Abstract: A communication system for inphase and quadrature modulation of low frequency signals wherein the modulated signal frequency bands lie close to a carrier, the system including a pilot tone having a fixed frequency ratio to the carrier, the system further including a demodulator employing the pilot tone in a phase locked loop for synthesizing a coarse replica of the carrier, a filter incorporating inphase and quadrature channels utilizing the coarse carrier for extracting the actual carrier from the modulated signal spectrum and synchronous detection of the signals by means of the extracted carrier.

Abstract: Means and method of reducing the probability of false indications of microphone activity in a multimicrophone system incorporating means for sensing the activity of microphones.The microphone, which is truly active, will receive its signal from its user before any of the other microphones receive the signal. The microphone circuit then sends a control wave to all of the other microphone channels reducing their amplifier's sensitivity and making them insensitive to false indication of activity.

Abstract: An acoustic telephone coupler for use in transmitting medical data over telephone lines and adapts to accommodate any telephone handset configuration. The slim coupler base further functions as a spool for a lengthy cable, and a dual purpose strap alternatively retains the cable and cable connector on the spool or a telephone handset against the transducers of the coupler.

Abstract: Disclosed is an automatically tunable notch filter and method for suppression of acoustical feedback in an audio signal. The apparatus includes a selectively tunable notch filter having a center frequency which is variable over at least a substantial portion of the audio frequency spectrum. The apparatus receives an audio signal which is substantially non-periodic in the absence of acoustical feedback and substantially periodic with an instantaneous dominant frequency in the presence of the same. The duration of successive periods are monitored and compared by an up/down counter to determine whether the audio input signal is substantially periodic and to determine the instantaneous dominant frequency of such audio signal. Upon detection of an audio signal which is substantially periodic, the notch filter is tuned to the instantaneous dominant frequency so as to suppress the acoustical feedback.

Abstract: A portable public address system for use in motor buses which can also be used inside buildings or out-of-doors including a first case and a second case. Within the first case, which is approximately the size of a pack of cigarettes, is an amplifier circuit with a Darlington configuration transistor pre-amplifier, a three-position switch, a volume control and a microphone. Within the second case is a loudspeaker and battery supply which are electrically connected to the amplifier circuit in the first case through the switch.

Abstract: A telephone conference amplifier consists of a plurality of identical channels connected to a common point. Each channel includes means for deriving an envelope signal from an incoming speech signal, and means for adding the envelope signal to the incoming signal to produce a unidirectional polarity signal which is applied through an ideal diode circuit to the common point. The signal at the common point is picked off by another ideal diode circuit in each channel and returned to the channel input. Each channel includes means for inhibiting the return of the signal when it is active. Each channel includes a level control circuit for controlling in dependence on the envelope signal the amplitude of the unidirectional polarity signal produced from an input signal so as to limit the range of variation.

Abstract: In the passenger compartment of an automobile or the like, a forwardly facing generally bucket-shaped loudspeaker enclosure is essentially sealed to the exposed lower corner or edge of the dashboard, and a rearwardly facing sound reflector is mounted under the dashboard and spaced from the speaker enclosure. The speaker is mounted within the enlarged end of the enclosure, the latter having an insulating lining.

Abstract: Control circuitry for a multi-channel audio system is disclosed which includes left and right signal processing channels and a DC control section. Each processing channel includes balance and volume sections which are connected with the DC control circuitry and controlled from respective balance and volume potentiometers connected with the DC control section. Each channel further includes a tone control section and more specifically treble and bass sections connected with the DC control circuitry and controlled from a tone potentiometer. The response of the bass section is also controlled as a function of the setting of the volume potentiometer to achieve a loudness function. The volume control circuitry is designed to produce a linear relationship between the DC control voltage and signal attenuation as measured in decibels. Current steering circuitry is employed in both amplitude and tone control.

Abstract: The present invention relates to a stethoscope, more particularly to a two-way stethoscope which permits hearing both direct and amplified sound from the human body through manipulation of a simple switching means at hand. The microphone for the electronic mode is located in the chestpiece, while the earpieces contain the speakers.

