EP1557820B1

EP1557820B1 - Voice activity detection operating with compressed speech signal parameters

Info

Publication number: EP1557820B1
Application number: EP04425031A
Authority: EP
Inventors: Matteo Aldrovandi
Original assignee: Siemens SpA
Current assignee: Siemens SpA
Priority date: 2004-01-22
Filing date: 2004-01-22
Publication date: 2006-10-18
Anticipated expiration: 2024-01-22
Also published as: DE602004002845T2; ATE343196T1; DE602004002845D1; EP1557820A1

Abstract

There is provided a voice activity detector (VAD) (40) for assisting the voice quality enhancement in the uplink direction of a mobile communication system in which the voice quality enhancement means (50) are embodied in the transcoding and rate adapting unit (TRAU) (2). The VAD (40) comprises means (41, 42) for performing both a spectral analysis and an energetic analysis on a received speech signal and means (43, 45) for processing the results of said analysis and taking a decision on audio segment nature. The VAD performs spectral analysis directly on the coded signal. <IMAGE>

Description

Field of the invention

The present invention refers to digital radio communication systems, in particular mobile communication systems, and more specifically it concerns a method of and device for voice activity detection in received speech signals in one such system.
Preferably, but not exclusively, the method and the device are intended for use in connection with voice quality enhancement.

Background of the Invention

Voice activity detectors (VADs) are devices that are supplied with a signal to detect therein periods of speech and periods of silence, where only noise is present. Possibly, the VADs are also arranged to distinguish among voiced/unvoiced sounds in speech periods.
A class of VADs performs detection through an energetic analysis and a spectral analysis of the input signal, the analysis results being combined to provide the classification of an analysed speech segment. An algorithm for classifying a speech segment as voiced speech, unvoiced speech or silence based on energetic and spectral analyses is disclosed in "Application of an LPC Distance Measure to the VoicedUnvoiced-Silence Detection Problem", by L.R. Rabiner and M.R. Sambur, IEEE Transactions on Acoustics, Speech and Signal Processing, Vol. ASSP-25, No. 4, August 1977, pages 338 - 343.
In mobile communication systems, VADs are typically used at the mobile terminals, in association with the speech coder, to drive a discontinuous transmission in which coded speech signals are transmitted during active speech periods whereas in silence periods the speech transmitter is inhibited and the so-called comfort noise is transmitted. This helps in saving power.
It has now been found that it is advantageous to use a VAD also in the control part of a mobile communication system, in particular for improving the Noise Reduction (NR) feature of the so-called Voice Quality Enhancement (VQE) function. An example of VAD-assisted noise reduction for the uplink direction of communication of a digital radio communication system is disclosed in EP-A 1 017 042.
The Applicant, as well as other manufacturers, integrate the VQE function in the units (such as the Transcoding and Rate Adapting Units, or TRAU, of the GSM system) adapting the speech signals from the requirements of the radio part of the system to the requirements of the control part and, if necessary, of the fixed telephone network, and vice versa. In the Applicant's VQE, the VAD is intended to drive the noise suppression in the uplink direction of communication. For several reasons, including cost and size of the apparatus, the Applicant wishes that the addition of a VAD-driven VQE in a TRAU does not entail changes in the hardware of the TRAU itself.
If the detection exploits both the spectrum and the energy characteristics of the received signal, the conventional approach, which is substantially as disclosed in the above-mentioned document by L.R. Rabiner and M.R. Sambur, entails that the speech signal is decoded and the spectral information (here the linear prediction coefficients LPCs) is recovered from the linear frames resulting from speech decoding. Yet, the spectral analysis of the linear signal to recover the LPCs is a heavy processing task. Taking into account that the TRAU processor generally operates in parallel on a plurality of channels, real time execution of the complete VAD algorithm on the same channels could compel to use a dedicated processor or a more powerful and hence more expensive processor than the one that would be used for the TRAU. Both solutions are in contrast with the goal of keeping the TRAU hardware unchanged.
To avoid the need for a VAD-dedicated or a more powerful processor, the spectral analysis could be dispensed with and the VAD could perform only the energetic analysis. Such a solution is disclosed in EP-A 1 017 042. The document teaches also that the energy estimation can be performed directly on the compressed signal, in order to dispense the speech decoder with the relevant processing tasks and to speed up the actual speech decoding.
Yet, by performing only the energetic analysis, only one feature of the received signal is exploited, and the detection, and hence the operation of the VAD-driven VQE, is less effective.
US-A 5 732 390 discloses a voice transmitting-receiving apparatus, in particular a portable telephone set, with a digital speech encoder and a digital speech decoder. The received sound volume is adjusted based on the results of the analysis of the performed by the speech encoder on a speech signal to be transmitted.

