also applied via line 11 to a speech level detector comprising an amplifier-rectifier and a capacitor holding circuit. These two circuits derive a voltage proportional to the peak of the applied signal. The holding circuit charges up rapidly but discharges slowly resulting in a circuit which tends to store a voltage proportional to the largest signal peaks. The holding circuit drives a transadmittance amplifier. The latter converts its input voltage to a control current to be applied through the variolosser for controlling the dynamic resistance of that device, which is always much smaller than R1 + R2.

Vin is also amplified and rectified by operational A3 of the speech level detector. Q1 is included in the feedback loop of A3 to help reduce the output impedance of the circuit. The important parameter is the peak of the received waveform. This signal appears at the junction of R25 and CR22 and is divided by R26 and R27 while C23 acts as a peak detector. R26 is much smaller than R27 so that the DC voltage at C23 is given by VR = the square root of 2VinR25/R21.

The transadmittance amplifier converts the DC voltage at C23 to a control current. This circuit consists of A4 and the Darlington amplifier Q2 and Q3. The base-to-emitter junctions of Q2 and Q3 are part of the feedback loop that includes R29. This arrangement reflects the base-to-emitter voltage drops o...