Author Topic: An Automatic Voice-Controlled Audio Amplifier  (Read 3616 times)

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An Automatic Voice-Controlled Audio Amplifier
« on: February 18, 2012, 02:23:43 am »
Author : Jonathan A. Enokela and Jonathan U. Agber
International Journal of Scientific & Engineering Research Volume 3, Issue 1, January-2012
ISSN 2229-5518
Download Full Paper : PDF

Abstract— The delivery of the proper quality of audio signals to the audience in the entertainment, public and other environments is of great, and sometimes critical, importance. This always requires that the audio signals be of the correct intensity to the hearing of the audience, especially if the signals come from different sources. This work presents a system which automatically fades out the main stream signal when signals from other sources are received. By arranging the circuit such that the signal from the other sources continuously drives a pair of bipolar junction transistors towards heavier saturation, the mainstream signal was attenuated by as much as 3 dB.

Index Terms— Audio Amplifier, Electronic Control, Attenuation, Voice Control, Public Address System, Audio Fading.

IN many instances in public addressing environment, radio stations, television houses and in other places, the need for two signals to be simultaneously sent to the listeners arises. In almost all cases the audio signals will have to operate in such a way that one source is attenuated while the other is amplified for the listeners to have their attention drawn to the one that is amplified momentarily. In a radio house, for instance, the announcer might want to put out an urgent message to the listeners while the music that he has been playing at the background will be attenuated. Most existing facilities require that the announcer use his hand to control the volume of the music being played at the background while he makes his announcements. This process has some drawbacks: in the first place, the degree of attenuation that the announcer imposes on the amplifier is highly subjective. This results in the background music being either too loud or too faint. Secondly, a manual control will wear away with time.
The system being proposed operates in such a way that the amount of attenuation will be proportional to the loudness of the announcer’s voice and immediately the announcer stops talking, the music being played would be restored to its original volume.

The block diagram of the proposed system is depicted in figure 1. Under normal conditions of operation, the signal input, designated as line input, is the signal that is transmitted to the output through the line and the mixer amplifiers. When a signal is input at the microphone (MIC) input, however, this signal is amplified by the block called MIC amplifier and is passed through the mixer amplifier to the output. Simultaneously the output signal from the MIC amplifier operates the attenuator which under the control of this signal attenuates the output from the line amplifier and reduces the amount of line input signal that is transmitted to the output. The amount of line input signal that is transmitted to the output depends on the strength of the signal from the MIC input.


A schematic diagram that can be used to realise the block diagram of figure 1 is depicted in figure 2. The line amplifier is built around the operational amplifier (Op Amp) IC1 [1], [2], [3] and there is a further amplification after attenuation by IC3, while IC5 is the mixer amplifier.
 The amplification of the MIC signal is done by IC2, while a further amplification by IC4 ensures enough signal level for rectification by the diodes. The positive half cycle of the signal is rectified by D2 and D3, while D1 and D4 rectify the negative half cycle. It is observed that distortion of the line signal results if only one half cycle is used for control. The transistors Q1 and Q2 form the controlled attenuator.
   Fig.2: Schematic Diagram of the Voice-Controlled Amplifier

The Voice-Controlled Amplifier (VCA) is expected to be incorporated into existing systems. This implies that the input and the output signal levels should be compatible with commercially available audio equipments [4]. Thus the following specifications are obtainable:

 Line input:    300mV, 10kΩ
 Mic. input:    20mV, 100Ω
 Output:          1V, 10kΩ
 Frequency Response:   20Hz – 18 kHz.

Each stage of the circuit can be isolated and analysed individually and then designed. Let us consider first the line input stage indicated in figure 3.
The circuit shown in Figure 3 is basically a non-inverting amplifier stage. The capacitor C3 controls the low frequency response while the high frequency response is controlled by C17. The capacitor C1 is chosen so that it has a very low reactance at the lowest frequency of interest. The gain of this amplifier stage is given by (1).

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