GB2332347A

GB2332347A - Digital communications device, method and systems

Info

Publication number: GB2332347A
Application number: GB9726288A
Authority: GB
Inventors: Pat Mcandrew
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1997-12-13
Filing date: 1997-12-13
Publication date: 1999-06-16
Anticipated expiration: 2017-12-13
Also published as: GB2332347B; GB2332347A9; GB9726288D0

Abstract

A digital communications system has an air interface utilising speech coding with discontinuous transmission. A first device 100 is arranged to communicate over the interface and has a first memory 130 containing sets of parameters, each set describing a comfort noise profile. A second device 200 is arranged to communicate with the first device 100 over the interface, and has a second memory 230 containing substantially the same sets of parameters as the first memory. The first device receives speech signals from a speech source 110 and identifies one set of parameters in the first memory 130 which most similarly reflects background noise of the speech source. A signal is transmitted to the second device 200 indicating a memory location corresponding to the identified set. The transmitted signal is used by the second device 200 to read from a corresponding location of the second memory 230 a comfort noise profile for comfort noise generation in the second device. At least part of the sets of parameters stored in the memory 130,230 may be updated.

Description

1 2332347 DIGITAL COMMUNICATIONS DEVICE,METHOD AND SYSTEM

Field of the Invention

This invention relates to digital communications systems and particularly but not exclusively to digital cellular communications systems.

Backeround of the Invention Current digital cellular communications systems use digital speech coding to code and reduce the bit-rate of digital transmissions between a transmitting station and a receiving station over an air interface, whilst maintaining an acceptable quality of speech reproduction at an earpiece or loudspeaker of a receiving station. The aim of digital speech coding is to reduce the bit rate as far as possible, whilst maintaining acceptable quality.

Furthermore, because of the constraint in a mobile station to also reduce operating power as far as possible, a minimisation of transmitted RF power is an advantage. This can be achieved by effectively disabling transmission when no speech is present. This situation occurs naturally in normal speech as periods of silence within the conversation gaps.

However, if the receiver encounters periods when there is no signal to send to the earpiece or loudspeaker, a listener will hear silence between normal speech, and this will appear unnatural and could seriously degrade speech intelligibility, due to the absence of any background noise.

A method is known by which the transmitter samples background noise in the absence of speech, encodes it in a predetermined manner and transmits this encoded signal to the receiver, where it is decoded and sent to the earpiece or loudspeaker during speech gaps to give the impression to the listener of the continuous presence of the transmitter's speech. This is known as 'comfort' noise.

Although this reduces the system overall bit rate sent, the comfort noise sequences are still of substantial size. Typically in GSM, a burst of 456 bits at 480mS intervals is used to transmit a model corresponding to the spectral shape of the background noise, and there is a need to further reduce the bit rate required for the transmission of the comfort hoise.

This invention seeks to provide a digital communications device, method and system which mitigate the above mentioned disadvantages.

SummM of the Invention

According to a first aspect of the present invention there is provided a digital communications system having an air interface utilising speech coding with discontinuous transmission, comprising: a first communications device arranged to communicate over the air interface and having a first memory arranged to contain a plurality of sets of parameters, each set of parameters describing a comfort noise profile, for use during discontinuous transmission; and, a second communications device arranged to communicate with the first communications device over the air interface, the second communications device having a second memory arranged to contain substantially the same sets of parameters as the first memory of the first communications device, wherein, during communication, the first device is arranged to receive speech signals from a speech source and to identify therefrom one set of the plurality of sets of parameters in the first memory which most similarly reflects background noise of the speech source, and is further arranged to transmit to the second device a signal indicating a memory location corresponding to the identified set of parameters, such that the transmitted signal is used by the second device to read from a corresponding memory location of the second memory a comfort noise profile for generation of comfort lioise in the second device.

