Abstract: This invention relates to a double talk detecting method suited to be used for an echo canceler, and more particularly to a method for correctly detecting whether or not the double talk is present, even in the case where a speech transmitting level difference between a far-end talker and a near-end talker is comparatively large. What is assumed in the present invention is an echo canceler which can be employed in a telephone line network for transmitting a speech of a far-end talker sent through a digital line and a speech of a near-end talker sent through an analog line. A double talk detecting method applicable to the echo canceler here extracts predetermined analyzed parameters respectively from a first speech signal corresponding to the speech of the far-end talker sent from the echo canceler to a hybrid circuit and the speech of the near-end talker input to the echo canceler through the hybrid circuit. The analyzed parameters herein used include, for example, speech pitches, frequency components, etc.

Abstract: A receiver which receives a signal transmitted while the symbol rate is changed over from among a plurality of different values, and which varies, in accordance with the symbol rate, the pass band width of the filter that removes unwanted signals from the detector output. By compensating for changes in delay produced by alteration of the pass band width of the filter, a receiver, according to this invention, recovers a clock that is synchronized with the symbol rate after changeover, without discontinuities occurring when the symbol rate is changed over.

Abstract: A CDMA receiver capable of effective orthogonalization even when the number of received signal vectors to be orthogonalized is very large. Received signals which have been spread by spreading codes are despread by despreading filters. Products of received signal levels and cross-correlations between the spreading codes are compared in a preliminary selector of the cross-correlations, and Ns products are selected in order of magnitude, where Ns is a predetermined integer. Ns received signals associated with the selected products have priority to undergo orthogonalization. The number of signals to be orthogonalized by decorrelators can be effectively decreased, and noise enhancement effect can be reduced on the reverse channels. The orthogonalized received signals are recovered through channel estimators, phase compensators, RAKE combiners and decision blocks.

Abstract: In a transmission circuit, part of the output signal of the transmission circuit is diverted by an output monitor circuit and subjected to envelope detection by an output detector circuit. The output signal of the output detector circuit is inputted to a peak hold circuit successively holding the peaks of the input signal, so that varying amplitude components are removed, and a DC value is detected. An error detector circuit compares the detected value with a reference signal from a reference signal generator circuit, and the error signal is used as a gain control signal to control the gain of a power amplifier circuit.

Abstract: A spread spectrum communication receiver which can obviate a highly accurate, highly stable VCO used in a local signal oscillator.