Intelligent Modem Corporation

Mark P Mallory

Donald L Chaffee

Graham H Brand

A high speed modem system which utilizes multiple carriers to successively transmit frames containing data such that the frames are transmitted so that the level of the signal is minimized, and is preferably zero, at the borders between individual frames thus, reducing or eliminating interframe discontinuities and the accompanying distortion and interframe interference. The modem system includes a circuit to successively combine together the modulated carrier signals of each set to produce a frame waveform which includes an inphase signal and a quadrature signal for each carrier signal as is known in the art. The components of the inphase signal and the quadrature signal are grouped to form an I odd/Q even group and an I even/Q odd group. In order to restore the missing components in the I odd/Q even group, the I odd and the Q even components are duplicated and combined with the I odd/Q even group. In order to restore the missing components in the I even/Q odd group, the I even component is negated and used to form the next higher numbered I component and the Q odd component is negated and used to form the next higher numbered Q component. The resulting waveforms are subjected to an Inverse Discrete Fourier Transform function. The resulting imaginary waveform is staggered by one-half frame and then added to the real waveform. The resulting waveform envelope reaches its maximum at each frame boundary thus keeping interframe discontinuities to a minimum and reducing or eliminating interframe interference and distortion.

Modulation method and apparatus for multicarrier data transmission

A system and method for reducing echoes on a duplex communication channel having both transmitted and received signals carried thereon, particularly a telephone channel having digital data transmitted thereon by way of a modem. The transmitted signal is converted from a time domain signal to a frequency domain transmitted signal by a fast fourier transform process. The frequency domain transmitted signal is multiplied by a set of filter coefficients to generate estimated echo coefficients. The estimated echo coefficients are converted from the frequency domain to the time domain by an inverse fast fourier transform process and then subtracted from the received signal to generate a residual echo signal. The residual echo signal is converted to a frequency domain residual echo signal by a fast fourier transform process and then multiplied by the complex conjugate of the frequency domain transmitted signal to generate correlation frequency components. The correlation frequency components are then used to update the filter coefficients so as to minimize the residual echo and at least partially cancel the echo present on the communication channel.

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Salt Lake City, UT, United States of America

Mark P Mallory