According to a second aspect of the present invention there is provided a method for providing speech coded discontinuous transmission between first and a second communications devices over an air interface, each of the first and second communications devices having a memory containing a plurality of sets of parameters, each set of param7eters describing a comfort noise profile for use during discontinuous transmission the method comprising the steps of. receiving, at the first device, speech signals from a speech source; identifying in the first device, from the speech source, one of the sets of parameters in the memory which most similarly reflects background noise of the speech source; transmitting from the first device to the second device a signal indicating a memory location corresponding to the identified set of parameters and, reading from a corresponding memory location of the second memory a comfort noise profile for generation of comfort noise in the second device.

Preferably one of the first and second devices is a base station and the other one of the first and second devices is a mobile station.

According to a third aspect of the present invention there is provided a communications device, for communicating over an air interface of a digital communications system utilising speech coding with discontinuous transmission, with a second communications device, the second communications device having a memory arranged to contain a plurality of sets of parameters, each set of parameters describing a comfort noise profile for use during discontinuous transmission, the communications device comprising: a first memory arranged to contain the same plurality of sets of parameters as the second memory of the second communications device, wherein, during communication, the communications device is arranged to receive speech signals from a speech source and to identify therefrom one of the sets of parameters in the memory which substantially reflect background noise of the speech source, and is further arranged to transmit to the second device a signal indicating a memory location corresponding to the identified set of parameters such that the transmitted signal is used by the second device to read from a corresponding memory location of the second memory a comfort noise profile for generation of comfort noise in the second device.

Preferably the plurality of sets of parameters include at least one set which is updatable over the air interface. The plurality U sets of parameters preferably include at least two sets which are updatable over the air interface.

Preferably the updating is performed in dependence apon a computed error between the background noise of the speech signal and each of the plurality of sets. One of the at least two sets of both memories is preferably selected to be overwritten by an updated set, the selection being made in dependence upon a computed error between the background noise of the speech signal and each of the at least two sets.

Preferably the selection is made at the first communications device, and the location of the set to be overwritten is transmitted to the second communications device. Alternatively the selection is made at both the first 20 and second communications devices.

In this way the bit-rate required for the provision of comfort noise is reduced.

Brief Description of the Drawings

An exemplary embodiment of the invention will now be described with reference to the drawing in which:

FIG. 1 shows a block diagram of preferred embodiment of a digital communications system in accordance with the invention, and FIG. 2 shows a block diagram of a portion of the preferred embodiment of FIG. 1.

Detailed DescriDtion of a Preferred Embodiment Referring to FIG. 1, there is shown a digital communi cations system 10, including a first communications device 100 and a second communications device 200, arranged to communicate over an air interface 150 using discontinuous transmission. In this way during periods of silence between speech, transmission is suspended such that transmit power is conserved by the transmitting device.

The first communications device, which may be a mobile station or a base station of the system 10, includes a microphone or other speech input device 110, a first processing unit 120, and a first memory 130.

Similarly the second communications device 200, which may also be a mobile station or a base station of the system 10, includes a loudspeaker or other speech output device 210, a second processing unit 220, and a second memory 230.

In operation, a number of vector quantised descriptions of typical comfort noise (CN) profiles or sequences are stored in corresponding memory locations of both the first and second memories 130 and 230 respectively.

When a CN transmission is required during a pause in speech, a sequence of the background noise of the microphone 110 is sampled by the first processing unit 120. The first processing unit 120 then compares the sampled sequence to the sequences stored in the first memory 130, in order to find the closest match. This operation is further described below.

The memory location of the closest matching sequence found in the first memory 130 is then transmitted over the air interface 150 to second communications device 200. The second processing unit 200 then reads the sequence found in the corresponding memory location of the second memory 230, and generates comfort noise from the read sequence, which is heard at the loudspeaker 210.

A typical size for the first and second memories 130 and 230 may be 32 or 64 locations. The more locations, the greater the number of CN profiles which can be stored. However, even with a very large memory, it may be desirable to have an adaptive arrangement, where the CN profiles can be updated from time to time. The following is a detailed description of the comparison operation and the update operation.

Referring now also to FIG. 2 there is shown an exemplary embodiment of a memory 30, to be used as the first memory 130 and the second memory 230 of FIG. 1. The memory 30 has a first fixed portion 40 and a second adaptive portion 50.

