WO1996004744A1

WO1996004744A1 - System and method for conducting conference calls in a communication system utilizing voice compression

Info

Publication number: WO1996004744A1
Application number: PCT/US1995/009935
Authority: WO
Inventors: David Almagor
Original assignee: National Semiconductor Corporation
Priority date: 1994-08-04
Filing date: 1995-08-04
Publication date: 1996-02-15
Also published as: KR960705440A

Abstract

A communication system and method for more cost-effectively conducting a conference call between multiple wired or wireless handset users and a remote conferee communicating over a telephone line (105). Each of the incoming compressed signals from the handset users are decompressed and attenuated, prior to being added together. The summed signals are then put through a digital-to-analog converter and sent to the remote conferee on the telephone line. The summed signals are also added to the output resulting from the operation of an analog-to-digital converter on the incoming signal from the remote conferee, and the entire summed signal is compressed and sent out to each of the handset users.

Description

SYSTEM AND METHOD FOR CONDUCTING CONFERENCE CALLS IN A COMMUNICATION SYSTEM UTILIZING VOICE COMPRESSION

TECHNICAL FIELD

The present invention relates to communication systems such as cellular phone systems, and more specifically, to a system and method for cost-effectively conducting conference calls in a communication system which uses voice compression techniques, by reducing the number of data compression stages required

BACKGROUND OF THE INVENTION

Personal communication systems which allow the transmission of information to another person in a distant location are rapidly becoming a common business tool. In such systems, portable handsets are used to connect users to each other through base stations which are linked to analog, digital, and mobile cellular networks. An example of such a system is a cellular phone system, which is a wireless system used by many people to allow them to place telephone calls while in their cars or away from their home or office. Personal communication systems can also be based on wired or fiber optic cable connections. and are capable of transmitting voice, data, or faxes between users.

These communication systems typically transmit digital signals which represent either analog or digital data. Analog information, such as voices, is converted to digital form and compressed before it is transmitted over the system. Digital signals are similarly compressed prior to being transmitted over the system. The compression stage is used in order to reduce the memory and bandwidth requirements of the system and to reduce the bandwidth requirements of the communication channel for each user This last aspect allows a communication channel to support a greater number of users for a given bandwidth of the spectrum.

A variety of data compression and decompression methods are well known in the communications industry. Standard compression techniques operate on the data in a non-linear fashion This means that a signal must be decompressed before other operations, such as combining data streams for purposes of carrying out a conference call, can be performed.

The use of data compression techniques requires that both the personal handsets and the base station be capable of compressing and decompressing (expanding) the voice data in accordance with a predetermined standard In addition, the base station needs to be capable of handling several compression decompression channels simultaneously to facilitate the use of the system by multiple parties

The need for a base station to simultaneously handle the compression/decompression operations for multiple channels places requirements on the semiconductor devices used in the station^'s equipment

These requirements may include lower limits on memory capacity, switching speed, and power usage, and can impact the complexity, size, and cost of the equipment used by the station The processing speed and load capacity of a cost-effective communications system are also affected by the multi-channel operation of the system.

One of the most intensive data processing operations performed by a base station involves a multi- handset conference call, wherein multiple communication system users are engaged in a conference call with a remote party connected to a standard telephone line. Although the following data processing stages are described with reference to a wireless communications system, many of the same operations would occur in a wired or fiber optic cable based system.

In a multi-handset conference call, the base station has to decompress the compressed digital signals coming from each of the conferee's handsets, combine them together, compress the combined signal, and then send it out on the telephone line to the remote conferee. A digital-to-analog stage which converts the compressed digital signal to analog form prior to transmission over the phone line may also be required. These steps provide the remote conferee's input signal for the conference call.

