GB2309851A

GB2309851A - A hands-free telephone arrangement for transmission of pulse width modulated voice signals

Info

Publication number: GB2309851A
Application number: GB9701836A
Authority: GB
Inventors: Osamu Sato
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 1996-01-29
Filing date: 1997-01-29
Publication date: 1997-08-06
Anticipated expiration: 2017-01-29
Also published as: JPH09205480A; HK1001450A1; GB9701836D0; AU709742B2; AU1238297A; GB2309851B

Abstract

The system comprises a radio telephone terminal 10 in the boot of a car which is connected to a hands-free telephone set 20, in the cabin of the car, by a cable 30. Both the radio telephone terminal 10 and hands-free set 20 are provided with a modem 106, 107, 203, 204 for transmitting Pulse Width Modulated (PWM) voice signals along the cable. This reduces the effects of noise due to the ignition system etc. The Hands-free set is powered by power supply 109 in the radio telephone terminal 10 which detects the connection of Hands-free set 20 by monitoring a detection line.

Description

TZLPHONE SYSM WITH WS-FEE EQUIPMENT BACKGROUND OF THE INVENTION 1. Field of the invention The present invention generally relates to a telephone terminal system and, in particular, to a telephone terminal having a hands-free telephone set connected thereto through a cable.

2. Description of the Related Art Recently, many motor vehicles have been provided with telephone equipment having a loudspeaking terminal (or a hands-free set) to enhance the convenience of telephone while driving. An example of the loudspeaking terminal is disclosed in United States Patent No. 4,513,177. In general, the hands-free set is installed in the trunk (boot) and is electrically connected to a telephone terminal through a transmission cable. Reception and transmission voice signals are transferred through the cable in analog form.

However, such signal transmission through the cable is easily affected by external noises. When the mixing of a noise into the voice signals occurs, the quality of communication is directly reduced. Especially, in the case of an external hands-free set installed within a car, the communication quality is frequently affected by noises caused by an ignition system and electric motors for use in an air conditioner and a wiper (see Fig. 4).

In order to avoid the loss of communication quality like this, it is necessary to use a shielded cable or specially install the hands-free set in the car so as to reduce the length of the cable. For example, the hands-free set is removed from the trunk (boot) to inside the car. It is apparent that the special installation like this and the shielded cable increase the cost of a telephone system carried on the car.

SUMMARY OF THE INVENTION An object of at least the preferred embodiment of the present invention is to provide a telephone terminal system which achieves a substantially noise-immune hands-free set with simple interconnection.

Another such object is to provide a transfer method of voice signals between a telephone terminal and a hands-free set without reducing communication quality.

Broadly, according to the present invention, a pulse width modulation scheme is used to transfer a voice signal between a telephone terminal and an external loudspeaking terminal, resulting in improved noise-immunity with simple interconnection.

and eliminating special installation.

According to the present invention, in a system where the external loudspeaking terminal is electrically connected to the telephone terminal through a cable, a modem is provided to modulate and demodulate a voice signal based on a pulse width modulation scheme to transfer the voice information through the cable. More specifically, the telephone terminal and the external loudspeaking terminal are both provided with a first modem and a second modem, respectively. The first modem and the second modem are connected through the cable.

Preferably, the telephone terminal-includes a telephone hand set comprising a receiver and a transmitter and a selector for selecting one of the first modem and the telephone hand set to transmitting and receiving the voice signal to and from a selected one. The telephone terminal may be detachably connected to the cable through a connector. A controller for controlling operations of the telephone terminal preferably controls the selector such that the selector select the first modem when the external loudspeaking terminal is connected to the telephone terminal through the connector.

Since the system employs the pulse width modulation scheme, a demodulator of the modem is formed with a relatively simple circuit comprising a comparator and an averaging circuit (or a low pass filter). The comparator compares an input pulse-width modulated (PWM) signal received from the cable with a predetermined voltage level to produce a PWM signal. The averaging circuit is formed with a resistor and a capacitor having a sufficient large time constant with respect to the pulse-width modulation frequency so as to produce an average of the PWM signal as a demodulated voice signal.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described merely by way of example with reference to the accompanying drawings, wherein Fig. 1 is a block diagram showing a radio telephone terminal system according to an embodiment of the present invention; Fig. 2 is a circuit diagram showing a pulse width demodulator used in the embodiment; Fig. 3 is a waveform chart showing an operation of the embodiment; and Fig. 4 is a waveform chart showing an operation of a conventional telephone terminal system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Fig. 1, there is shown a mobile telephone terminal system in accordance with the present invention. The telephone terminal system is comprised of a mobile telephone terminal 10 and a hands-free set 20 which are electrically connected through a cable 30. The cable 30 is detachably connected to the telephone terminal 10 by a connector 40. The telephone terminal 10 includes a radio system 101 for radio transmission and reception and a speech processor 102 performing speech processing of transmission and reception voice signals, for instance, volume control.

