Noise suppression

Details Noise suppression Noise suppression

2000-11-15

2004-10-26

Nguyen, Duc M. (Department: 2685)

Telecommunications

Transmitter and receiver at same station

Radiotelephone equipment detail

C455S063100, C455S296000, C370S286000, C379S406010

Reexamination Certificate

active

06810273

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a noise suppressor and a noise suppression method. It relates particularly to a mobile terminal incorporating a noise suppressor for suppressing noise in a speech signal. A noise suppressor according to the invention can be used for suppressing acoustic background noise, particularly in a mobile terminal operating in a cellular network.
BACKGROUND OF THE INVENTION
One purpose of noise suppression or speech enhancement in a mobile telephone terminal is to reduce the impact of environmental noise on a speech signal and thus to improve the quality of communication. In the case of an up-link (transmission, TX) signal, it is also desired to minimise detrimental effects in the speech coding process caused by this noise.
In face-to-face communication, acoustic background noise disturbs a listener and makes it more difficult to understand speech. Intelligibility is improved by a speaker raising his or her voice so that it is louder than the background noise. In the case of telephony, background noise is troublesome because there is no additional information provided by facial expressions and gestures.
In digital telephony, a speech signal is first converted into a sequence of digital samples in an analogue-to-digital (A/D) converter and then compressed for transmission using a speech codec. The term codec is used to describe a speech encoder/decoder pair. In this description, the term “speech encoder” is used to denote the encoding side of the speech codec and the term “speech decoder” is used to denote the decoding functions of the speech codec. It should be appreciated that a general speech codec may be implemented as a single functional unit, or as separate elements that implement the encoding and decoding operations.
In digital telephony, the deleterious effect of background noise can be great. This is due to the fact that speech codecs are generally optimised for efficient compression and acceptable reconstruction of speech and their performance can be impaired if noise is present in the speech signal, or errors occur in speech transmission or reception. In addition, the presence of noise itself can lead to distortion to the background noise signal when it is encoded and transmitted.
Impaired performance of a speech codec reduces both the intelligibility of the transmitted speech and its subjective quality. Distortion of the transmitted background noise signal degrades the quality of the transmitted signal, making it more annoying to listen to and rendering contextual information less recognisable by changing the nature of the background noise signal. Consequently, work in the field of speech enhancement has concentrated on studying the effect of noise on speech coding performance and producing pre-processing methods to reduce the impact of noise on speech codecs.
The problems discussed above relate to arrangements in which only one microphone is present to provide only one signal. In such arrangements a noise suppressor is provided which can interpret the one-channel signal to decide which parts of it represent underlying speech and which represent noise.
When a digital mobile terminal receives an encoded speech signal, it is decoded by the decoding part of the terminal's speech codec and supplied to a loudspeaker or earpiece for the user of the terminal to hear. A noise suppressor may be provided in the speech decoding path, after the speech decoder, in order to reduce the noise component in the received and decoded speech signal. However, in noisy conditions the performance of the speech decoder may be affected detrimentally, resulting in one or more of the following effects:
1. The speech component of the signal may sound less natural or harsh, as critical information required by the speech codec in order to correctly decode the speech signal is altered by the presence of noise.
2. The background noise may sound unnatural because codecs are generally optimised for compressing speech rather than noise. Typically this gives rise to increased periodicity in the background noise component and may be sufficiently severe to cause the loss of contextual information carried by the background noise signal.
Information about an encoded speech signal may also be lost or corrupted during transmission and reception, for example due to transmission channel errors. This situation may give rise to further deterioration in the speech decoder output, causing additional artefacts to become apparent in the decoded speech signal. When a noise suppressor is used in the speech decoding path, after a speech decoder, non-optimal performance of the speech decoder may in turn cause the noise suppressor to operate in a less than optimal manner.
Therefore special care must be taken when implementing noise suppressors intended to operate on decoded speech signals. In particular, two conflicting factors have to be balanced. If the noise suppressor provides too much noise attenuation, this may reveal the deterioration in speech quality caused by the speech codec. However, due to the intrinsic properties of typical speech codecs, which are optimised for the encoding and decoding of speech, decoded background noise can sound more annoying than the original noise signal and so it should be attenuated as much as possible. Thus, in practice, it is found that a slightly lower level of noise reduction may be optimal for decoded speech signals, compared with that which can be applied to speech signals prior to encoding.
It is generally desirable that when noise suppression is used during speech encoding and/or decoding, it should reduce the level of background noise, minimise the speech distortion caused by the noise reduction process and preserve the original nature of the input background noise.
An embodiment of a mobile terminal comprising a noise suppressor according to prior art will now be described with reference to FIG.
1
. The mobile terminal and the wireless system with which it communicates operate according to the Global System for Mobile telecommunications (GSM) standard.
FIG. 1
shows a mobile terminal
10
comprises a transmitting (speech encoding) branch
12
and a receiving (speech decoding) branch
14
.
In the transmitting (speech encoding) branch, a speech signal is picked up by a microphone
16
and sampled by an analogue-to-digital (A/D) converter
18
and noise suppressed in a noise suppressor
20
to produce an enhanced signal. This requires the spectrum of the background noise to be estimated so that background noise in the sampled signal can be suppressed. A typical noise suppressor operates in the frequency domain. The time domain signal is first transformed to the frequency domain, which can be carried out efficiently using a Fast Fourier Transform (FFT). In the frequency domain, voice activity has to be distinguished from background noise, and when there is no voice activity, the spectrum of the background noise is estimated. Noise suppression gain coefficients are then calculated on the basis of the current input signal spectrum and the background noise estimate. Finally, the signal is transformed back to the time domain using an inverse FFT (IFFT).
The enhanced (noise suppressed) signal is encoded by a speech encoder
22
to extract a set of speech parameters which are and then channel encoded in a channel encoder
24
where redundancy is added to the encoded speech signal in order to provide some degree of error protection. The resultant signal is then up-converted into a radio frequency (RF) signal and transmitted by a transmitting/receiving unit
26
. The transmitting/receiving unit
26
comprises a duplex filter (not shown) connected to an antenna to enable both transmission and reception to occur.
A noise suppressor suitable for use in the mobile terminal of
FIG. 1
is described in published document WO97/22116.
In order to lengthen battery life, different kinds of input signal-dependent low power operation modes are typically applied in mobile telecommunication systems. These arrangements are commonly referred to as discon

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