Browse Prior Art Database

Signal Presence Detector with Speech Tone Discrimination

IP.com Disclosure Number: IPCOM000092132D
Original Publication Date: 1968-Sep-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 3 page(s) / 55K

Publishing Venue

IBM

Related People

Hellwarth, GA: AUTHOR [+3]

Abstract

This system rapidly detects the presence of an input audio signal in a variable noise background. The system also determines if that signal has characteristics of speech or of a steady-state tone. The system is self-monitoring since it provides validation of the signal presence decision and is insensitive to random noise-like pulses. The system drawing is divided into three major component parts and shows a signal presence detector, a signal validation circuit, and a speech-tone discriminating circuit. The signal presence detector detects the presence of an input signal by a fixed signal-to-noise ratio threshold. Its presence is indicated when the input exceeds the average noise level by a prescribed number of decibels. The average noise level is established during intervals when the signal is absent.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 52% of the total text.

Page 1 of 3

Signal Presence Detector with Speech Tone Discrimination

This system rapidly detects the presence of an input audio signal in a variable noise background. The system also determines if that signal has characteristics of speech or of a steady-state tone. The system is self-monitoring since it provides validation of the signal presence decision and is insensitive to random noise-like pulses. The system drawing is divided into three major component parts and shows a signal presence detector, a signal validation circuit, and a speech-tone discriminating circuit. The signal presence detector detects the presence of an input signal by a fixed signal-to-noise ratio threshold. Its presence is indicated when the input exceeds the average noise level by a prescribed number of decibels. The average noise level is established during intervals when the signal is absent.

Signal presence detection is accomplished first by full-wave rectification of the input and taking the logarithm of the magnitude of the resulting positive signal. The logarithm of the input signal magnitude is then low-pass filtered to remove ripple and then applied to a non-linear noise averaging circuit such as is shown in U. S. Patent 3,387, 222. When the input is noise, switch S is closed and the average of the noise NA appears across capacitor C. When the input exceeds NA by a threshold ET, the comparator changes state and opens switch
S. The value of the noise average is held constant until the signal again drops below the threshold, closing S. Noise averaging is then resumed.

At the onset of a signal, the signal presence detector output rises, setting latch L1. The duration of the initial portion of the signal is determined to be less than or greater than the prescribed time T1 by the timing circuit consisting of resistor R1 and capacitor C1 and voltage comparator VC1. If the initial onset exceeds time T1, the increasing voltage across C1 has sufficient time to overcome the threshold ET1, causing comparator VC1 to change state indicating that a valid signal is received. This condition sets latch L2 which indicates a valid signal. Latch L2 disables And A2 thus preventing a false start indication.

If the initial duration of the signal is less than T1, latch L2 remains reset. When the signal disappears, And A1 is enabled which, in turn, enables And A2, indicating that the initial signal detection is invalid or is a false...