Electrical audio signal processing systems and devices – Noise or distortion suppression
Reexamination Certificate
1999-05-20
2004-02-10
Harvey, Minsun Oh (Department: 2644)
Electrical audio signal processing systems and devices
Noise or distortion suppression
C455S222000, C455S296000, C381S013000
Reexamination Certificate
active
06690805
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an audio signal noise reduction system and, more particularly, a noise reduction system capable of removing a pulse-like noise (referred to as “pulse-like noise” or “noise” hereinafter) in a car radio, etc., which is caused by an ignition plug of an engine, an window open/close driving motor, etc.
FIG. 53
is a block diagram of a pulse-like noise reduction system in the prior art, which is set forth in Patent Application Publication (KOKAI) Sho 63-87026, for example. In
FIG. 53
, when receives an FM intermediate frequency signal of an FM receiver, an FM detector circuit
1
outputs a detected signal to a delay circuit
2
formed of an LPF (low-pass filter) which then delays the signal. Then, an output of the delay circuit
2
is supplied to a stereo demodulator circuit
5
via a gate circuit
3
and a level hold circuit
4
. The detected signal is supplied to an HPF (high-pass filter)
6
to detect a noise. A noise signal passed through the HPF
6
is amplified by a noise amplifier
7
and then supplied to a noise detector circuit
8
.
The noise detector circuit
8
is composed of a rectifier circuit which rectifies an output signal of the noise amplifier
7
. An output of the noise detector circuit
8
is supplied to a waveform shaping circuit
9
and an integration circuit
10
. The waveform shaping circuit
9
converts the output of the noise detector circuit
8
into a pulse with a predetermined time width and supplies the pulse to the gate circuit
3
. The gate circuit
3
, when driven by the pulse which is supplied from the waveform shaping circuit
9
to the gate circuit
3
, is brought into its signal cut-off state. In this signal cut-off state, a delayed output level being detected immediately before the signal cut-off is held by the level hold circuit
4
and then supplied to the stereo demodulator circuit
5
. Hence, generation of a spike due to sudden change in potential can be prevented. An integration circuit
10
outputs a direct current signal in answer to the noise level by smoothing the output of the noise detector circuit
8
and then feeds back it to the noise amplifier
7
, whereby an AGC loop is constructed.
The delay circuit
2
is provided to compensate a time period ranging from a time point when the pulse-like noise is supplied to the HPF
6
to a time point when the gate circuit
3
is brought into its cut-off state. Since an Lch (left channel) signal and an Rch (right channel) which are balanced-modulated around (Lch+Rch)/2 by using a frequency of 38 kHz, as shown in
FIG. 54
, are input into the stereo demodulator circuit
5
, Lch and Rch signals can be picked out separately from the stereo demodulator circuit
5
by virtue of the time division at 38 kHz, for example.
FIG. 55
is a view showing an operation of the noise reduction system in the prior art. Assume that the output signal of the FM detector circuit
1
is such a signal that includes the pulse-like noise (symbol A) shown in
FIG. 55A
, a high frequency component of the output signal of the FM detector circuit
1
is extracted by the HPF
6
, so that a signal shown in
FIG. 55B
can be detected. An output signal of the HPF
6
is amplified by the noise amplifier
7
, then rectified by the noise detector circuit
8
, and then converted into the pulse with a predetermined time width, as shown in
FIG. 55C
, by the waveform shaping circuit
9
. The gate circuit
3
brings a period of the pulse-like noise (referred to as a “pulse-like noise period” or “noise period” hereinafter) of the signal which is delayed by a predetermined time by the delay circuit
2
, as shown in
FIG. 55D
, into a cut-off state, as shown in FIG.
55
E. The level hold circuit
4
removes the pulse-like noise contained in the original signal by holding the level, which is detected immediately before the cut-off, during the cut-off period of the output of the gate circuit
3
, as shown in FIG.
55
F.
Since the pulse-like noise reduction system in the prior art is constructed as mentioned above, the pulse-like noise can be rejected. However, if the signal has an amplitude to some extent, discontinuity of the signal occurs at the end of the holding period, as shown in
FIG. 56B
, because of preceding value holding process, i.e., because a value detected immediately before the start of the holding period is held. As a result, the noise cannot be satisfactorily removed.
In addition, if the signal contains a high frequency component, the signal becomes discontinuous at both the beginning and end of the holding period, as shown in
FIG. 56C
, because of the preceding value holding process. Therefore, in some cases the presence of the noise rejection process is conspicuous in hearing sense.
Moreover, since the preceding value holding process is carried out before the stereo demodulation, there has been such a problem that, if there is a difference between the signals on both channels, signal portions which are largely different from the beginning and end of the process are caused on one channel, as shown in
FIG. 56D
, because of the preceding value holding process, and others.
SUMMARY OF THE INVENTION
The present invention has been made to overcome the above problems, and it is an object of the present invention to provide a noise reduction system which is capable of removing pulse-like noises from an audio signal without fail after stereo demodulation, and also causing no discontinuity between removed regions and their preceding and succeeding regions even if the audio signal contains a high frequency component.
An audio signal noise reduction system according to the present invention comprises a noise detecting means for detecting a noise of an audio signal and outputting a detection signal indicating a start time and an end time of a noise period of the noise; a first filter means for extracting a low frequency component of the audio signal; a low frequency band interpolation means for polynomial-interpolating the noise period of the low frequency component being extracted; a second filter means for extracting intermediate and high frequency components of the audio signal; a means for suppressing a level of the noise period of the intermediate and high frequency components being extracted; and a signal synthesizing means for synthesizing the low frequency component, whose noise period is polynomial-interpolated, and the intermediate and high frequency components, the level of whose noise period is suppressed, to output the audio signal.
In this audio signal noise reduction system, since the low frequency component of the audio signal is extracted by the low-pass filter means and then the noise period from which the pulse-like noise is detected is polynomial-interpolated, the protruded interpolation signal is never derived and also the signal does not become extremely discontinuous before and after the noise period at all. In addition, since the intermediate and high frequency components of the audio signal are extracted by the intermediate/high-pass filter, then the level of the noise period of the intermediate and high frequency components is suppressed, and then the intermediate and high frequency components are synthesized with the low frequency component which is polynomial-interpolated, the audio signal from which the noise is completely removed can be derived.
Further, since the noise period is interpolated and suppressed independently every channel after the stereo demodulation, no discrepancy is caused before and after the noise period even if there is difference between signals over a plurality of channels.
An audio signal noise reduction system comprises a noise detecting means for detecting a noise of an audio signal and outputting a detection signal indicating a start time and an end time of a noise period of the noise; a first filter means for extracting a low frequency component of the audio signal; a limiter means for limiting a degree of an inclination of a segment serving as a start point of the noise period of the low frequency component b
Ishida Masayuki
Taura Ken-ichi
Tsuji Masayuki
Tsujishita Masahiro
Birch & Stewart Kolasch & Birch, LLP
Harvey Minsun Oh
Mitsubishi Denki & Kabushiki Kaisha
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