GB2218302A

GB2218302A - Hands-free telephone

Info

Publication number: GB2218302A
Application number: GB8810882A
Authority: GB
Inventors: Roger Bennett Thorp
Original assignee: Crystalate Electronics Ltd
Current assignee: Crystalate Electronics Ltd
Priority date: 1988-05-07
Filing date: 1988-05-07
Publication date: 1989-11-08
Also published as: GB8810882D0

Abstract

A hands-free or loudspeaking telephone arrangement comprises a three-sided housing (21) dimensioned to accommodate the head of a user (22). Two loudspeakers (2, 3) are located in side portions (21A, 21B) of the housing, in line with the ears of the user (22) and a microphone (1) is located in a portion (21C) of the housing in front of the mouth of the user (22). The loudspeakers (2, 3) are interconnected and arranged so as to produce sound outputs with a predetermined mutual phase relationship which combine at the location of the microphone (1) to produce minimum acoustic effect on the microphone. The arrangement is particularly applicable for road-side use and prevents undesirable feed-back between the loudspeakers and microphone. In the case of a pressure-sensitive microphone, the arrangement is such that at the microphone location (1) sound pressure variations resulting from the loudspeakers are minimised. <IMAGE>

Description

SPECIFICATION TITLE: HANDS FREE TELEPHONE ARRANGEMENT This invention relates to hands-free telephones. Such telephones, which are also known as loudspeaking telephones, may be provided as free-standing devices connected by switches and leads to conventional handset telephones. Alternatively they may be incorporated within such handset telephones, with switching into a 'hands-free' mode being effected by a user. A hands-free telephone usually incorporates circuitry to separate incoming signals, which are delivered to the user by way of a loudspeaker, from outgoing signals which are derived from a microphone. Such circuitry normally operates to compare the levels of incoming and outgoing signals and switch to that mode exhibiting the highest signal level.

A major operational problem with a hands-free instrument is feedback from the loudspeaker to the microphone. Such feedback can be either seismic or acoustic in nature and causes instability in the circuitry, resulting in undesirable oscillations creating an unpleasant howling sound. If a user of a hands-free telephone is listening to incoming speech and wishes to speak to the caller, this may be achieved by speaking sufficiently loudly for the microphone and its associated circuitry to establish, at a signal comparator, a higher level than that of the incoming signal from the caller. At this point the instrument switches from a 'receive' mode to a 'send' mode. Such instruments normally operate in a 'simplex' mode, whereby they either send or receive, but not simultaneously.

Normal telephony operates in a 'duplex' mode whereby it is possible to send and receive signals simultaneously. It would be desirable if hands-free telephones could operate in duplex mode or simplex mode without the problem of unwanted feedback referred to above and it is an object of the present invention to provide a means whereby this problem is overcome or minimized.

The present invention provides a hands-free or loudspeaking telephone arrangement incorporating a microphone and a receiving means and characterised in that the receiving means comprises two or more interconnected loudspeakers located in predetermined positions with respect to the microphone and adapted and arranged to co-operate in such a way as to produce sound outputs therefrom with a predetermined mutual phase relationship which combine at the location of the microphone to produce minimum acoustic effect on the microphone.

In the case of a pressure-sensitive microphone, the loudspeakers are suitably adapted and arranged such that at the location of the microphone sound pressure variations resulting from the loudspeakers are minimised.

The loudspeakers are suitably interconnected in series or in parallel, or in any other appropriate arrangement.

The microphone and loudspeakers are suitably located on a planar, curved or multi-sided, e.g., three-sided, mounting means. Such a mounting means may comprise, or form part of, or be located in, a housing for the telephone arrangement.

In a particular embodiment, the microphone and loudspeakers are arranged such that the head of a user can be accommodated between the loudspeakers with the loudspeakers facing one another and such that the mouth of the user is in front of the microphone. Such an arrangement is particularly advantageous in a noisy environment such as a factory, airport, railway station or roadside location. Ambient noise can be effectively reduced by means of housings provided for the loudspeakers, while the user's head assists in preventing some of the unwanted noise from reaching the microphone. It is to be noted that this arrangement is not a headphone where speakers are in contact with the user's ears and where external acoustic insulation is normally provided. In the present invention clearance of several inches will typically be provided between the loudspeakers and a user's head.