Object of the Invention

it is an object of the invention to provide a method and a device for voice activity detection, which are suitable for mobile communication systems and in particular are intended to drive a voice quality enhancement of the uplink signals integrated in a unit performing speech rate and coding adaptation in parallel for a plurality of channels, which method and device perform a robust detection and allow using the same processor as would be required for performing said adaptation in parallel for said plurality of channels.

Summary of the Invention

According to the invention, there is provided a method according to claim 1 of detecting voice activity voice activity in a digitally coded speech signal propagating in uplink direction in a mobile communication system and received at a receiver belonging to a control part of said system, in which said detection is performed by operating directly on the received speech signal and a signal representative of the presence or absence of voice activity is generated by submitting the speech signal to an energetic analysis: The detection also includes a spectral analysis performed directly on the received coded speech signal, by exploiting only the information contained in the received signal itself; and spectrum and energy information resulting from said spectral analysis and said energetic analysis, respectively, are jointly processed for generating said signal representative of the presence or absence of voice.
The invention also concerns a voice activity detector according to claim 5 the detector comprising means for performing an energetic analysis on the received speech signal and for supplying energy information to means for generating a signal representative of the presence or absence of voice activity. The detector also includes means for performing a spectral analysis on the received speech signal, which spectral analysis means are directly connected with a detector input where coded speech signals are present and are arranged to perform spectral analysis by exploiting only information contained in the received signal itself and to supply and to supply said generating means with the spectral information resulting from the spectral analysis, whereby said generating means are arranged to generate said signal representative of the presence or absence of voice activity by jointly processing spectral and energy information.
In the preferred application, the voice activity detector drives a noise reduction operation, within a voice quality enhancement function performed on speech signals propagating in the uplink communication direction in the mobile communication system and embodied in units, like the so-called TRAU (Transcoding and Rate Adapting Unit), which adapt the uplink directed speech signals to the requirements of the control part of the mobile system and possibly of the fixed telephone network and adapt downlink directed speech signals to the requirements of the radio part of the mobile system.
Therefore, the invention provides also a method of voice quality enhancement according to claim 10 in a mobile communication system, in which a voice quality enhancement including a noise reduction operation is performed at least for speech signals propagating in uplink direction, in which said noise reduction operation is driven by a signal representative of the presence or absence of voice activity generated by a method of and device for voice activity detection as defined above.

Brief description of the drawings

A preferred embodiment of the invention, given by way of non-limiting example, will now be described with reference to the accompanying drawings, in which:

Fig. 1 is a schematic block diagram of a TRAU embodying a VQE unit and of its connections inside the mobile communications system; and
Fig. 2 is a schematic block diagram of the invention.

