METHOD AND APPARATUS FOR REDUCING NOISE IN PAM TIME DIVISION NETWORKS

摘要

METHOD AND APPARATUS FOR REDUCING NOISE IN PAM TIME DIVISION NETWORKS In pulse amplitude modulated time division switching systems, noise and crosstalk often result because the time division switches used to connect individual channels to a common bus are not perfect open circuits when they are in the off-state. The switches in the off-state possess a small parasitic capacitance which couples energy on the common bus to the channel resulting in a noise signal during time slots which are not associated with the channel. A method is disclosed for substantially reducing crosstalk and noise induced by stray capacitances without increasing the complexity of the time division switch. Each pulse on the common bus is converted into a bipolar pulse in which positive and negative portions have equal average values. In addition, the timing of each of the time division switches is modified to extract the desired signal by sampling the first portion of the bipolar pulse in the appropriate time slot for that channel, and rejecting the second portion of the bipolar pulse. The noise energy which is coupled from the common bus to the channel by stray switch capacitance during other time slots remains essentially bipolar in form and tends to cancel itself. the noise power at low frequencies is therefore substantially reduced. Apparatus is also disclosed for converting pulses on the common bus into the bipolar format required by the invention.