The station must also provide the conference call to each of the conferees using the wireless communication system, with each conferee receiving the signal from the remote conferee and the other wireless system conferees. However, it is desireable that each wireless system user not receive their own signal to prevent their hearing an echo which would degrade the quality of the received signal. Thus, for each wireless handset conferee, the base station must generate a signal equal to the sum of the signal from the remote conferee plus the decompressed signals from all of the wireless handset conferees, except for the one to whom the combined signal will be sent. This combined signal must be compressed for each wireless handset conferee separately since each combined signal is different, and sent to the proper part

An analog-to-digital stage which converts the incoming signal from the remote conferee to a digital signal prior to summing it with the decompressed signals from the wireless handset users may also be required. These steps provide the handset users^' input signals for the conference call.

This scenario is depicted in Fig. 1. which shows a schematic diagram for a standard implementation of the processing stages needed for a conference call between four wireless handset users and one remote conferee. The diagram of Fig. 1 represents the functional features of a conference call system of the type described in literature discussing conferencing in PCM based wireless or wired systems.

As shown in Fig. 1, each of the four wireless handset users are identified by a slot or signal channel having a number from one to four. Each wireless handset includes means for converting the analog signal representing the voice of the user to a digital signal, compressing the digital signal, and transmitting the compressed signal to the base station. The digital signal coming into the station in compressed form from each of the handsets is decompressed or expanded by data decompression units 10. The expanded signals are summed by adder 12 and then put through a digital-to-analog converter (not shown) and sent out on a telephone line 14 to the remote conferee. This provides the incoming conference call signal for the remote conferee.

The expanded signals from the wireless handsets are also provided as inputs to adders 20. 22, 24. and 26. However, as shown in Fig. 1. the expanded signals provided to each adder do not include the one corresponding to the slot or user for which that summed signal is intended. Thus, adder 20 does not receive as an input the expanded signal corresponding to slot one. The same relationship holds for the other adders. Note that in order to prevent data overflow during the add operations, the expanded signals may be attenuated prior to summing by adder 12 or adders 20. 22. 24. and 26

The incoming signal on telephone line 14 from the remote conferee is put through an analog-to- digital converter (not shown) and then routed to each of adders 20. 22. 24. and 26. where that signal is summed with the partial sums from the expanded signals transmitted by the wireless handsets Each of the resulting sums is put through a data compression unit 30 and then transmitted to the appropriate user

Upon receipt by the wireless handset users, the digital data is decompressed and converted to an analog signal.

A variety of data compression/decompression schemes are currently used in communication systems, including the PCM scheme mentioned previously. These schemes are typically implemented in hardware which has an equal number of compression and decompression channels, and may include analog-to-digital and digital-to-analog converters.

The conference call system shown in Fig. 1 can be expanded to serve any number of conference call participants. As indicated by the figure, for M number of conferees using handsets, the base station requires M data decompressors and M compressors. This imposes lower limits on the amount of processing capability, memory, and power required for a communication system which is designed to handle conference calls with multiple users. As all of the noted requirements increase the cost and development time of such systems, it is apparent that a way of reducing these requirements while maintaining the operational features of the system would be beneficial.

Although not directed to a wireless communications system. European Patent Application EP 0 135 383, published March 27, 1985, and entitled "Conference Bridge Circuit Arrangements2 teaches a circuit for allowing a multi-user conference call to be conducted over a PCM telephone system. The system uses wires or data buses to carry signals from a plurality of telephone set users to a central phone station. At the station, signals from one or more of the buses are selected, the selected signals are serially added, the output obtained by serially adding the signals is multiplexed, and the multiplexed signals are retransmitted to the conference call participants. Although the circuitry described in the EP application permits multiple parties using the telephone system to participate in a conference call, its is believed that the circuit design disclosed introduces a serious disadvantage during the conduct of such a call. In the circuit, the signals which are serially added are not subject to attenuation prior to being added. The lack of such attenuation can create two possible problems. Firstly, saturation of the adder can occur due to data overflow. Although the circuit does incorporate a means of checking for overflow, the system^'s response to an overflow condition results in a corruption of the signal data.