Description of the preferred embodiment

The preferred embodiment disclosed here concerns a VAD intended to drive the noise reduction feature in a voice quality enhancement performed in the uplink direction of communication in a mobile communication system, in case the VQE function is incorporated into the units performing transcoding and/or rate adaptation in the control part of such a system.
Referring to Fig. 1, there is schematically shown a Transcoding and Rate Adapting Unit (TRAU) 1 of a mobile communication system, for instance a GSM system. The TRAU is connected to the Mobile Switching Centre (MSC) 2 and the Base station Controller (BSC) 3 through interfaces A and Asub and embodies a VAD-driven Voice Quality Enhancement function performed in block 4 labelled VAD & VQE.
In the most general case, a VQE includes the well-known features of Acoustic Echo Cancellation, Noise Reduction and Acoustic Level Control. In the preferred application of the invention, all of said features are provided for the uplink direction of communication only and the VAD drives the Noise Reduction (NR) feature. In downlink direction, only the Acoustic Level Control is performed, which is not concerned by the present invention.
The drawing only shows the units that, in TRAU 1, are directly concerned with the transcoding function, namely a speech coder 5 and a speech decoder 6 on the Asubinterface side, and an A/µ law expander 7 and an A/µ law compander 8 on the A-interface side.
In downlink direction, the TRAU receives A-law PCM signals from MSC through a line 10, sends the expanded signals to the In_Down input of VAD&VQE block 4 through line 11. The signals outgoing from the Out_Down output of block 4 are fed to coder 5 through line 12, are coded according to the desired coding technique (full-rate, enhanced full-rate, half-rate or adaptive multi-rate) and the coded signals are then forwarded to base station controller 3 through line 13.
In uplink direction, the coded signals arriving from base station controller 3 through line 14 are fed to both decoder 6 and VAD&VQE block 4. The decoded signals are fed to the In_Down input of VAD&VQE block 4 through line 15. The decoded signals having undergone voice quality enhancement are fed from Out_Up output of VAD&VQE to A/µ law compander 8 through line 16 and hence to MSC through line 17.
It is not necessary to provide here details on the organisation of the coded speech signals in a mobile communication system, which depends on the kind of system and on the chosen coding rate. On the other hand, for any given system and rate, such organisation is well known to the skilled in the art and can be found in the relevant standards. It is sufficient here to recall that the coded speech signals include spectral information, such as the LPCs or a representation thereof.
In Fig. 2 block VAD&VQE is decomposed into its constituent blocks, namely VAD 40 and VQE 50. VAD 40 has been schematised by a spectral analyser 41 determining the LPC coefficients, an energy analyser 42 and Joint Processing Means including including an Hard Decision Unit 43 and a Soft Decision Unit 45. Said Joint Processing Means being adapted to combine the results of the two analyses and emitting on line 44 a signal indicating the nature of the received speech frame (the so-called VAD flag), which is an input to the Noise Reduction feature of VQE 50.
According to the invention, LPC analyser 41 is directly fed with the coded speech signal frames present on line 14, whereas energy estimator 42 is fed with the decoded signal outgoing from decoder 6 through line 15. The LPC analysis of course depends on the manner in which the LPCs are represented in the coded signal. The energy evaluation and the decision may be performed according to any technique known in the art, for instance as disclosed in the above-mentioned paper of L.R. Rabiner et al.
Performing the LPC analysis directly on the coded signal affords a number of advantages in terms of processing power requirements. In particular, there is no need of dedicating processing power to the reconstruction of the LPC coefficients from the decoded signal: it is sufficient to extract them from the relevant information included in the coded speech signal, which is available on the same board. Besides the greater processing simplicity, also a reduction to at least of one fifth or even less of the information amount to be processed is achieved: indeed, at most 244 bits are to be processed to obtain the LPC coefficients from the coded signal, whereas 1280 bits are to be processed when the linear signal is used.
Under such conditions, the same processor used on the TRAU board for performing all TRAU functions and for managing the so-called tandem free operation (i.e. for dispensing with the transcoding in case of communication between two mobile terminals), for a plurality of speech channels in parallel (for instance 12 channels), can perform in real time, for the same channels, also the voice activity detection by exploiting both the spectral and the energy information, and the subsequent VQE. The resulting detection is more accurate than when only the energy information is exploited and hence also the noise suppression operation is more accurate.
It is to be appreciated that, according to the existing GSM standards, the LPC information in the coded signal is updated every 5 ms. The energy information is computed on the same interval of 5 ms and then the two contributions are jointly processed to take a decision on the nature of audio segment. This is denoted by the presence of said Joint Processing Means 43 and 45. For voice quality enhancement the high rate of decisions available at the output of said Hard decision Unit 43 is not necessary and has often a negative impact on audition. Therefore these "hard" decisions are softened through a smoothing process which aims to redefine as "voice" eventual isolated segments of noise among a group of segments of voice and to redefine as "noise" eventual isolated segments of voice among a group of segments of noise. This is the aim of Soft Decision Unit 45 sited immediately after said Hard Decision Unit 43.
It is clear that the above description is given only by way of non-limiting example and that variations and modifications are possible without departing from the scope of the invention. In particular, even if reference has been made to a TRAU unit in a GSM system, what has been said can be applied also to mobile communication systems operating according to other standards. In such case, the energy analysis and spectral analysis should be adapted to the specific requirements of that system. The invention could be used also in other radio communication signals in which a digital coding of speech is adopted and the digitally coded speech signals include spectral information. Moreover, also the energy analysis could be performed on the coded signal, as disclosed in the above-mentioned EP-A 1017042.

