Abstract: An adaptive Infinite Impulse Response (IIR) filter is provided that can adaptively detect the presence of one or more tones in its input stream. The tones to be detected may be of arbitrary frequency, subject only to a limitation that such tones fall within a frequency band consistent with accepted sampling principles (e.g., a maximum frequency of interest no greater than one-half the sampling frequency--Nyquist sampling criteria). An IIR filter developed according to the method of the invention will adaptively locate the frequencies of tones to be detected, thereby allowing for frequency drift from nominal expected frequency values with no loss in accuracy. Such a filter will also process the input signal sample-by-sample, thereby avoiding the blocking problem of FFT-based filter approaches. With the filter of the invention, an application can identify the frequencies, associated power levels, SNR and duration of the tones.

Abstract: A reference source of uniformly spaced pulses is coupled to the input of a smooth sequence generator (SSG) which is capable of dividing the reference pulse sequence by any proper, predetermined, rational fraction (e.g., 193/512). The SSG comprises a plurality of routing circuits connected in tandem. Each routing circuit has a first and second output and a control terminal which serves to route a pulse presented at the routing circuit input to either said first or second output. The first output of each routing circuit is connected to the next tandem-connected routing circuit. A feedback connection from the first output of each routing circuit to the control terminal of the preceding routing circuit serves to control the routing of a pulse presented to the input of the latter. A synchronization circuit is coupled to the output of said reference source, via a divider circuit, and to the first output of the last tandem-connected routing circuit.

Abstract: The detection sensitivity and noise rejection of an arrangement for detecting speech in the presence of noise is improved by accumulating the weighted differences between input signal samples and their short-term running average. The detector thus tracks ambient noise, providing an adaptive detection threshold such that detection sensitivity is increased in low noise environments without excessive false operation on high level noise. The peak average attained during an interval of speech is used to provide variable hangover upon cessation of speech, yielding greater hangover for weak talkers than for loud talkers. In an illustrative embodiment of the speech detector used in a speech interpolation system, protection is afforded also against false transmission path operation due to detection of speech echo.

Abstract: During echo suppression in a system utilizing a full echo suppressor, noise is typically inserted in the receive line of the near-end party to assure him that the line has not "gone dead". The need for a separate noise source for this purpose is eliminated in a digital system by using the existing coded incoming signals on the line. Thus, during echo suppression the bits of the coded incoming signal are altered to provide a random noise signal of normal background amplitude which is then inserted on the receive line. In an illustrative companded sign magnitude code embodiment, the sign bit is replaced by the least significant bit, the least and next-to-least significant bits are interchanged and the remaining bits are zeroed, thereby producing an unintelligible background noise signal of appropriate amplitude.

Abstract: Active and standby clock signals are phase synchronized to each other within a predetermined phase tolerance by synchronizing each clock signal to the same reference pulse stream. A first synchronizing pulse signal is derived from the active clock signal and a second synchronizing pulse signal is derived from the standby clock signal. For each synchronizing pulse signal, the pulse widths exceed the reference signal pulse widths by an amount related to the predetermined phase tolerance. Synchronization between the active and standby clock signals is achieved when the phases of the active and standby clock signals are adjusted such that the pulse widths of the reference pulses lie entirely within the time domain of the pulse widths of the synchronizing pulse signals.

Abstract: During echo suppression in a system utilizing a split echo suppressor, suppression is removed when the near-end party breaks in (i.e., during double talking) and is typically replaced with a small fixed loss inserted in the receive line. To minimize loud echoes without unduly penalizing soft talkers at the far end, the loss inserted in the receive line can be varied advantageously according to the incoming signal level by using a compression circuit. The present disclosure relates to a simple arrangement for providing instantaneous compression loss in a digital system by modifying the incoming encoded signal in a predetermined manner. In an illustrative embodiment, one-half the value of the input signal is added to one-half of a selected value when the input signal level equals or exceeds the selected value.