Northern Telecom Limited

Edward M Sich

Guy J Chaput

Beverley W Gumb

An electret microphone has a metal shield around it in order to guard against electromagnetic interference. Acoustic waves reach the electret element through a passage in the shield. The electret response is undesirably affected by an electric field produced by body capacitance when the microphone is brought close to a user's mouth. To overcome this effect, the microphone has a conductive film, transparent to acoustic waves, positioned between the passage and the electret element, the film contacting a grounded part of the casing. The conductive film can form an integral part of a microphone moisture barrier.

Electret microphone shield

A porcelain coated metal substrate bearing a printed circuit and in which a capacitive coupling is created between the substrate and the circuit to eliminate unwanted frequencies. This coupling is formed by the inclusion of a conductive element on the porcelain. The conductive element is out of direct electrical contact with the printed circuit and has a means to connect the element to ground. Connection to ground provides the desired capacitive coupling and another capacitive coupling from the metal substrate to the conductive element, the two couplings being in series.

Electrical circuit on porcelain coated metal substrates

Telephone circuit using a transconductance switch in a high current line with the voice network, and a dial network in a low current line with the control input of the switch to provide dial input signals at the control input and send dial pulse signalling along the high current line through the voice network. The voice network includes a current clamp to stabilize the high current through the switch to the voice network sufficiently quickly to establish the desired waveform during dialling.

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Ottawa, Canada

Edward M Sich