GB2256342A

GB2256342A - Feedback reducing telephone handset

Info

Publication number: GB2256342A
Application number: GB9209843A
Authority: GB
Inventors: Robin Christopher Cross
Original assignee: British Telecommunications PLC
Current assignee: British Telecommunications PLC
Priority date: 1991-05-08
Filing date: 1992-05-07
Publication date: 1992-12-02
Anticipated expiration: 2012-05-07
Also published as: GB9109895D0; GB9209843D0; GB2256342B

Abstract

A telephone handset has a housing 2 and a receiver 1 mounted within the housing adjacent to a resiliently-mounted cover plate 3b forming part of the housing. The arrangement is such as to permit relative movement of the receiver 1 and the cover plate 3b between a first operating position in which the cover plate substantially blocks off the acoustic output of the receiver, and a second operating position in which the acoustic output of the receiver is substantially unblocked. The relative motion is caused by placing the handset against the user's head. Thus the arrangement prevents acoustic feedback when the handset is used away from the head. A further embodiment is disclosed (Figs 4 to 6). <IMAGE>

Description

TELEPHONE HANDSET This invention relates to a telephone handset, and in particular to a mobile telephone handset.

With the advent of mobile telephone handsets, and the proposed Group Systems Mobile (GSM) mobile digital network, it has become clear that the terminal coupling loss (TCL) of most mobile telephone handsets is inadequate. TCL is the frequency dependent loss between the receiver and the microphone of a telephone handset, and is inversely proportional to the amount of signal fed back from the receiver to the microphone.

When in normal use, the earpiece of a normal telephone handset is pressed against the ear of the user, whereby the user' s ear constitutes a block to the external air feedback path, so that there is low feedback and hence a high TCL.

Unfortunately, mobile handsets are not usually shaped to conform with the contours of a user' 5 head, so the earpiece of such a telephone is not usually pressed against the user's ear. Consequently, the external air feedback path to the microphone is only partially blocked, so the handset effectively has a low TCL. Worse still, dialling is invariably done with the handset away from the user' s head, so that, as soon as the connection is made, the external air feedback path is completely unbroken, and hence the TCL is very low. The GSM network is fully digital, and includes (for access requirements) a delay of 100 to 150 milliseconds in each speech path. If the TCL of a mobile telephone handset is low, this delay means that the user of the handset will hear an echo if his/her voice or even that howl-around will occur.Thus, control of echo at a mobile handset of a digital network is necessary to provide satisfactory echo performance to the user at the other connection end.

The aim of the invention is to provide a mobile telephone handset with a TCL that is sufficiently high to provide echo-free performance without the use of additional echo control devices, such as echo cancellers, in the network. In this connection, it should be noted that a minimum TCL of 45dB is required to provide such a performance, and that known handsets do not meet this requirement.

The present invention provides a telephone handset having a housing, a receiver and a microphone mounted within the housing, the microphone and the receiver being associated with the ends of an external air feedback path for acoustic signals between the receiver and the microphone, and isolation means for isolating the receiver from the external air feedback path, wherein the isolation means comprises a cover plate positioned adjacent to the receiver, the arrangement being such as to permit relative movement between the receiver and the cover plate between a first operating position, in which the cover plate substantially blocks off the acoustic output of the receiver, and a second operating position, in which the acoustic output of the receiver is substantially unblocked.

Because the acoustic output of this handset is blocked unless the cover plate is moved relative to the receiver, it has a high TCL when the connection is made following dialling carried out with the handset away from the user' s head. Moreover, the user will not receive the acoustic output of the receiver unless the receiver is pressed against his/her ear so as to move the cover plate relative to the receiver. However, the act of pressing the receiver against the ear blocks the external air feedback path from the handset receiver to its microphone, so again a high TCt results.

Advantageously, the receiver is fixed within the housing, and the cover plate is movable relative to the receiver. Preferably, the handset further comprises an additional cover plate fixed to the housing, the cover plate and the additional cover plate forming a cover plate assembly, and the cover plate being movable relative to the additional cover plate thereby constituting said relative movement of the receiver and the cover plate. The cover plate may be slidable or pivotable relative to the additional cover plate.

Conveniently, the cover plate is spring biased toward its first operating position.

Two forms of mobile telephone handset, each of which is constructed in accordance with the invention, will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which Figure 1 is a perspective view of the receiver portion of the first form of handset; Figure 2 is a part-sectional side elevation of the receiver portion of Figure 1 in a first operating position; Figure 3 is a part-sectional side elevation of the receiver portion of Figure 1 in a second operating position; Figure 4 is a perspective view of the receiver portion of the second form of handset; Figure 5 is a part-sectional side elevation of the receiver portion of Figure 4 in a first operating position; and Figure 6 is a part-sectional side elevation of the receiver portion of Figure 4 in a second operating position.

Referring to the drawings, Figures 1 to 3 show the receiver portion of the first form of mobile telephone handset, the receiver portion including a low acoustic output impedance receiver 1 mounted within a housing 2.

The housing 2 is provided with sound holes 2a for facilitating sound reception. A two part cover plate assembly, indicated generally by the reference numeral 3, fits over the housing 2 so as to overlie the sound holes 2a. The cover plate assembly 3 has a fixed cover plate 3a and a movable cover plate 3b. The movable cover plate 3b is mounted on a pair of locating pins 4 fixed to the housing 2 for movement towards (and away from) the housing.

