Data processing: speech signal processing – linguistics – language – Speech signal processing – For storage or transmission
Reexamination Certificate
2001-10-02
2003-12-09
Chawan, Vijay (Department: 2654)
Data processing: speech signal processing, linguistics, language
Speech signal processing
For storage or transmission
C704S210000, C704S226000, C704S227000, C704S233000
Reexamination Certificate
active
06662155
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to speech communication and, more particularly, to comfort noise generation in discontinuous transmission.
BACKGROUND OF THE INVENTION
In a normal telephone conversation, one user speaks at a time and the other listens. At times, neither of the users speak. The silent periods could result in a situation where average speech activity is below 50%. In these silent periods, only acoustic noise from the background is likely to be heard. The background noise does not usually have any informative content and it is not necessary to transmit the exact background noise from the transmit side (TX) to the receive side (RX). In mobile communication, a procedure known as discontinuous transmission (DTX) takes advantage of this fact to save power in the mobile equipment. In particular, the TX DTX mechanism has a low state (DTX Low) in which the radio transmission from the mobile station (MS) to the base station (BS) is switched off most of the time during speech pauses to save power in the MS and to reduce the overall interference level in the air interface.
A basic problem when using DTX is that the background acoustic noise, present with the speech during speech periods, would disappear when the radio transmission is switched off, resulting in discontinuities of the background noise. Since the DTX switching can take place rapidly, it has been found that this effect can be very annoying for the listener. Furthermore, if the voice activity detector (VAD) occasionally classifies the noise as speech, some parts of the background noise are reconstructed during speech synthesis, while other parts remain silent. Not only is the sudden appearance and disappearance of the background noise very disturbing and annoying, it also decreases the intelligibility of the conversation, especially when the energy level of the noise is high, as it is inside a moving vehicle. In order to reduce this disturbing effect, a synthetic noise similar to the background noise on the transmit side is generated on the receive side. The synthetic noise is called comfort noise (CN) because it makes listening more comfortable.
In order for the receive side to simulate the background noise on the transmit side, the comfort noise parameters are estimated on the transmit side and transmitted to the receive side using Silence Descriptor (SID) frames. The transmission takes place before transitioning to the DTX Low state and at an MS defined rate afterwards. The TX DTX handler decides what kind of parameters to compute and whether to generate a speech frame or a SID frame.
FIG. 1
describes the logical operation of TX DTX. This operation is carried out with the help of a voice activity detector (VAD), which indicates whether or not the current frame contains speech. The output of the VAD algorithm is a Boolean flag marked with ‘true’ if speech is detected, and ‘false’ otherwise. The TX DTX also contains the speech encoder and comfort noise generation modules.
The basic operation of the TX DTX handler is as follows. A Boolean speech (SP) flag indicates whether the frame is a speech frame or a SID frame. During a speech period, the SP flag is set ‘true’ and a speech frame is generated using the speech coding algorithm. If the speech period has been sustained for a sufficiently long period of time before the VAD flag changes to ‘false’, there exists a hangover period (see FIG.
2
). This time period is used for the computation of the average background noise parameters. During the hangover period, normal speech frames are transmitted to the receive side, although the coded signal contains only background noise. The value of SP flag remains ‘true’ in the hangover period. After the hangover period, the comfort noise (CN) period starts. During the CN period, the SP flag is marked with ‘false’ and the SID frames are generated.
During the hangover period, the spectrum, S, and power level, E, of each frame is saved. After the hangover, the averages of the saved parameters, S
ave
and E
ave
, are computed. The averaging length is one frame longer than the length of the hangover period. Therefore, the first comfort noise parameters are the averages from the hangover period and the first frame after it.
During the comfort noise period, SID frames are generated every frame, but they are not all sent. The TX radio subsystem (RSS) controls the scheduling of the SID frame transmission based on the SP flag. When a speech period ends, the transmission is cut off after the first SID frame. Afterward, one SID frame is occasionally transmitted in order to update the estimation of the comfort noise.
FIG. 3
describes the logical operation of the RX DTX. If errors have been detected in the received frame, the bad frame indication (BFI) flag is set ‘true’. Similar to the SP flag in the transmit side, a SID flag in the receive side is used to describe whether the received frame is a SID frame or a speech frame.
The RX DTX handler is responsible for the overall RX DTX operation. It classifies whether the received frame is a valid frame or an invalid frame (BFI=0 or BFI=1, respectively) and whether the received frame is a SID frame or a speech frame (SID=1 or SID=0, respectively). When a valid speech frame is received, the RX DTX handler passes it directly to the speech decoder. When an erroneous speech frame is received or the frame is lost during a speech period, the speech decoder uses the speech related parameters from the latest good speech frame for speech synthesis and, at the same time, the decoder starts to gradually mute the output signal.
When a valid SID frame is received, comfort noise is generated until a new valid SID frame is received. The process repeats itself in the same manner. However, if the received frame is classified as an invalid SID frame, the last valid SID is used. During the comfort noise period, the decoder receives transmission channel noise between SID frames that have never been sent. To synthesize signals for those frames, comfort noise is generated with the parameters interpolated from the two previously received valid SID frames for comfort noise updating. The RX DTX handler ignores the unsent frames during the CN period because it is presumably due to a transmission break.
Comfort noise is generated using analyzed information from the background noise. The background noise can have very different characteristics depending on its source. Therefore, there is no general way to find a set of parameters that would adequately describe the characteristics of all types of background noise, and could also be transmitted just a few times per second using a small number of bits. Because speech synthesis in speech communication is based on the human speech generation system, the speech synthesis algorithms cannot be used for the comfort noise generation in the same way. Furthermore, unlike speech related parameters, the parameters in the SID frames are not transmitted every frame. It is known that the human auditory system concentrates more on the amplitude spectrum of the signal than to the phase response. Accordingly, it is sufficient to transmit only information about the average spectrum and power of the background noise for comfort noise generation. Comfort noise is, therefore, generated using these two parameters. While this type of comfort noise generation actually introduces much distortion in the time domain, it resembles the background noise in the frequency domain. This is enough to reduce the annoying effects in the transition interval between a speech period and a comfort noise period. Comfort noise generation that works well has a very soothing effect and the comfort noise does not draw attention to itself. Because the comfort noise generation decreases the transmission rate while introducing only small perceptual error, the concept is well accepted. However, when the characteristics of the generated comfort noise differ significantly from the true background noise, the transition between comfort noise and true background no
Mikkola Hannu
Rotola-Pukkila Jani
Vainio Janne
Chawan Vijay
Green Bradford
Harper V. Paul
Ware Fressola Van Der Sluys & Adolphson LLP
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