Automatic gain control (AGC) circuits are used within communication systems, such as telephonic communication systems, in order to maintain transmitted speech signals at comfortably audible levels. In order to maintain a specified average or peak level of speech signals, while minimizing noise content, automatic gain control circuits use a speech detector for discriminating between speech and noise signals. Typically, a speech detector evaluates a single statistical property of the transmitted signal, compares the statistical property value with a predetermined reference and provides a logical output signal indicating the presence or absence of speech in the transmitted signal. The AGC circuit responds to the logical output signal by adjusting the applied gain depending on whether a logical output signal indicates the presence of speech.
One problem with traditional speech detectors is that reliance upon a single statistical determination renders such speech detectors vulnerable to making false determinations when evaluating noise to noise signals that possess the requisite statistical property at a level sufficient to indicate speech detection. Another problem is that the production of a single logical output obscures the degree of confidence with which the presence of speech was determined by the speech detector. It would be desirable to provide a speech detector that utilizes more than a single statistical criterion in order to determine the presence of speech in a transmitted telephone signal. It would further be desirable to provide a speech detector that produces a detection signal from which the degree of confidence in the determination can be taken into account in adjusting the gain.