A compression spring 5 is mounted between the movable cover plate 3b and the adjacent portion of the housing 2, the spring biassing the movable cover plate away from the housing into a "receiver closed-off'1 position (see Figure 2), in which the adjacent edges of the two cover plates 3a and 3b are substantially contiguous.

The spring 5 is rated so that light pressure (indicated by the arrows A in Figure 3) is sufficient to move the cover plate 3b towards the adjacent portion of the housing 2, thereby exposing the sound holes 2a. Thus, when the handset is in use, and the receiver portion is pressed against the user' s ear, the acoustic output of the receiver is unblocked (via the sound holes 2a and the gap between the two cover plates 3a and 3b). However, because it is necessary to press the receiver portion of the handset firmly against the ear for the user to hear acoustic signals emitted by the receiver, the user's ear forms an effective block to the external air feedback path to the microphone of the handset. Consequently, the TCL of the handset is high, and the user will not experience echo or howl-around problems. Moreover, after dialling and the connection is made, but before the handset is positioned against the user' s ear, there will be no pressure on the movable cover plate 3b, so this will block the acoustic output of the receiver, thereby blocking the external air feedback path to the microphone. Once again, therefore, the TCL of the handset will be high, thereby preventing echo and/or howl-around problems.

Figures 4 to 6 show the receiver portion of the second form of mobile telephone handset. This handset is similar to that of Figures 1 to 3, so like reference numerals will be used, and only the modified parts will be described in detail. Thus, the handset receiver portion of this embodiment has a cover plate assembly 13 having a central, circular fixed cover plate 13a, and a movable cover plate 13b which surrounds the cover plate 13a. The movable cover plate 13b is pivotably mounted on the housing 2 by means of a pivot pin 14 fixed to the housing.A compression spring 15 is mounted between the housing 2 and that portion of the movable cover plate 13b on the opposite side of the fixed cover plate 13a to the pivot pin 14, the spring biassing the movable cover plate away from the housing into a receiver closed-off" position (see Figure 5), in which the adjacent edges of the two cover plates are substantially contiguous.

As with the embodiment of Figures 1 to 3, the light pressure exerted on the movable cover plate 13b by pressing the handset receiver portion against a user' s ear is sufficient to move the cover plate 13b towards the adjacent portion of the housing 2, thereby exposing the sound holes 2a. Consequently, the TCL of the handset is high when it is being used properly, and also after dialling when the connection is made.

An important advantage of each of the handsets described above is that the control of the acoustic output of the receiver gives sufficient flexibility of circuit design to permit changes in gain. Thus, in noisy environments (such as car interiors) more gain is required to obtain a predetermined volume level at the users ear, and this would normally result in high feedback when the external air path is not closed by the user's head (as is usual for car phones when used in "hands-off" mode). The control of the receiver output allows car 'phones to have an increased gain without this adversely affecting the 'phones TCL when in the "hands-off" mode.

It will be apparent that modifications could be made to the handsets described above. In particular, there are many ways of constructing a movable cover plate for controllably blocking off the handset receiver. Obviously, the most practical solutions will involve the least number of moving parts and the lowest tolerances on the mouldings.

The mechanism can also be made to control smoothly the volume of sound output by the receiver with a relativelylarge amount of travel of the movable cover plate, or to provide an on/off facility. In either case, mobile handsets can be made with a maximum increase in TCL of between 20dB and 30dB. This increase in TCL is sufficient to meet the GSM requirement for mobile handsets to have a TCL of 45dB. A further modification would be to move the receiver backwards from a "blocked" position to allow its output to be unblocked.

For technical reasons, low acoustic output receivers operate best with the types of handset described above, as the change in acoustic output that occurs, in use, is not accompanied by a change in the frequency response of incoming speech.

Claims

1. A telephone handset having a housing, a receiver and a microphone mounted within the housing, the microphone and the receiver being associated with the ends of an external air feedback path for acoustic signals between the receiver and the microphone, and isolation means for isolating the receiver from the external air feedback path, wherein the isolation means comprises a cover plate posi.tioned adjacent to the receiver, the arrangement being such as to permit relative movement between the receiver and the cover plate between a first operating position, in which the cover plate substantially blocks off the acoustic output of the receiver, and a second operating position, in which the acoustic output of the receiver is substantially unblocked.

2. A handset as claimed in claim 1, wherein the receiver is fixed within the housing, and the cover plate is movable relative to the receiver.

3. A handset as claimed in claim 1 or claim 2, further comprising an additional cover plate fixed to the housing, the cover plate and the additional cover plate forming a cover plate assembly, and the cover plate being movable relative to the additional cover plate thereby constituting said relative movement between the receiver and the cover plate.

4. A handset as claimed in claim 3, wherein the cover plate is slidable relative to the additional cover plate.

5. A handset as claimed in claim 3, wherein the cover plate is pivotable relative to the additional cover plate.

6. A handset as claimed in any one of claims 1 to 5, wherein the cover plate is spring biassed toward its first operating position.

7. A telephone handset substantially as hereinbefore described with reference to, and as illustrated by, Figures 1 to 3 or Figures 4 to 6 of the accompanying drawings.