Claims

A method of detecting voice activity in a digitally coded speech signal propagating in uplink direction for a mobile communication system , said coded speech signal comprising a representation of a spectral information and received at a receiver (4, 6) belonging to a control part (1, 3) of said system, in which said detection is performed by operating directly on the received speech signal and a signal representative of the presence or absence of voice activity is generated by submitting the speech signal to an energetic analysis, characterised in that said detection also includes a spectral analysis performed directly on the received coded speech signal, by exploiting only the spectral information contained in the received signal itself; and spectrum and energy information resulting from said spectral analysis and said energetic analysis, respectively, are jointly processed for generating said signal representative of the presence or absence of voice.
A method as claimed in claim 1, characterised in that said energetic analysis is performed on decoded received speech signals.
A method as claimed in claim 1 or 2, wherein spectral information in said coded signal is periodically updated, characterised in that said joint processing includes:
- taking a preliminary hard decision, at each updating period of said spectral information, about the presence or absence of voice activity; and

- generating said signal representative of the presence or absence of voice activity through a soft decision step adapted to smooth the decision rate.
A method as claimed in any preceding claim, wherein said voice activity detection is for assisting a voice quality enhancement function for the uplink direction of said system, characterised in that said detection is performed in parallel on a plurality of speech channels, along with an adaptation of the uplink speech signals of said channels to the requirements of the control part of the mobile system and possibly of a fixed network and with an adaptation of the downlink speech signals of said channels to the requirements of the radio part of the mobile system and by using the same processor as would be provided for performing speech signal adaptation and voice quality enhancement in parallel for said plurality of speech channels.
A voice activity detector (40) for detecting voice activity in a digitally coded speech signal propagating in uplink direction for a mobile communication system, said coded speech signal comprising a representation of a spectral information and received at a receiver (4, 6) belonging to a control part (1, 3) of said system, the detector (40) comprising means (42) for performing an energetic analysis on the received speech signal and for supplying energy information to means (43, 45) for generating a signal representative of the presence or absence of voice activity,
characterised in that said detector (40) also includes means (41) for performing a spectral analysis on the received speech signal, said spectral analysis means (41) being connected directly with a detector input (14) where coded speech signals are present and being arranged to perform spectral analysis by exploiting only the spectral information contained in the received signal itself and to supply said generating means (43, 45) with spectral information resulting from said spectral analysis, whereby said generating means (43, 45) are arranged to generate said signal representative of the presence or absence of voice activity by a joint processing of the spectral and energy information.
A voice activity detector as claimed in claim 5, characterised in that said means (42) for performing an energetic analysis on the received speech signal are connected downstream a speech decoder (6).
A voice activity detector as claimed in claim 5 or 6, wherein spectral information in said coded signals is periodically updated, characterised in that said means (43, 45) for generating a signal representative of the presence or absence of voice activity include:
- a hard decision unit (43) connected to the output of said means (42) for performing an energetic analysis and said means (41) for performing a spectral analysis, said unit being adapted to joint processing, at each updating period, an input speech segment and to output a hard noise or voice decision;

- a soft decision unit (45) connected to the output of said hard decision unit (43) and adapted to perform a smoothing process in order to redefine as "voice" possible isolated noise segments among a group of voice segments and to redefine as "noise" possible isolated voice segments among a group of noise segments.
A voice activity detector (40) as claimed in any of claims 5 to 7, wherein said detector (40) is located upstream of means (50) performing a voice quality enhancement on uplink speech signals and supplies said voice quality enhancement means (50) with said signal representative of the presence or absence of voice activity, characterised in that it is arranged to perform detection in parallel for the uplink speech signals transmitted over a plurality of speech channels.
A voice activity detector as claimed in claim 8, characterised in that said detector (40) is part, together with said voice quality enhancement means (50), of a unit (1) performing, for said plurality of channels, an adaptation of the uplink directed speech signals to the requirements of the control part (1, 2) of the mobile system and possibly of a fixed network and an adaptation of the downlink directed speech signals to the requirements of the radio part of the mobile system, and is implemented by the same processor that would be provided for performing speech signal adaptation and voice quality enhancement in parallel for said plurality of speech channels.
A method of voice quality enhancement for a mobile communication system, in which a voice quality enhancement including a noise reduction operation is performed at least for speech signals propagating in uplink direction, characterised in that said noise reduction operation is driven by a signal representative of the presence or absence of voice activity generated by a method of voice activity detection as claimed in any of claims 1 to 4 and/or by a voice activity detector as claimed in any of claims 5 to 9.