The telephone terminal 10 is further comprised of a switch circuit 103 comprising a reception switch SWR and a transmission switch SUNS. The switch circuit 103 electrically connects the speech processor 102 to either a hand set including a receiver 104 and a transmitter 105 or a pair of a pulse width modulator 106 and a pulse width demodulator 107 which are connected to the hands-free set 20 through the connector 40 and the cable 30.

A controller 108 controls the operations of the telephone terminal 10 including switching control of the switch circuit 103.

The switch circuit 103 selectively connects the speech processor 102 to one of the hand set and the hands-free set 20 under the control of the controller 108. Power is supplied from a power supply 109 of the telephone terminal 10 to the hands-free set 20 through a power supply line of the cable 30. In this embodiment, the power supply voltage of the power supply line is returns to the connector 40 through a hands-free set detection line of the cable 30 and then it is monitored by the controller 108. Therefore, when the cable 30 of the hands-free set 20 is connected to the connector 40 of the telephone terminal 10, the controller 108 can detect its connection by monitoring a change of the voltage of the hands-free set detection line. When the controller 108 detects the connection of the hands-free set 20, the switch circuit 103 connects the speech processor 102 to the hands-free set 20 under the control of the controller 108.

The hands-free set 20 is comprised of a transmission system and a reception system. The transmission system includes a microphone 201, a microphone amplifier 202, and a pulse width modulator 203 and the reception system includes a pulse width demodulator 204, a speaker amplifier 205, and a speaker 206. The pulse width modulator 203 and the pulse width demodulator 204 are connected to the pulse width demodulator 107 and the pulse width modulator 106 of the telephone terminal 10 through the cable 30, respectively Therefore, in a transmission system, when receiving a transmission voice signal from the microphone amplifier 202, the pulse width modulator 203 produces a transmission PWM signal which travels over the cable 30 to the pulse width demodulator 107 of the telephone terminal 10. In a reception system, when receiving a reception voice signal from the speech processor 102, the pulse width modulator 106 produces a reception PWM signal which travels over the cable 30 to the pulse width demodulator 204 of the hands-free set 20.

As shown in Fig. 2, the pulse width demodulators 107 and 204 can be formed with a simple circuit including a comparator 301 and an RC circuit (a low pass filter or a smoothing circuit) consisting of a resistor 302 and a capacitor 303. More specifically, the input voltage V of a PWM (pulse width modulation) signal received from the cable 30 is compared with a threshold voltage VS generated by a reference voltage source.

When the input voltage VZN is greater than the threshold voltage v=, the comparator 301 produces a high-voltage level output.

Contrarily, when the input voltage V is not greater than the threshold voltage V, the comparator 301 produces a low-voltage level output. Therefore, the comparator 301 removes mixed noise components from the PWM signal by comparing it to the threshold voltage Vw to shape its rectangular waveform.

The time constant defined by a resistance R of the resistor 302 and a capacitance C of the capacitor 303 is set to an enough large value with respect to the frequency of the pulse width modulation so that the RC circuit can produce an average of the output voltage Vo of the comparator 301. In this manner, a voice signal sup is demodulated from the PWM signal. As described above, employing the pulse width modulation scheme results in the simplified circuit configuration and the sufficient performance of the demodulator with very low cost.

Bereinafter, a reception and transmission operation of the telephone terminal system mentioned above will be described ref erring to Figs. 1 and 3. In the case where the hands-free set 20 is used for communication, the switch circuit 103 is set at the state of connecting the speech processor 102 to the pulse width modulator and demodulator 106 and 107. A downstream radio signal is received from a base station (not shown) and is demodulated into a baseband signal by the radio system 101. The speech processor 102 reproduces a reception voice signal with controlling volume from the baseband signal and it is transferred to the pulse width modulator 106 through the switch SWR of the switch circuit 103.

Referring to Fig. 3, the pulse width modulator 106 modulates the reception voice signal to.produce the reception PWM signal.

the reception PWM signal is transferred from the pulse width modulator 106 to the pulse width demodulator 203 of the hands-free set 20 through the connector 40 and the cable 30. As described before, there is a possibility that some noise, for instance, an ignition noise and the like, is mixed into the reception PWM signal transmitting through the cable 30 (see (c) of the figure). The pulse width demodulator 204, however, removes the noise from the reception PWM signal by comparing the reception PWM signal to the threshold voltage VS (see (d)) and then demodulates the reception voice signal Sw (see (e)). The reception voice signal is amplified by the amplifier 205 and is converted to sound waves by the speaker 206.

on the other hand, user's voice is converted to an electric transmission voice signal by the microphone 201 of the handsfree set 20. After the transmission voice signal is amplified by the amplifier 202, the pulse width modulator 203 modulates the transmission voice signal to produce the transmission PWM signal.

The transmission PWM signal is transferred from the pulse width modulator 203 to the pulse width demodulator 107 of the telephone terminal 10 through the connector 40 and the cable 30. As described before, there is a possibility that some noise, for instance, an ignition noise and the like, is mixed into the transmission PWM signal transmitting through the cable 30 (see (c) of the figure). The pulse width demodulator 107, however, removes the noise from the transmission PWM signal by comparing the transmission PWM signal to the threshold voltage V= (see (d)) and then demodulates the transmission voice signal Sw (see (e) ) - The transmission voice signal is output to the speech processor 102 through the switch SWsof the switch circuit 103. The speech processor 102 produces an encoded speech signal with controlling volume and then the radio system 101 produces a transmitting radio signal based on the encoded speech signal to transmit it to the base station.

Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features.

The text of the abstract filed herewith is repeated here as part of the specification.

In a telephone terminal system where a hands-free set is electrically connected to a telephone terminal through a cable, they are both provided with a modem for modulating and demodulating a voice signal based on a pulse width modulation scheme to transfer the voice information through the cable.

Claims

What is claimed is:

1. A telephone terminal system comprising: a telephone terminal; an external loudspeaking terminal comprising a microphone and a speaker, the external loudspeaking terminal being electrically connected to the telephone terminal through a cable for transferring voice information therebetween; and modem means for modulating and demodulating a voice signal based on a pulse width modulation scheme to transfer the voice information through the cable.

2. The telephone terminal system according to claim 1, wherein the modem means comprises: a first modem provided in the telephone terminal, the first modem being electrically connected to one end of the cable; and a second modem provided in the external loudspeaking terminal, the second modem being electrically connected to another end of the cable.

3. The telephone terminal system according to claim 2, wherein the telephone terminal comprises: a telephone hand set comprising a receiver and a transmitter; and a selector for selecting one of the first modem and the telephone hand set to transmitting and receiving the voice signal to and from a selected one.

4. The telephone terminal system according to any of claims 1-3, wherein the telephone terminal further comprises a connector which is detachably connected to the cable.

5. The telephone terminal system according to claim 3, wherein the telephone terminal further comprises a connector and a controller, the connector being detachably connected to the cable, and the controller controlling operations of the telephone terminal, the controller controlling the selector such that the selector selects the first modem when the external loudspeaking terminal is connected to the telephone terminal.

6. The telephone terminal system according to claim 2, wherein the external loudspeaking terminal further comprises: a first amplifier for amplifying a transmission voice signal received from the microphone to produce the voice signal to be modulated by a modulator of the second modem; and a second amplifier for amplifying a reception voice signal received from a demodulator of the second modem to drive the speaker

7. The telephone terminal system according to claim 1, wherein a demodulator of the modem means comprises: a comparator for comparing an input pulse-width modulated (PWM) signal received from the cable with a predetermined voltage level to produce a PWM signal; and an averaging circuit for averaging the PWM signal to produce the voice signal.

8. The telephone terminal system according to any of claims 1-7, wherein the telephone terminal is a mobile telephone terminal carried on a mobile vehicle.

9. A transfer method for transferring voice information between a telephone terminal and an external loudspeaking terminal through a cable, the method comprising the steps of: modulating a voice signal carrying the voice information based on a pulse width modulation scheme to produce a PWM signal at a transmitting side; transferring the PWM signal over the cable; and demodulating the voice signal from the PWM signal received from the cable based on the pulse width modulation scheme at a receiving side.

10. A transfer method for transferring voice information between a telephone terminal and an external loudspeaking terminal through a cable, the method comprising the steps of: at the telephone terminal, receiving a radio signal from a base station to produce a reception voice signal; modulating the reception voice signal based on a pulse width modulation scheme to produce a reception PWM signal; and transferring the reception PWM signal to the external loudspeaking terminal over the cable, at the external loudspeaking terminal, demodulating the reception voice signal from the reception PWM signal received from the cable based on the pulse width modulation scheme at the external loudspeaking terminal; and producing sound waves corresponding to the reception voice signal using a speaker of the external loudspeaking terminal.

11. The transfer method according to claim 10, further comprising the steps of: at the external loudspeaking terminal, receiving sound waves by using a microphone of the external loudspeaking terminal to produce a transmission voice signal; modulating the transmission voice signal based on the pulse width modulation scheme to produce a transmission PWM signal at the external loudspeaking terminal; and transferring the transmission PWM signal to the telephone terminal over the cable, at the telephone terminal, demodulating the transmission voice signal from the transmission PWM signal received from the cable based on the pulse width modulation scheme at the telephone terminal; and transmitting a radio signal produced based on the transmission voice signal to a base station.

12. The telephone terminal system according to claim 3, wherein the telephone terminal further comprises a connector which is detachably connected to the cable, the cable including a power supply line through which power is supplied from the telephone terminal to the external loudspeaking terminal.

13. The telephone terminal system according to claim 12, the telephone terminal further comprises a controller for controlling operations of the telephone terminal, the controller controlling the selector such that the selector selects the first modem when the external loudspeaking terminal is connected to the telephone terminal.

14. The telephone terminal system according to claim 13, wherein the connector includes a detection line which is electrically connected to the power supply line in a cable side to return a power supply voltage to the telephone terminal so that the controller detects connection of the external loudspeaking terminal by monitoring the detection line.

15. The telephone terminal system according to claim 1, wherein the external loudspeaking terminal is a hands-free set.

16. A telephone terminal system or a transfer method substantially as herein described with reference to figures 1 to 3 of the accompanying drawings.