Pulse or digital communications – Transceivers
Reexamination Certificate
1998-08-06
2002-07-09
Vo, Don N. (Department: 2631)
Pulse or digital communications
Transceivers
C375S222000, C375S232000, C375S285000, C375S296000, C455S570000
Reexamination Certificate
active
06418159
ABSTRACT:
TECHNICAL FIELD
This invention relates to a portable radio telephone device used as a mobile station, such as a cellular system, PHS (personal handy phone system), or the like, and a control method for same, and in particular, it relates to a radio telephone device and control method for same, which is improved such that deterioration of call quality due to unstable operation of the noise canceller, which removes ambient noise, is prevented.
BACKGROUND ART
TDMA (Time Division Multiple Access) is known as one communications method used in recent digital portable telephone system. This method is extremely valuable in increasing channel capacity by enabling a plurality of channels to be transmitted on the same radio channel by compressing voice signals by means of digital signal processing.
Furthermore, VSELP (Vector Sum Excited Linear Prediction) is commonly used as a digital voice processing method in a full-rate codec.
Since a voice processing system of this kind involves only having a code book for voice signals, an unnatural transmission signal is produced when sound other than a voice, such as noise, for example, is transmitted, and in circumstances where there is a high level of ambient noise, or. the like, this has a significant effect on the quality of the communicated voice signal.
As a countermeasure against this, generally, a method whereby the ambient noise is reduced by providing a noise canceller function is employed. In particular, in PSI-CELP (Pitch Synchronous Innovation CELP) systems which have a high compression rate and are used in a half-rate codec, voice pitch information is used in the encoding process, and therefore sound distortion due to noise occurs readily, so a noise canceller is indispensable for maintaining call quality.
Incidentally, a noise canceller cancels noise by extracting noise components from an input sound signal containing ambient noise by means of an adaptive filter, and mixing components of opposite phase into the aforementioned input sound signal, and generally, a Carman filter is used as the adaptive filter and a structure is adopted whereby a learning operation is carried out according to the ambient noise component, such that the filter tap coefficient is an optimum value.
Therefore, if there is not sufficient learning time, for example, the amount of noise cancellation will decline and it will not be possible to implement an effective noise cancelling operation.
For example, immediately after the start of a call, the noise canceller has only just started to operate and the tap coefficient of the adaptive filter is in an unspecified (not yet converged) state, meaning that there is inadequate noise cancellation and therefore unwanted noise is audible to the other party, which obstructs the call.
Usually, a time period of approximately four seconds is required for complete convergence, but it is known that after approximately two seconds, at least, it is possible to cancel the noise component to a certain level.
However, with regard to call quality, the fact that a noise component is present in the call signal for two seconds even represents a very significant demerit.
Furthermore, since the convergence of the noise canceller produces an effect whereby, in the next step, the noise component suddenly reduces, the call sounds very unnatural to the other party.
Moreover, whilst it is common for portable radio telephones used in this type of system to incorporate a function whereby the user can mute the transmitted sound consciously by his/her own operation, since the input of ambient noise to the noise canceller is temporarily prohibited when this mute function is operated, the learning operation of the noise canceller is interrupted temporarily and the learning operation must start again after the mute function has been cancelled, thereby causing unavoidable deterioration of call quality due to insufficient convergence of the noise canceller, similarly to the start of a call.
Furthermore, if the portable radio telephone moves during a call, or the radio state deteriorates, a so-called hand-over operation for changing the base station, or resynchronization processing for resynchronizing with the base station, is implemented, but since the input of ambient noise to the noise canceller is temporarily prohibited in this case also, the learning operation of the noise canceller is interrupted temporarily and the learning operation must start again after the mute function has been cancelled, thereby causing unavoidable deterioration of call quality due to insufficient convergence of the noise canceller, similarly to the start of a call.
In this way, in the conventional device described above, the noise canceller does not converge and the amount of reduction of the noise component is insufficient immediately after the start of communications, when a call is made or received, thereby causing unnatural noise to be audible to the other party.
Moreover, when a mute function is engaged by the user, or when hand-over or resynchronization processing is implemented, the call quality is impaired dramatically by generation of unnatural noise when the mute function is cancelled, or when the hand-over or resynchronization processing is completed.
DISCLOSURE OF THE INVENTION
Therefore, it is an object of the present invention to provide a portable radio telephone and a control method for same, whereby call quality can be improved by eliminating unstable operation of the noise canceller at the start of communications, when a call is made or received, when the mute function is cancelled during communications, or when a hand-over or resynchronization operation is completed.
In order to achieve the aforementioned objects, this invention comprises: a noise canceller for removing noise components by extracting the noise components from a transmission voice signal and mixing components of opposite phase to the extracted noise components into the transmission voice signal; voice encoding means for encoding the transmission voice signal from which the noise components have been removed by the noise canceller; and control means for activating the noise canceller before the start of operation of the voice encoding means.
Here, a composition may be adopted, wherein the noise canceller extracts the noise components by a learning process, and the control means starts the learning extraction operation of the noise components by the noise canceller before the start of operation of the encoding means by at least a time period required for extraction of the noise components.
Here, a composition may be adopted, wherein the noise canceller comprises an adaptive filter for extracting noise components from a digital transmission voice signal, and the adaptive filter determines a filter tap coefficient by means of a learning operation such that it is optimal with respect to the noise components contained in the digital transmission voice signal.
Furthermore, a composition may also be adopted, wherein, during start-up of a communications channel relating to call origination or call reception, the control means sets the noise canceller to an operational state before the start of operation of the voice encoding means, and sets the voice encoding means to an operational state after a response by a user on the communications channel.
Moreover, a composition may be adopted, wherein the control means sets the voice encoding means to an idle state and the noise canceller to an operational state in response to a muting operation during a call, and sets the voice encoding means to an operational state in response to cancellation of the muting operation.
Furthermore, a composition may be adopted, wherein the control means sets the voice encoding means to an idle state and the noise canceller to an operational state in the event of hand-over or resynchronization processing during a call, and sets the voice encoding means to an operational state in response to completion of the hand-over or resynchronization processing.
Moreover, this invention comprises: a noise canceller for removing nois
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Kabushiki Kaisha Toshiba
Vo Don N.
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