The second disadvantage is that each conferee will hear its own signal at full strength with a given system delay after its transmission. This can produce a confusing and distracting echo signal which degrades the quality of the conference call. The echo may be removed by using a delay echo canceler in the conferees' handsets. This increases the cost and complexity of the system, however, it may be worthwhile in some situations. If the system delay is sufficiently small, the echo will instead be heard as a sidetone. a feature which is required in many communication systems. However, the sidetone produced by the system described in the EP patent application is at a fixed level which may be a disadvantage in using it within some communication systems, since different systems have different sidetone level specifications.

Another disadvantage of the EP application conference call circuit is that it is designed to be implemented in a dedicated piece of hardware which operates within a wired telephone system This places limits on its expandibility, which can become a problem as the number of users of the svstem increases. What is desired is a system and method for conducting conference calls between multiple users of a wired or wireless personal communication system and a remote conferee which reduces the number of signal compression and decompression stages required in present systems

SUMMARY OF THE INVENTION

The present invention is directed to a communication system and method for cost-effectivel conducting a conference call between multiple wired or wireless handset users and a remote conferee communicating over a telephone line. Each of the incoming compressed signals from the handset users are decompressed and attenuated, prior to being added together. The summed signals are then put through a digital-to-analog converter and sent to the remote conferee on the telephone line This provides the incoming signal for the remote conferee. The summed signals are also added to the output signal resulting from the operation of an analog- to-digital converter on the incoming signal from the remote conferee, and the entire summed signal is compressed and sent out to each of the handset users. The attenuation of the incoming signals from the handset users allows them to be summed without data overflow and provides the added benefit of a sidetone which is required by most communication systems. The result is a communication svstem which is capable of handling multi-user conference calls while requiring fewer data compression units than other currently used svstems. Further objects and advantages of the present invention will become apparent from the following detailed description and accompanying drawings

BRIEF DESCRIPTION OF THE DRAWINGS

Fig 1 shows a schematic diagram for a standard implementation of the processing stages needed for 5 a conference call between four wireless handset users and one remote conferee

Fig 2 shows a schematic diagram for an implementation of the processing stages needed for a conference call between four wireless handset users and one remote conferee according to the present invention

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

10 Although the following description is directed towards a conference call conducted between wireless handset users and a remote conferee, the present invention mav also be applied to a wired or fiber optic based communications svstem in which data compression techniques are used to compress and decompress digital data

The inventor of the present invention has recognized that the amount of processing capabihtv

15 memory and power required for a commumcation svstem which is designed to permit the conduct of conference calls can be reduced bv replacing the individual data compression stage usuallv required for each user channel bv a smgle data compression stage The mput for this single compression stage is the sum of the signal from the remote conferee and the properlv attenuated decompressed signals from all of the handset users For a conference call mvolvmg M wireless handset users and one remote conferee this

20 enables the number of data compression stages to be decreased from M to 1 a savings of M-l stages or devices

Fig 2 shows a schematic diagram for an implementation of the processing stages needed for a conference call between four wireless handset users and one remote conferee according to the present invention As shown m Fig 2, each of the four wireless handset users are identified bv a slot or signal

25 channel havmg a number from one to four The digital signal commg to the station in compressed form from each of the handsets is decompressed or expanded bv decompression units 100 The expanded signals are then attenuated bv attenuators 102 which attenuate the signals from the wireless handset users in order to prevent data overflow and reduce the signal strength hen the summed signals are later provided to each of the users As will be discussed later the signal strength reduction caused bv the ⁾ attenuation stage can be compensated for bv circuitrv in the handsets This insures that the signal heard bv the handset users is at an appropnate level

The attenuation factor is normallv chosen to be the reciprocal of the number of handset users Thus for the case of four such users shown in Fig 2 the attenuation factor is one-fourth l e the v oltage corresponding to the incoming signal is reduced to one-fourth its v alue In general terms the attenuators