Abstract: The known process for transmitting two or more audio signals over a single transmission channel by eliminating alternate small segments from a first signal and filling the gaps with alternate small segments of a second signal and then filling in the gaps in the received signals with repeated segments, is found to be significantly improved with respect to comprehension or understandability, etc., if the audio signals are divided into a low frequency subband and one or more high frequency subbands and if the lowest frequency subband is transmitted substantially continuously while the higher frequency subbands are segmented and transmitted as before. Processes and apparatuses for splitting the audible signals into high and low frequency subbands and for treating the signals both on the sending and receiving sides to take advantage of the above discovery are disclosed.

Abstract: A combination acoustic and electronic stethoscope which can be operated with one hand is disclosed. The stethoscope has a combination slide valve-electrical switch located in the chestpiece for conversion from an acoustic to an electronic mode of operation. The microphone for the electronic mode is located in the chestpiece and the speaker is located in the Y connector of the earpieces. A volume control and frequency selector control are operative in the electronic mode.

Abstract: Messages are collected by a central polling station from a plurality of remote terminal stations on a multiparty line. The central polling station is arranged to initially query stations in groups, probing successively smaller size subgroups of each group if a station in the group returns an acknowledgement that it has a message and eventually addressing individual stations to collect the messages. In heavily loaded periods, when a large quantity of messages are available at the terminals, the polling station initially queries a smaller size subgroup (or in the extreme, initially addresses individual stations) while in lightly loaded periods, when few or no messages are available, the polling station initially queries a larger size group (or in the extreme, initially broadcasts a query to all of the terminals). Loading of the system is determined by measuring the duration of time required for polling and collecting all the available messages.

Abstract: In a communications system such as a time assignment speech interpolation (TASI) system for concentrating signals from N trunks onto C channels (C < N), and for expanding same, it is common for a speech signal to pass through a plurality of transmitter/receiver terminals, perhaps connected for tandem operation. Unfortunately, transmitter speech detector clipping of the speech signal leads to a distortion accumulation problem. Also, while a calling trunk is inactive, an inserted noise signal is commonly furnished by a receiver to the called trunk to avoid an aural "too quiet" condition. Unfortunately, actual calling trunk noise and the called trunk inserted noise may differ, leading to a noise signal contrast problem. To mitigate the signal contrast problem, communications apparatus including an improved noise signal inserting arrangement, responsive to a measure of calling trunk noise, supplies an adaptively adjusted, deterministic, pseudorandom noise signal to the called trunk.

Abstract: A system and method for detecting the presence of useful speech information in telephone voice channels capable of containing noise as well as such useful speech information for optimizing the telephone transmission of such speech information. Two segments of the envelope of a given voice channel are compared against each other over two different time domains in order to determine if a predetermined magnitude of difference exists between these envelopes. The presence of such magnitude of difference is indicative of the presence of such useful speech information in the voice channel thereby enabling transmission thereof by the system, whereas the absence of such magnitude of difference is indicative of the presence of solely noise thereby preventing the transmission thereof by the system.

Abstract: An adaptive coder and decoder for reducing the bit rate required to trans digitally encoded speech signals. The invention relies on the fact that the speech pattern of the average talker contains significant numbers of inter-syllable and inter-word pauses. The coder includes circuitry that monitors the idle pattern code generated by the coder's analog-to-digital converter and a code word generator that generates one or more special code words that are substituted for idle pattern code sequences of predetermined length. The speech signal and special code words are then fed into an elastic buffer and transmitted to the distant receiver at a lower bit rate than was employed for the encoding process.

Abstract: A digital voice switch for detecting speech signals in the presence of noise on a communication channel. The voice switch employs a threshold adjustment circuitry and three threshold detectors which include a speech detector, a noise detector and a disabling detector. The speech detector having a variable speech threshold level detects the presence of speech signals in the communication channel. The noise detector having a variable noise threshold level detects the presence of noise. The threshold adjustment circuitry, which is capable of providing rapid threshold adjustment, operates in conjunction with the noise detector to detect the noise level and to adjust the speech and noise threshold levels according to the level of the noise present in the communication channel. The disabling detector having a fixed maximum threshold level operates to disable the function of the threshold adjustment circuit while speech is present.

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.