The overall number of memory locations are N, and this is. a fixed number which is also a binary power of two. i.e. N = 2".

Let i be a memory location index, such that i = 0.. N - 1 The length of each memory location (a vector) is also fixed (L).

Then, size[CB(i)l= L where CBW is the memory (or CodeBook) location i. Let the reference time domain background noise for the current frame be:

s(k) and it's corresponding fourier transform representation be: SM At start-up or at a given synchronisation point, the fixed portion 40 of the memory 30 is filled with a pre-determined set of sequences or vectors. The adaptive portion 50 of the memory 30 may either be filled accordingly or may be set to zero. As in CELP (Code Excited Linear predictive Coding) speech coders, CN vectors may be organised in various manners to reduce searching complexity to below that of a full exhaustive search.

The vector v for each memory location, may be either: a spectrally shaped time-domain sequence CB(i) = v(k) oraparameterset CB(i)=f[S(w)g(k)] where each is representative of typical background noise sequences and g(k)is a required gain factor.

In order to determine which vector description to transmit, the first processing unit 130 of the first communicatiuns device 100 performs a search through all the memory locations of the memory 30 (or 130) to find the closest match against the current background noise sequence. The matching function used depends upon the type of vectors used. For the time-domain vectors the best match may be determined by a correlation search. The closest match would be determined by the highest correlation value. In this case the transmitted description at time tx(t), would include both the corresponding memory location and the quantised correlation lag (1).

tx (t) = 0 ' 1) For the other case of the paramatised vectors, the closest match would be determined by the minimum Mean Squared Error (MSE) between s(k)and the output of a model (M) described by tr(t). In this case, X (t) = CQ, g) or if the gain parameter is encoded within each memory location, X (t) = C0 The second processing unit 220 of the second communications device 200 uses the transmitted description to generate the estimated comfort noise sequence. In the case of the time-domain vectors, the estimated sequence is given directly by (k) = CB [i 1k - 1) modulo L For the case of paramatised vectors, the estimated sequence is the output of the model (M) described by these vectors. Hence, (k) = M [CB(i g] The adaptive portion of the memory 30 allows the system 10 to adapt to new sequences as the spectral characteristics of the environment change. If the first processing unit 120 of the first communications device 100 does not find a comfort noise match better than a predetermined threshold, then message may be sent over the air interface 150 to the second communications device 200, indicating that a M sequence was being transmitted, for the purposes of updating one of the memory locations of the adaptive portion 40. The corresponding M sequence is transmitted to the second communications device 200, spread over a number of transmissions if required. Additionally, the CN sequence is used to update the same memory location in the first communications device, such that the two memories are synchronised.

The updating of the adaptive portion 50 of the memory 30 is performed as follows. A scalar threshold (Ta) is pre-determined and is used to compare the quality of the closest match. If the closest match is worse than Ta, then the current background sequence is used to update the adaptive portion 50 of the memory 30. The choice to update is made as follows: for the time- domain vectors if, Tamaxirvs(k)l where rvs(k)is the cross-correlation between v(k) and s(k) or for the paramatised vectors if, Ta(min(MSE) If the decision to update the adaptive portion 50 of the memory 30 has been taken, then the replacement memory location is chosen to correspond to the location of. for the time-domain vectors, or for the paramatised vectors, max(MSE) c= CB 50 (') - minlrvs;.1k: c= CB 50) where CB50 represents the adaptive portion 50 of the memory 30 In other words, the least closest match currently stored in the adaptive portion 50 of the memory 30 is overwritten. The processing unit 120 of the first communications device 100 will update this location in first memory 130, and either transmit the memory location to the second processing unit 220 of the second communications device 200 or allow the second processing unit 220 to compute the required location in a manner identical to that performed by the first processing unit 120. Transmission of the updating location index allows operation with minimised error propagation, whilst computing this location locally at the decoder, minimises the transmission bit-rate at the expense of increases risk of error propagation.

