WO1997004577A1

WO1997004577A1 - Loudspeaking telephone systems

Info

Publication number: WO1997004577A1
Application number: PCT/GB1996/001641
Authority: WO
Inventors: David Connolly
Original assignee: Hong Kong Telecommunications Limited
Priority date: 1995-07-17
Filing date: 1996-07-09
Publication date: 1997-02-06
Also published as: AU6366396A; GB2317535B; GB9800665D0; GB2317535A; GB9514553D0

Abstract

A loudspeaking telephone network has an echo cancelling device (EC) for serving a number of telephones. Each telephone has a microphone (MIC) and a loudspeaker (LS) which are separately connectable by transmission channels to supply voice signals to the echo cancelling device and to receive voice signals from the echo cancelling device. In this way high quality echo cancelling is provided for any telephone on demand that is connectable to the network and the relatively high cost of echo cancelling devices is shared in effect by many telephones.

Description

L0UD5PEAKI G TELEPHONE SYSTEMS

The invention relates to networks for loudspeaking telephone systems.

Loudspeaking telephone systems provide ease and convenience to users with their handsfree operation. However, the design of handsfree is complicated by the need to prevent instability or "howl round" and to prevent the sound of "hollowness" experienced by the distant talker, who usually uses a conventional telephone set.

In loudspeaking telephones, the additional amplification required for microphone and loudspeaker tend to cause instability because some of the loudspeaker signals are returned to the microphone due to acoustic reflections in the room, then amplified and reflected at the hybrid in the telephone.

In earlier designs, this instability was prevented using a voice switch, in which either the microphone signal was used to switch off a loudspeaker amplifier, or the received speaker signal was used to switch off a microphone amplifier. In this case, the telephone waε unsatisfactory in situations where both parties in a conversation talked, one of them would be switched off, introducing the unnatural symptom of "clipping". In more recent loudspeaking telephone designs, the technique of echo cancellation is used in v:hich the signal received by the microphone is correlated with the signal sent to the loudspeaker, such that element of signals resulting from room echo can be cancelled while the talker signals are not.

Certain more sophisticated echo cancelling devices are a known for telephones to prevent electrical echo, particulary on long distance and satellite calls. In a loudspeaking telephone itself, the acoustic echo path requires significantly more processing and memory in the echo cancellation process, and only recently have acoustic echo cancelling loudspeaking telephones been introduced, giving more natural conversation capability and a significant reduction in difficulty experienced by the distant party. Though acoustic echo cancelling loudspeaking telephones have satisfactory performance, the cancellation device adds a significant cost to the telephone, making it suitable only for limited applications such as conference rooms.

It is an object of the invention to overcome or at least reduce this problem.

According to the invention there is provided a loudspeaking telephone network including an echo cancelling device, in which the echo cancelling device is provided for serving a plurality of telephones, each telephone being connectable to the echo cancelling device by transmission channels arranged to separately transmit outgoing and incoming signals to echo cancelling device.

The echo cancelling device may be arranged to transmit and receive signals via a digital radio interface.

The signals transmitted between the echo cancelling device and the telephones may have a frequency bandwidth of around 7 KHz or more.

The echo cancelling device may be connected to other networks by two types of transmission channels that normally operate at different bandwidth frequencies and is arranged to selectively receive and transmit signals from the two types of channels.

Loudspeaking telephone networks according to the invention will now be described by way of example with reference to the accompanying schematic drawings in which:

Figure 1 shows a known form of network;

Figure 2 shows another known form of network;

Figure 3 shows a network according to the invention;

Figure 4 shows a network according _.to the invention for a cordless telephone; Figure 5 shows another network according to the invention.

Referring to the drawings, in Figure 1 a handsfree telephone has a microphone (MIC) and a loudspeaker (LS) connected by transmission channels to a hybrid unit 10. A voice signal detector (DET) and switchable amplifier (SWA) is arranged in each channel so that the microphone signals can be used to switch OFF the loudspeaker amplifier and the loudspeaker signals used to switch OFF the microphone amplifier. This prevents amplification of acoustic reflections which are otherwise reflected at the hybrid unit 10.

In Figure 2, a similar arrangement is shown. An echo cancelling device (EC) is connected between the microphone and loudspeaker channels. In thiε case, the echo cancelling device is arranged to correlate the signals sent to the loudspeaker and the signals received by the microphone. Discrimination can therefore be made between the telephone speaker's voice and the incoming signals received at the loudspeaker. Elements of any signals acoustically reflected in the room are illi inated by the echo cancelling device applying appropriate controls to a cancelling amplifier (CA) . Each telephone is provided with a dedicated echo cancelling device (EC) .

