Browse Prior Art Database

Low-Complexity One-Way Speaker Phone

IP.com Disclosure Number: IPCOM000037113D
Original Publication Date: 1989-Nov-01
Included in the Prior Art Database: 2005-Jan-29
Document File: 4 page(s) / 95K

Publishing Venue

IBM

Related People

Chang, PC: AUTHOR [+3]

Abstract

This article describes a low-complexity one-way speaker phone which provides useful speakerphone functions, such as hands-free monitoring and on-hook dialing, at a cost comparable to a basic telephone.

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Low-Complexity One-Way Speaker Phone

This article describes a low-complexity one-way speaker phone which provides useful speakerphone functions, such as hands-free monitoring and on- hook dialing, at a cost comparable to a basic telephone.

The new one-way speaker phone is based on the basic telephone set and has the following additional components: a control logic circuit, three switches (analog switches or relays), a loudspeaker (may use the ringing circuit), a speaker (SP) push button, and an LED indicator. A block diagram providing an overview of the one-way speaker phone is given in Fig. 1. A typical basic telephone has only one switch, the line switch (LINE SW), which is completely controlled by the hook status, i.e., whether the handset is on-hook or off-hook. In the new phone, two switches, the microphone switch (MIC SW) and the speaker switch (SP SW), are added to provide the flexibility of disabling the microphone and enabling the loudspeaker to do on-hook monitoring and dialing. Since the connectivity to the subscriber loop needs to be maintained even when the handset is on-hook, the LINE SW can no longer be controlled only by the hook status. It, like the other two switches, is now controlled by a control logic, which takes into consideration the SP button signal, the hook status, and the internal state.

The control logic is responsible for the generation of control signals for these switches. It is a finite state machine whose input, output, and state signals are described in Fig. 2. Each input/output signal has a binary value. The control logic has four states, and hence each state is represented by 2 bits. Determined by its internal state and input signals, the control logic generates control signals for three switches, LINE SW, SP SW, and MIC SW. LINE SW connects the telephone set to the subscriber loop. SP SW and MIC SW enable the loudspeaker and the microphone, respectively. Note that the loudspeaker and the microphone operate exclusively to avoid the echo from the loudspeaker to the microphone. Therefore, only two output control signals, LINE and SP, need to be considered in the control logic.

The state transition diagram of a control logic which provides loudspeaker monitoring functions is given in Fig. 3. The state machine is asynchronous in the sense that there are no clock signals. There are no undefined state transitions because all possible combinations of inputs and states are considered. It has four states, i.e., idle, two-way conversation, on-hook monitoring, and off-hook monitoring. Normally, each state tr...