Abstract: This invention applies to the means whereby capacitance changes due to varying temperature and/or pressure in a piezoelectric transducer used for acoustic telemetry in a drilling environment is dynamically offset by modifying one or more parameters associated with the drive or control circuitry of said transducer. The object of the invention is to closely maintain the transducer in a resonant mode, thereby ensuring optimum energy consumption.

Abstract: A double echo cancelling system includes comparators which monitor, respectively, "unmodified" signals from the near end or far end and "modified" signals from the other end. The unmodified signals are the signals received by the system and the modified signals are those signals which have been subjected to echo cancellation and attenuation. A comparator determines that a caller at one end is speaking if the associated unmodified signals exceed by more than a predetermined value the modified signals associated with the other end. The comparator which detects near end speech controls the adaptation of an associated adaptive filter and the operation of a cancellation monitor, which, in turn, controls the adaptation of the adaptive filter associated with the far end. When this comparator detects near end speech, it enables the cancellation monitor.