Pre-Amplifier and Noise Cancellation

Abstract

This Bachelor graduation project has the goal to create a device which is able of automatic volume control, to be used for enhancing speech intelligibility. To tackle the intelligibility of speech through Public Address Systems (PA Systems), an Intelligibility-Enhancing Automatic Volume Control system was proposed. The total system to be made must be able to alter a clean speech signal according to a noise estimation. Then the altered, enhanced, signal should be amplified before being sent to an existing Public Address System. A subsystem is added in order to dampen the outside noise in a car-like environment. The whole project is divided into three parts: Noise Statistics Estimation, Intelligibility Enhancement and Amplifier and Noise Cancellation. These parts have been performed by three different subgroups. In this report, the Amplifier and Noise Cancellation is discussed. The other parts are explained in the respective reports [1,2]. The Amplifier and Noise Cancellation group will amplify the enhanced audio signal with the use of a pre-amplifier. This group also introduces an additional noise cancellation subsystem for usage in enclosed spaces, like a car. It does so by inverting the recorded environment noise below 500 Hz, and adding this to the to be amplified signal before sending it to the PA System. This thesis is divided in two main design sections: the design of the pre-amplifier and the design of the active noise cancellation circuit. At the heart of both circuits lies a LM386 Audio Operational Amplifier (Op-Amp) but they both have different objectives. The pre-amplifier is designed to have a flat transfer function in audio range, 20 Hz to 20 kHz. The output level of the pre-amplifier is a standard level for consumer electronics, being 447 mVpp. The pre-amplifier inverts the signal from the microphone and the audio signal to achieve noise cancellation. The noise cancellation circuit features a microphone amplifier and a Low Pass Filter (LPF). The microphone amplifier amplifies the signal so that the microphone circuit’s output level is at the same level of the audio input of the pre-amplifier (200 mVpp). The filter makes sure only sounds below 500 Hz are passed to the pre-amplifier. With the inverting capabilities of the pre-amplifier and both signals being completely out of phase, a theoretical cancellation of sound signals is possible. Because of the LPF used in the microphone amplifier this cancellation is done for signals below 500 Hz. At the end of the project, a system was built which met most of the requirements. Some of the requirements can not be satisfied due to incapability of the test equipment available. The system does amplify the signal to the desired amplitude and is capable of slightly cancelling noise in a car. However, improvements of the product are needed to function more optimally.