Referring to Figure 3, the network includes an echo cancelling device (EC) which can serve a number of telephones in an otherwise normal fashion. Each telephone includes a microphone (MIC) and a digital coder (COD) , and a loudspeaker and a digital decoder (DEC) . Transmission channels for the microphone and loudspeaker of each telephone are connected separately through a network terminating unit (NTU) to the echo cancelling device (EC) by a copper pair line, such as an ISDN Basic Rate Access Line. The echo cancelling device is connected to transmit and receive signals to the remainder of network via a separate digital coder (COD) and digital decoder (DEC) as shown. Thus, the described echo cancelling device (EC) can serve a number of telephones and provide on demand echo cancelling facilities for all telephones connectable to the network.

In Figure 4, the network is generally the same as shown in Figure 3 except the telephone in Figure 4 incorporates a cordless telephone connected to the network via a digital radio interface. The transmission channels of the radio interface inherently retains outgoing microphone signals and incoming loudspeaker signals separate from one another to allow the echo cancelling device (EC) to make the necessary discrimination to cancel any echoes.

In Figure 5, the echo cancelling device (EC) is connected to a normal public switch telephone network (PSTN) and to a special network (ISDN) for higher quality transmission, such as presently available or used for Group 4 facsimile transmissions, for example. For normal public switch telephone communications bandwidth frequencies up to 3400 Hz are used and for the "special" networks an increased bandwidth can be achieved by using ADPCM coding of 7 KHz at 64 Kbps, rather than a PCM coding of 3.4 Kbps within the 64 Kbps digital system. In Figure 5, the telephone (not shown) and signals in the communication channels to the echo cancelling device (EC) can operate at frequencies up to 7 KHz giving superior quality (and superior echo cancelling) compared to the normal 3.4 KHz telephone service. Thus, the echo cancelling device is connected separately for outgoing and incoming calls to the public switching telephone network via PCM-ADPCM transcoders to public switch telephone networks and directly to high quality networks (ISDN) . In the case where there is a radio interface which normally operate at 32 Kbps with ADPCM, improved quality can be achieved by establishing two channels with a total 64 Kbps capacity.

The echo cancelling device (EC) can connect calls either to normal networks (PSTN) or to "special" networks (ISDN) operating at 64 Kbps or above. For normal networks, the echo cancelling device converts 7 KHz signals to "conventional" signals at 3.4 KHz PCM. For the "special" networks, the full 7 KHz capability is maintained using ADPCM at 64 Kbps. This enables, the same loudspeaking telephone to be used at normal quality for connection to normal networks (PSTN) and at higher quality over the "special" networks (ISDN) . Automatic selection of normal or higher quality channels is achieved by dial code prefixes for outgoing calls, and by separate appropriate PSTN and ISDN identifiable numbers for incoming calls.

In all embodiments of the invention, generally sophisticated and relatively expensive echo cancelling devices, already available for use in individual telephones, are incorporated within each network to serve a number of telephones. Thus, the overall cost of each echo cancelling device is effectively shared by a number of subscribers. Each subscriber can nevertheless make us of on demand, as and when required, the superior quality and operation of the echo cancelling device in the network; the quality of echo cancelling provided being much superior for the networks than shown in Figure 1, for example. Embodiments of the invention require each telephone to be connected by separate channels, or channels where the incoming and outgoing signals are themselves separated, to the echo cancelling device of its network. However, the separate connections has the advantage that signals can be transmitted at higher than "normal" frequencies with more kilobits per second and so provide much higher quality for the voice signals.

Claims

1. A loudspeaking telephone network including an echo cancelling device, in which the echo cancelling device is provided for serving a plurality of telephones, each telephone being connectable to the echo cancelling device by transmission channels arranged to separately transmit outgoing and incoming signals to echo cancelling device.

2. A network according to claim 1, in which the echo cancelling device is arranged to transmit and receive signals via a digital radio interface.

3. A network according to claim 1 or 2, in which signals transmitted between the echo cancelling device and the telephones have a frequency bandwidth of around 7 KHz or more.

4. A network according to any of claims 1 to 3 , in which the echo cancelling device is connected to other networks by two types _. of transmission channels that normally operate at different bandwidth frequencies and is arranged to selectively receive and transmit signals from the two types of channels.