Browse Prior Art Database

Full Duplex Speakerphone

IP.com Disclosure Number: IPCOM000039368D
Original Publication Date: 1987-May-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 5 page(s) / 44K

Publishing Venue

IBM

Related People

Einkauf, MA: AUTHOR [+3]

Abstract

A speakerphone receives input from a local party through a microphone, processes that input, and outputs the resulting signal to the telephone line. Meanwhile, the speakerphone receives input from the remote party through the telephone line, processes that input, and outputs the resulting signal to the speaker. The main problem which must be solved by any speakerphone implementation is the reduction or elimination of echoes which occur at the telephone line interface, and between the speaker and microphone. The telephone line echoes are caused primarily by hybrid transformers used to connect the 4-wire speakerphone (+/- connections for input (mike) and +/- connections for output (speaker)) to the 2-wire (tip and ring) telephone line path which is predominantly used throughout national telephone networks.

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Full Duplex Speakerphone

A speakerphone receives input from a local party through a microphone, processes that input, and outputs the resulting signal to the telephone line. Meanwhile, the speakerphone receives input from the remote party through the telephone line, processes that input, and outputs the resulting signal to the speaker. The main problem which must be solved by any speakerphone implementation is the reduction or elimination of echoes which occur at the telephone line interface, and between the speaker and microphone. The telephone line echoes are caused primarily by hybrid transformers used to connect the 4-wire speakerphone (+/- connections for input (mike) and +/- connections for output (speaker)) to the 2-wire (tip and ring) telephone line path which is predominantly used throughout national telephone networks. The speaker/mike echo is caused by the feedback of the speaker output into the microphone input. If these echoes are not eliminated or significantly reduced, they may produce sustained oscillations in the audio frequency band which will render the speakerphone unusable. Traditional speakerphones eliminate the echo problem by allowing only one path to be active at any one time. That is, while the mike input is output to the telephone line, no telephone line input is accepted, and, likewise, while the telephone line input is output to the speaker, no mike input is accepted. In this way, the echo paths are eliminated, but at the sacrifice of natural conversational flow (one party has difficulty interrupting the other, the beginning and ending of statements may be eliminated due to switching delay, etc.). A full duplex speakerphone is described which is implemented as a digital signal processing program. It sufficiently reduces the echo caused by the telephone line interface to prevent sustained oscillations, while allowing both party's inputs (mike for local user(s) and telephone line input for remote user(s)) to be active at all times. Significantly reducing the speaker/mike echo is considerably more difficult than reducing the telephone line echo and requires processing power beyond that available on the hardware on which the full duplex speakerphone is implemented. However, by maintaining a minimum separation between the speaker and mike, and limiting mike input and speaker output amplitudes (i.e., volumes) to reasonable levels, the speaker/ mike echo is insignificant so that no reduction of this echo is required.

The basic function of the full duplex speakerphone is to reduce the telephone line interface echo. This is accomplished by passing the same signal that is output to the telephone line through an adaptive filter, which attempts to model the characteristics of the telephone line interface. The output of the adaptive filter then closely approximates the telephone line input which is the echo of the previous telephone line output. By subtracting the adaptive filter output from the telephone line inpu...