35 function as a 1/N divider here N is the number of handset users The attenuation factor can also be adiusted so that the signals are attenuated sufficientlv to reduce the signal bv anv desired amount

The attenuated signals are then summed bv adder 104 put through a digital-to-analog co erter (not shown), and sent out on a telephone lme 105 to the remote conferee (not shown) This prov ides the incoming conference call signal for the remote conferee

40 The output of adder 104 is also prov ided as an mput to adder 106 The incoming signal on telephone lme 105 from the remote conferee is put through an analog-to-digital converter (not shown) and then serves as the other input for adder 106 The output of adder 106 i e the sum of the inputs is put through a smgle data compression unit 110 and then provided to each of the wireless handset users as the incoming conference call signal An important aspect of the present invention is that the use of attenuators 102 provides two pnmarv benefits, firstly, it permits the incoming signals to be summed m adder 104 without creating data overflow problems, and secondly, under appropriate conditions it provides a sidetone so that each handset user receives a signal having as one component a reduced amplitude version of their own transmission As the existence of an appropnate level sidetone is a requirement of manv commumcation svstems the svstem shown m Fig 2 not onlv reduces the number of data compression units required but also helps to satisfv the commumcation svstem specifications

The use of a 1/N divider as an attenuation unit provides a sidetone havmg a level ithin the required boundanes for a range of users For example, m die DECT (Digital European Cordless Telecommunications) svstem, the sidetone level attenuation is required to be 13 ± 5 dB, or between 8 and

18 dB A 1 N divider provides an attenuation in signal equal to 20 log_1Q (1 N dB for N users Therefore for 3 users the attenuation is 9 6 dB, while for 7 users the attenuation is 16 9 dB Thus, for this example, the appropnate level sidetone can be provided for between 3 and 7 users This shows that the same aspect of the svstem provides both a means of controlling data overflow and generating an appropnate lev el sidetone

The present invention can be implemented bv means of am standard programmable digital signal processor This makes the svstem more flexible (easilv reconfigurable) and cost efficient than a hardware based svstem which unnecessary and costlv equipment mav be mcluded Note that most wireless handsets mclude an automatic gam control (AGC) stage which compensates for fluctuauons in signal level and. if necessary . acts to overcome the signal strength reduction caused bv the attenuators Thus, anv signal strength problems caused bv the use of the attenuation stage can be compensated for bv the AGC circuitrv the handsets

Although Fig 2 depicts onlv four wireless handset users engaged in a conference call the number mav be increased bv simplv adding a data decompression unit and attenuator for each user The attenuation factor is altered as mentioned so that it equals the reciprocal of the number of wireless handset users Thus, accordance with the present invention, for M wireless handset users engaged m a conference call with one remote conferee communicating bv means of a standard telephone lme the svstem requires M data decompression units one (1) data compression unit and M attenuators hav g an attenuauon factor of 1/M Depending upon the number of wireless handset conferees other attenuauon factors mav also be used to maintain the signals at a desired level in order to satisfv the commumcation svstem specifications In conjunction w ith the AGC circuitrv in the handsets this also prov ides a means for producmg a desired signal lev el for the handset users

The method of the present inv ention reduces the number of data compression stages/units needed for multiple conferees and provides a savings in cost and complexity of the svstem With the data compression and decompression steps being implemented in the form of software the cost sa ings of M-

1 units of data compression hardware can be substantial

Delavs in both wired and wireless commumcation sv stems tvpicallv do not exceed ten milliseconds Such delavs are small enough that a partv s own signal hich is attenuated and then sent back to that partv is heard as a sidetone and not as an echo In the situation in which the time delav introduced bv the commumcation svstem is sufficientlv long that the attenuated signals sent back to the handset users cause an undesirable echo instead of a sidetone a delav echo canceler can be mcluded in each handset Such a dev ice acts to subtract from the received signal a delav ed v ersion of the signal transmitted bv that handset This w ill effectiv elv remov e the echo although it is at the cost of added circuit complexity Whether the cost of this additional feature offsets the sav ings prov ided bv the reduction m the number of data compression units depends upon the characteπstics of the commumcation svstem in which the present invention is being implemented The terms and expressions which have been employed herein are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding equivalents of the features shown and described, or portions thereof, it being recognized that various modifications are possible within the scope of the invention claimed.

