Data processing: speech signal processing – linguistics – language – Speech signal processing – For storage or transmission
Reexamination Certificate
1999-08-10
2003-08-12
Abebe, Daniel (Department: 2654)
Data processing: speech signal processing, linguistics, language
Speech signal processing
For storage or transmission
C704S226000
Reexamination Certificate
active
06606593
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the field of speech communication and, more particularly, to discontinuous transmission (DTX) and to improving the quality of comfort noise (CN) during discontinuous transmission.
BACKGROUND OF THE INVENTION
Discontinuous transmission is used in mobile communication systems to switch the radio transmitter off during speech pauses. The use of DTX saves power in the mobile station and increases the time required between battery recharging. It also reduces the general interference level and thus improves transmission quality.
However, during speech pauses the background noise which is transmitted with the speech also disappears if the channel is cut off completely. The result is an unnatural sounding audio signal (silence) at the receiving end of the communication.
It is known in the art, instead of completely switching the transmission off during speech pauses, to generate parameters that characterize the background noise, and to send these parameters over the air interface at a low rate in Silence Descriptor (SID) frames. These parameters are used at the receive side to regenerate background noise which reflects, as well as possible, the spectral and temporal content of the background noise at the transmit side. These parameters that characterize the background noise are referred to as comfort noise (CN) parameters. The comfort noise parameters typically include a subset of speech coding parameters: in particular synthesis filter coefficients and gain parameters.
It should be noted, however, that in some comfort noise evaluation schemes of some speech codecs, part of the comfort noise parameters are derived from speech coding parameters while other comfort noise parameter(s) are derived from, for example, signals that are available in the speech coder but that are not transmitted over the air interface.
It is assumed in prior-art DTX systems that the excitation can be approximated sufficiently well by spectrally flat noise (i.e., white noise). In prior art DTX systems, the comfort noise is generated by feeding locally generated, spectrally flat noise through a speech coder synthesis filter. However, such white noise sequences are unable to produce high quality comfort noise. This is because the optimal excitation sequences are not spectrally flat, but may have spectral tilt or even a stronger deviation from flat spectral characteristics. Depending on the type of background noise, the spectra of the optimal excitation sequences may, for example, have lowpass or highpass characteristics. Because of this mismatch between the random excitation and the correct or optimal excitation the comfort noise generated at the receive side sounds different from the background noise on the transmit side. The generated comfort noise may, for example, sound considerably “brighter” or “darker” than it should be. During DTX, the spectral content of the background noise thus changes between active speech (i.e., speech coding on) and speech pauses (i.e., comfort noise generation on). This audible difference in the comfort noise thus causes a reduction in the transmission quality which can be perceived by a user.
In speech coding systems, such as in the full rate (FR), half rate (HR), and enhanced full rate (EFR) speech channels of the GSM system, the comfort noise parameters are transmitted at a low rate. By example, in the FR and EFR channels this rate is only once per every 24 frames (i.e., every 480 milliseconds). This means that comfort noise parameters are updated only about twice per second. This low transmission rate cannot accurately represent the spectral and temporal characteristics of the background noise and, therefore, some degradation in the quality of background noise is unavoidable during DTX.
A further problem that arises during DTX in digital cellular systems, such as GSM, relates to a hangover period of a few speech frames that is introduced after a speech burst, and before the actual transmission is terminated. If the speech burst is below some threshold duration, it can be interpreted as a background noise spike, and in this case the speech burst is not followed by a hangover period. The hangover period is used for computing an estimate of the characteristics of the background noise on the transmit side to be transmitted to the receive side in a comfort noise parameter message (or Silence Descriptor (SID) frame), before the transmission is terminated. As was described above, the transmitted background noise estimate is used on the receive side to generate comfort noise with characteristics similar to the transmit side background noise at the time the transmission is terminated.
In known types of DTX mechanisms similar to those of GSM FR and HR, non-predictive comfort noise quantization schemes are employed. Due to this, the receive side does not have to know if a hangover period exists at the end of a speech burst. However, in GSM EFR, efficient predictive comfort noise quantization schemes are employed, and the existence of a hangover period is locally evaluated at the receive side to assist in comfort noise dequantization. This involves a small computational load and a number of program instructions to be executed.
Another problem arises if the background noise on the transmit side is not stationary but varies considerably. In this case there may exist a single frame or a small number of frames within an averaging period for which some or all of the speech coding parameters provide a poor characterization of the typical background noise. A similar situation may occur when a Voice Activity Detection or VAD algorithm interprets the unvoiced end of the period of active speech as “no speech”, or the stationary background noise contains strong impulse-type noise bursts. Because of the short duration of the averaging periods in known types of DTX systems such ill-conditioned speech coding parameters may change the result of the averaging significantly enough that the resulting averaged CN parameters do not accurately characterize the background noise. This results in a mismatch either in the level or in the spectrum, or both, between the background noise and the comfort noise. The quality of transmission is thus impaired as the background noise sounds different to the user depending on whether it is received during speech (normal speech coding of speech and background noise) or during speech pauses (produced by comfort noise generation).
In greater detail, during the DTX hangover period any frames declared by the VAD algorithm as being “no speech” frames are sent over the air interface, and the speech coding parameters are buffered to be able to evaluate the comfort noise parameters for a first SID frame. The first SID frame is transmitted immediately after the end of the DTX hangover period. The length of the DTX hangover period is thus determined by the length of the averaging period. Therefore, to minimize the channel activity of the system, the averaging period should be fixed at a relatively short length.
Before describing the present invention, it will be instructive to review conventional circuitry and methods for generating comfort noise parameters on the transmit side, and for generating comfort noise on the receive side. In this regard reference is thus first made to
FIGS. 1
a
-
1
d.
Referring to
FIG. 1
a
, short term spectral parameters
102
are calculated from a speech signal
100
in a Linear Predictive Coding (LPC) analysis block
101
. LPC is a method well known in the prior art. For simplicity, discussed herein is only the case where the synthesis filter has only a short term synthesis filter, it being realized that in most prior art systems, such as in GSM FR, HR and EFR coders, the synthesis filter is constructed as a cascade of a short term synthesis filter and a long term synthesis filter. However, for the purposes of this description a discussion of the long term synthesis filter is not necessary. Furthermore, the long term synthesis filter is typically switched off during comfort noise generation in
Jarvinen Kari
Kapanen Pekka
Rotola-Pukkila Jani
Ruoppila Vesa
Abebe Daniel
Nokia Mobile Phones Ltd.
Perman & Green LLP
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