Block 60 of FIG. 2 shows the closest fit measurement step, and block 70 shows the closest match computation step. This is either transmitted to the remote device (arrow 75), or if the closest match is worse than Ta, the adaptive portion 50 of the memory 30 is updated, as shown by block 80, and the updated sequence is tranmi d (arrow-90).

It will be appreciated that alternative embodiments to the one described above are possible. For example, various other statistical and computational methods may be used to facilitate the comparison operations described above.

Furthermore, the precise dimensions of the memories and the nature of the transmissions described above may differ from those indicated.

Claims

1. A digital communications system having an air interface utilising speech coding with discontinuous transmission, comprising: a first communications device arranged to communicate over the air interface and having a first memory arranged to contain a plurality of sets of parameters, each set of parameters describing a comfort noise profile, for use during discontinuous transmission; and, a second communications device arranged to communicate with the first communications device over the air interface, the second communications device having a second memory arranged to contain substantially the same sets of parameters as the first memory of the first communications device, wherein, during communication, the first device is arranged to receive speech signals from a speech source and to identify therefrom one set of the plurality of sets of parameters in the first memory which most similarly reflects background noise of the speech source, and is further arranged to transmit to the second device a signal indicating a memory location corresponding to the identified set of parameters, such that the transmitted signal is used by the second device to read from a corresponding memory location of the second memory a comfort noise profile for generation of comfort noise in the second device.

2. A method for providing speech coded discontinuous transmission between first and a second communications devices over an air interface, each of the first and second communications devices having a memory containing a plurality of sets of parameters, each set of parameters describing a comfort noise profile for use during discontinuous transmission the method comprising the steps of.

receiving, at the first device, speech signals from a speech source; identifying in the first device, from the speech source, one of the sets of parameters in the memory which most similarl y reflects'background noise of the speech source; transmitting from the first device to the second device a signal indicating a memory location corresponding to the identified set of parameters and, reading from a corresponding memory location of the second memory a comfort noise profile for generation of comfort noise in the second device.

3. The system or method of claim 1 or claim 2-respectively, wherein one of the first and second devices is a base station and the other one of the first and second devices is a mobile station.

4. A communications device, for communicating over an air interface of a digital communications system utilising speech coding with discontinuous transmission, with a secolid communications device, the second communications device having a memory arranged to contain a plurality of sets of parameters, each set of parameters describing a comfort noise profile for use during discontinuous transmission, the communications device comprising: a first memory arranged to contain the same plurality of sets of parameters as the second memory of the second communications device, wherein, during communication, the communications device is arranged to receive speech signals from a speech source and to identify therefrom one of the sets of parameters in the memory which substantially reflect background noise of the speech source, and is further arranged to transmit to the second device a signal indicating a memory location corresponding to the identified set of parameters such that the transmitted signal is used by the second device to read from a corresponding memory location of the second memory a comfort noise profile for generation of comfort noise in the second device.

5. The system, method or device of any preceding claim wherein the plurality of sets of parameters include at least one set which is updatable over the air interface.

6. The system, method or station of any preceding claim wherein the plurality of sets of parameters include at least two sets which are updatable over the air interface.

7. The system, method or station of claim 5 or claim 6 wherein the updating is performed in dependence upon a computed error between the background noise of the speech signal and each of the plurality of sets.

8. The system, method or station of claim 6 or claim 7 when dependent on claim 6 wherein one of the at least two sets of both memories is selected to be overwritten by an updated set, the selection being made in dependence upon a computed error between the background noise of the speech signal and each of the at least two sets.

9. The system, method or station of claim 8 wherein the selection is made at the first communications device, and the location of the set to be overwritten is transmitted to the second communications device.

10. The system, method or station of claim 8 wherein the selection is made at both the first and second communications devices.

11. A digital communications system substantially as hereinbefore described and with reference to the drawings of FIG.s 1 and 2.

12. A method substantially as hereinbefore described and with reference to the drawings of FIG.s 1 and 2.

13. A communications device substantially as hereinbefore described and with reference to the drawings of FIG.s 1 and 2.