Indeed, in suitable conditions it is possible to operate with a user's head positioned at a distance of several feet from the microphone and loudspeakers.

The invention is now described by way of example with reference to the accompanying drawings in which: Figures 1 and IA represent plan views of alternative embodiments of a telephone arrangement according to the invention; Figure 2 represents a cross-sectional view of a further embodiment of telephone arrangement according to the invention; Figure 3 represents a perspective view of the telephone arrangement of Figure 2; Figure 4 represents a diagram of an embodiment of circuit for use with a telephone arrangement of the invention.

Referring to Figure 1, a hands-free, or loudspeaking, telephone arrangement includes a flat plate 20 on which are mounted two substantially identical loudspeakers 2, 3, arranged to face a user (not shown). A microphone is located in an area 1 on the plate 20 and arranged to face the user and so as to be substantially equidistant from the two loudspeakers 2, 3. The loudspeakers 2, 3 are interconnected and arranged in such a way as to produce sound outputs with a predetermined mutual phase relationship which combine at the location of the microphone to produce minimum acoustic effect on the microphone. If the microphone at location 1 is of the usual pressure-sensitive type, the arrangement is such that at this location 1 sound pressure variations resulting from the loudspeakers are minimised.

If loudspeakers 2, 3 having different characteristics are employed, each producing different sound pressures, an arrangement as shown in Figure 1A is adopted whereby the microphone location 1 is offset towards one of the loudspeakers 3 to ensure that resultant minimum acoustic effect is produced on the microphone. With the arrangements shown in Figures 1 and 1A, problems of feedback from the loudspeakers to the microphone are minimised or reduced.

Figures 2 and 3 show an alternative arrangement of hands-free telephone which is particularly suitable for use in noisy environments and is particularly applicable as a telephone arrangement for road-side use. A three-sided housing 21 is provided of suitable dimensions to accommodate the head of a user 22. The two loudspeakers 2, 3 are located in side portions 21A and 21B of the housing, substantially in line with the ears of the user 22 and the microphone 1 is located in a portion 21C of the housing, in front of the mouth of the user 22. The housing 21 provides screening from external ambient noise while the user's head 22 assists in preventing some of the unwanted noise from reaching the microphone 1. The arrangement operates in similar manner to that described with reference to Figure 1.

'On' and 'Off' switches 23, 24 are provided for starting and terminating calls and other buttons or switches 25 are provided for number selection, e.g., as in a normal keypad. The buttons or switches 25 may alternatively be arranged such that predetermined numbers can be obtained by single button or switch operation.

An automatic call-terminating or 'time-out' arrangement (not shown) may be provided to operate if a user 22 neglects to terminate a call.

Such an arrangement serves to prevent unnecessarily protracted use of exchange equipment by providing automatic disconnection from the exchange equipment after a predetermined time interval. Provision may be made for extending the duration of a call by further operation of the switch 23 provided for switching-on the equipment, before the automatic call terminating arrangement operates.

Means (not shown) may be provided to give an audible or visual indication that a call is about to be automatically terminated.

The telephone arrangement of the invention can be embodied for coin or charge-card operation, i.e., as a 'payphone', but finds particular application on a free-of-charge basis where operation is dedicated to an organisation providing a service, e.g., police, fire or ambulance services or road rescue services. The invention is particularly advantageous in providing a hands-free telephone for roadside use. Memory means (not shown) may be provided for storing data such as telephone area codes and numbers. Such memory means may be provided with battery support means. Provision may be made for programming appropriate numbers and codes into the memory means by way of a user-inaccessible keypad which may either be located inside a suitable housing or temporarily plugged into the telephone arrangement for programming purposes.This ensures that users are only able to call one or more dedicated numbers.

The telephone arrangement of the invention can be provided for operation in simplex or duplex mode and is capable of operating over any length of telephone line commonly used without requiring additional power to be provided over that supplied as normal line current from an exchange.

