Abstract: With the presently described echo canceller, a sparse adaptive transversal filter is used to model and cancel only the active regions of the echo impulse response. Since the tap coefficients, in the dormant regions, are set to zero and have no random fluctuation this approach reduces computation time and excess meansquare error in the tap coefficients. Also, the present design provides for increased speed of convergence of the sparse canceller as compared to the full-tap canceller, since convergence is proportional to the number of active taps in the LMS algorithm.

Abstract: There is provided a method of reducing the effects of transmission errors in a digital cellular mobile radio system in which R-S symbols are transmitted over a radio channel in two bursts, separated in time. This technique provides sufficient R-S error protection to recover the data if one of the bursts is entirely erased due to channel errors. With a TDMA channel format, speech data is sent in bursts over the radio channel. Each transmission bursts contain only half of the R-S symbols for a given R-S code block. The two bursts, comprising a complete R-S block, are separated in time so that a short Raleigh fade in the transmission channel is only likely to affect one of the bursts, allowing the data to be completely recovered from the unaffected burst. In order to provide sufficient error correcting power for this purpose, the R-S algorithm assumes that the number of protection symbols is at least as large as the number of data symbols.

Abstract: A two-wire transmission path repeater includes, between two hybrid circuits coupled to the path, an echo canceller and an amplifier for each direction of transmission, each echo canceller including a transversal filter and a subtracting circuit. The power ratio between signals before and after the subtracting circuit of each echo canceller is used to distinguish between simplex and duplex signals. If the power ratio exceeds a given value, which is dependent upon the input signal power, for only one transmission direction then a simplex signal in the opposite direction is determined, the gain of the amplifier for the one transmission direction is reduced, and adaptation of the transversal filter coefficients for the opposite direction is prevented.

Abstract: An adaptive differential pulse code modulation system (ADPCM), with adaptive prediction, has a transmitter, which substracts the output of its predictor from the original input signal and transmits a numeric representation of the quantized difference, and a receiver which reconstructs the original input signal by the reverse process. Mistracking, whereby the receiver fails to faithfully reproduce the original input signal because its pole-based predictor coefficients differ from those of the transmitter predictor, which can result from transmission errors when the original signal comprises two pure tones, is avoided by eliminating the dependence of the adaptation process upon the predictor output. This is achieved by deriving the coefficients using the difference signal but not the reconstructed signal.

Abstract: An improvement in torque characteristics is provided for a turboshaft engine of the type having a compressor rotor driven by a gas generator turbine that is mechanically independent of the power turbine. The improvement comprises a rotatable compressor stator casing which is interconnected with the power turbine rotor to allow rotation of the compressor casing in the same direction as the compressor rotor at a lower speed. At reduced output shaft speed, the relative speed between the compressor rotor blades and stator blades is increased for increased gas generator output and hence torque, for a predetermined compressor rotor speed.