Level One Communications, Inc.

Kenneth G Buttle

Hiroshi Takatori

James W Everitt

Daniel L Ray

A timing recovery apparatus for recovering the timing from sparse timing information in multi-level or partial response codes. The timing recovery apparatus includes a switch for sampling an incoming line code signal according to a selectable sample rate, a feed forward equalizer for filtering the sampled signal, a decision feedback equalizer for cancelling intersymbol interference in the filtered signal and for recovering the timing in the sampled signal. The timing recovery circuit creates a phase correction signal in response to a signal received from the feed forward equalizer and thereby control the sample rate of the sample switch so that the signal-to-noise ratio at the node before the decision is maximized. The voltage controlled crystal oscillator may be controlled within a certain frequency range by using a second phase detector which compares the phase of the signal controlling the sampling of the incoming line code with a reference clock.

Phase-locked loop timing recovery circuit

A circuit for comparing transmitted and received data frames containing appropriate stuff symbols to detect when a channel is disconnected. Transmit data symbols consisting of frames of user data, each with a constant number of symbols, are separated by a sequence of symbols known as the frame pattern. At the end of each data frame, just before the next frame pattern, a small number of stuff symbols are added into the frame. The stuff symbols injected in the transmit signal are chosen to be identical to the last scrambled data symbol in the frame of user data. Stuff symbols in a recovered receive signal are chosen to look identical to data symbols which were scrambled at the remote transceiver. A frame-by-frame comparison between the transmit stuff symbols and the next received stuff symbols is carried out on a continual basis. When the two sets of symbols are equal in each frame for a number of consecutive frames N, then the circuit indicates that only the transmit signal is being received and channel disconnection is detected.

Line disconnection detection circuit for echo-cancelling transceivers

An adaptive convergent decision feedback equalizer apparatus and method is disclosed for reducing intersymbol interference (ISI) in a data communication system. ISI is cancelled by generating and subtracting an estimation of the interference from a received signal. the estimation is generated by a N-tap transversal filter in which individual delayed received signals stored in the taps are multiplied by the respective adaptable tap coefficient and summed to form a digital representation of the ISI present. The present invention takes two steps to reduce the probability of coefficient adaptation from diverging. First, the DFE performs a coefficient modification only when the delayed received signal is a particular filter tap is a maximum or minimum level of the selected line code. A second step to eliminate divergence addresses the start up coefficient determination and errors in ISI estimation as the filter coefficients grow in size. On start up, only the first P number of DFE taps are chosen for modification and the remaining taps are held at zero. P is a number of taps for which the sum of the uncancelled ISI and the voltage error of an incorrect ISI estimation yields less than 0.5 probability of an error. After a minimum time (t.sub.min) which insures convergence, the remaining taps (N-P) are enabled.

Adaptive convergent decision feedback equalizer

A method and apparatus for echo cancelling linear and nonlinear echo components from a received signal by temporally and spatially separating the overall echo canceller into a fast, transversal filter based linear echo canceller which quickly cancels the linear echoes and a slow, look-up table based nonlinear echo canceller which cancels the nonlinear echo components.

Non-linear echo canceller

A transversal filter apparatus having a shift register, an averaging circuit for averaging data stored in sequential shift registers, and a multiplier for multiplying the averaged data. Additionally, a method and apparatus for echo cancelling a transhybrid response having a well behaved portion by using a transversal filter based echo canceller wherein the data stored in adjacent registers of the shift register which correspond to the well behaved portion of the transhybrid response are averaged together prior to being applied to a multiplier. This apparatus and method makes possible a reduction in the amount of hardware required to achieve a given level of echo cancellation.

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Orangevale, CA, United States of America

Kenneth G Buttle