Figure 4 shows a circuit arrangement for use in the telephone arrangement of the invention. Sounds detected by the microphone 1 are amplified by an amplifier 30 and the resulting amplified signal is compared, by means of a comparator 4, with a threshold setpoint generated by a threshold amplifier 5. The threshold setpoint is adjustable, by a user, to a required level by means of a control 12 in order to suit the environment in which the telephone arrangement is being used. This threshold setpoint level suitably corresponds to a signal level expected from the microphone as a result of speech from a user's mouth in very close proximity thereto.The comparator 4 has an output from which a transmitted signal, resulting from sounds above a pre-set level detected by the microphone 1, is fed simultaneously to a transmitted signal attenuator 7, a noise discriminating circuit 6 and a comparator 9 for transmitted and received signals. The noise discriminating circuit 6 produces an output to an attenuator control 8 such that the latter causes the transmitted signal attenuator 7 to be high impedance to the transmitted signal, in the case of background noise being detected by the microphone 1, and low impedance to the transmitted signal in the case of speech from a user's mouth in very close proximity to the microphone 1. An attenuator 11 is provided for incoming signals received over a telephone network 16.The attenuator control 8 has a pair of outputs which are arranged to be complementary such that the transmitted signal attenuator 7 is high impedance, transmitted signal attenuator 7 is high impedance, and the received signal attenuator 11 is low impedance, or vice versa. This ensures that when a signal is being transmitted from the microphone, no signal will be received at the receiver. The attenuator control 8 has, as one of its inputs, an output from the comparator 9 which compares the relative levels of a transmitted signal from the output of the comparator 4 and a signal received over the network 16 and amplified by an amplifier 14. The output from the comparator 9 accordingly determines the state of operation of the attenuator control 8.

When the transmitted signal attenuator 7 is in its low impedance state, signals resulting from a person speaking close to the microphone 1 are transmitted over the network 16. when the received signal attenuator 11 is in its low impedance state, signals received over the network 16 pass through the attenuator 11 and are amplified by an amplifier 10 to an appropriate level to suit the loudspeakers 2, 3. Volume control means may be provided whereby a user may set the volume of the received signals to a desired level.

A 2-wire to 4-wire converter 13 is provided for interfacing the signals transmitted by the microphone, and also the received signals, with the signal processing circuits. Converter 13 may be of well-known hybrid form, involving a multi-winding transformer, or may comprise other known forms of integrated transmission circuits.

Appropriate interfacing circuitry 15 may also be provided for connecting the telephone arrangement to the network 16.

Although the attenuator control 8 may provide two-way transmission by means of a simplex path, in which the telephone arrangement may either receive or transmit, but not both simultaneously, it may also be arranged to provide transmission by means of a duplex or semi-duplex path, in which the arrangement is capable of transmitting and receiving simultaneously, either in full or at reduced level.

Claims

1 A hands-free or loudspeaking telephone arrangement incorporating a microphone and a receiving means and characterised in that the receiving means comprises two or more interconnected loudspeakers located in predetermined positions with respect to the microphone and adapted and arranged to co-operate in such a way as to produce sound outputs therefrom with a predetermined mutual phase relationship which combine at the location of the microphone to produce minimum acoustic effect on the microphone.

2 An arrangement according to Claim 1, in which the microphone is pressure-sensitive and the loudspeakers are adapted and arranged such that at the location of the microphone, sound pressure variations resulting from the loudspeakers are minimised.

3 An arrangement according to Claim 1, or 2, in which the microphone and loudspeakers are located on a planar, curved or multi-sided mounting means.

4 An arrangement according to Claim 3 in which the mounting means comprises, or forms part of, or is located in, a housing for the telephone arrangement.

5 An arrangement according to any preceding Claim in which the microphone and loudspeakers are arranged such that the head of a user can be accommodated between the loudspeakers with the loudspeakers facing one another and such that the mouth of the user is in front of the microphone.

6 A hands-free or loudspeaking telephone arrangement constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.