Claims

I claim:

1. A method for providing an incoming signal to each of a plurality of participating users of a communications system who are conducting a conference call between themselves and a remote conferee, the method comprising: decompressing a plurality of compressed input signals provided by the participating users of the communications system: attenuating each of the decompressed signals: adding the plurality of attenuated signals, adding the sum of the attenuated signals to a signal from a remote conferee. compressing the sum of the attenuated signals and the signal from the remote conferee to form a compressed sum signal; and transmitting the compressed sum signal to each of the participating users of the communications system.

2 The method of claim 1. wherein the communications system is a wireless system

3. The method of claim 1, wherein the attenuation factor used to attenuate the sum of the decompressed signals is proportional to the inverse of the number of participating users of the communication system.

4. A communications system for providing an incoming signal to each of a plurality of participating users who are conducting a conference call between themselves and a remote conferee, the system comprising: means for decompressing a plurality of compressed input signals provided by the participating users of the communications system; attenuation means for attenuating each of the decompressed signals, adding means for summing the attenuated signals. means for adding the summed attenuated signals to a signal from the remote conferee data compression means for compressing the sum of the attenuated signals and the signal from the remote conferee to form a compressed sum signal, and transmitting means for transmitting the compressed sum signal to each of the participating users of the commumcations system

5 The system of claim 4. where the communicauons system is a wireless svstem

6 A method for providing a first incoming signal to each of a plurality of participatmg users of a communications system and a second incoming signal to a remote conferee communicating ith the participating users by means of a telephone lme dunng a conference call betw een the participating users and the remote conferee, the method comprising decompressing a plurality of compressed input signals provided by the participatmg users of the communications system. attenuating each of the decompressed signals, adding the plurality of attenuated signals. transmitting the sum of the attenuated signals to the remote conferee over the telephone line. adding the sum of the attenuated signals to a signal from the remote conferee. compressing the sum of die attenuated signals and the signal from the remote conferee to form a compressed sum signal: and transmitting the compressed sum signal to each of the participating users of the communication system.

7. The method of claim 6, wherein the communications system is a wireless system.

8. The method of claim 6, wherein the attenuation factor used to attenuate the sum of the decompressed signals is proportional to the inverse of the number of participating users of the communication system.

9. The method of claim 6. further comprising the step of: using a digital-to-analog converter to convert the sum of the attenuated signals to an analog signal prior to transmitting the sum of the attenuated signals to the remote conferee over the telephone line.

10. The method of claim 6, further comprising the step of: using an analog-to-digital converter to convert the signal from the remote conferee to a digital signal prior to adding the sum of the anenuated signals to the signal from the remote conferee

11. A communications system for providing a first incoming signal to each of a plurality of participating users of a communications system and a second incoming signal to a remote conferee communicating with the participating users by means of a telephone line during a conference call between the participating users and the remote conferee, the system comprising. means for decompressing a plurality of compressed input signals provided by die participating users of the communications system; attenuation means for attenuating each of the decompressed signals: adding means for summing the attenuated signals: transmitting means for sending the sum of the attenuated signals to the remote conferee over the telephone line; means for adding the summed attenuated signals to a signal from the remote conferee: data compression means for compressing the sum of the attenuated signals and the signal from the remote conferee to form a compressed sum signal: and transmitting means for transmitting the compressed sum signal to each of the participating users of the communication system.

12. The svstem of claim 1 1. wherein the communications svstem is a wireless svstem.