Noise suppression device, noise suppression method, and reception device and reception method using same
A noise suppression device includes: a DFT executor that expands a baseband signal into a discrete Fourier series X 0 (n), the baseband signal being generated by mixing an AM broadcast wave signal including a carrier wave of the angular frequency ω C with a complex sine wave of the same frequency; and an amplitude spectrum calculator that calculates an amplitude spectrum |X 0 (n)| from X 0 (n). The noise suppression device also includes: an asymmetric component detector that detects an asymmetric component in |X 0 (n)|; a suppressor that calculates a discrete Fourier series X 1 (n) by multiplying the value corresponding to the asymmetric frequency bin by a first factor and multiplying the other values by a second factor in X 0 (n); and an IDFT executor that performs inverse discrete Fourier transform on X 1 (n) to obtain a discrete-time signal.
1. A noise suppression device, comprising:
a discrete Fourier transform executor that expands a baseband signal into a discrete Fourier series, the baseband signal being generated by quadrature-demodulating a received signal having an upper sideband signal and a lower sideband signal located symmetrically on a frequency axis with respect to a first angular frequency;
an amplitude spectrum calculator that calculates an amplitude spectrum of frequency bins of the baseband signal expanded into the discrete Fourier series;
an asymmetric component detector that detects, as an asymmetric frequency bin, a frequency bin corresponding to an asymmetric component in the amplitude spectrum by evaluating symmetry of the amplitude spectrum with respect to a center frequency bin corresponding to the first angular frequency;
a suppressor that, in the discrete Fourier series expanded from the baseband signal, multiplies a value corresponding to the asymmetric frequency bin by a first factor, and multiplies a value corresponding to a frequency bin other than the asymmetric frequency bin by a second factor larger than the first factor; and
an inverse discrete Fourier transform executor that performs inverse discrete Fourier transform on the discrete Fourier series processed by the suppressor and obtains a discrete-time signal.
2. The noise suppression device according to claim 1 , further comprising:
a noise component detector that compares an amplitude of a center frequency bin corresponding to an angular frequency of a carrier wave signal included in the received signal and an amplitude of a frequency bin other than the center frequency bin, and detects a noise frequency bin having an amplitude whose ratio to the amplitude of the center frequency bin is higher than a predetermined value;
a second suppressor that, in the discrete Fourier series expanded from the baseband signal, multiplies a value corresponding to the noise frequency bin by a third factor, and multiplies a value corresponding to a frequency bin having an amplitude whose ratio to the amplitude of the center frequency bin is not higher than the predetermined value by a fourth factor larger than the third factor; and
an adjuster that interpolates the discrete Fourier series processed by the second suppressor into the discrete Fourier series processed by the suppressor and calculates a new discrete Fourier series,
wherein the discrete-time signal is obtained by the inverse discrete Fourier transform executor performing inverse discrete Fourier transform on the new discrete Fourier series.
3. The noise suppression device according to claim 2 , wherein
the noise frequency bin comprises a plurality of noise frequency bins, and
the third factor is capable of taking an individual value for each noise frequency bin.
4. The noise suppression device according to claim 2 , wherein
the frequency bin other than the noise frequency bin comprises a plurality of frequency bins, and
the fourth factor is capable of taking an individual value for each frequency bin other than the noise frequency bin.
5. The noise suppression device according to claim 1 , wherein
the asymmetric frequency bin comprises a plurality of asymmetric frequency bins, and
the first factor is capable of taking an individual value for each asymmetric frequency bin.
6. The noise suppression device according to claim 1 , wherein
the frequency bin other than the asymmetric frequency bin comprises a plurality of frequency bins, and
the second factor is capable of taking an individual value for each frequency bin other than the asymmetric frequency bin.
7. The noise suppression device according to claim 1 , wherein the asymmetric component detector detects the asymmetric frequency bin by evaluating the symmetry of the amplitude spectrum based on comparison between an amplitude of one frequency bin and the greatest amplitude among amplitudes of sequential frequency bins including a frequency bin located symmetrically to the one frequency bin with respect to the center frequency bin.
8. The noise suppression device according to claim 1 , wherein the center frequency bin is a frequency bin having the greatest amplitude near a DC component in the amplitude spectrum.
9. A reception device, comprising:
an antenna that receives an AM broadcast wave signal output from a broadcast station;
an amplifier that amplifies the AM broadcast wave signal received from the antenna;
an analog-to-digital converter that converts the amplified AM broadcast wave signal into a digital signal;
a quadrature demodulator that quadrature-demodulates the digital signal to generate a baseband signal;
a noise suppression device according to claim 1 that suppresses a noise included in the baseband signal to generate a discrete-time signal; and
a demodulator that demodulates the discrete-time signal into a sound signal.
10. A noise suppression device, comprising:
a discrete Fourier transform executor that expands a baseband signal into a discrete Fourier series, the baseband signal being generated by quadrature-demodulating a received signal having a carrier wave signal and a pair of sideband signals;
an amplitude spectrum calculator that calculates an amplitude spectrum of frequency bins of the baseband signal expanded into the discrete Fourier series;
a noise component detector that compares an amplitude of a center frequency bin corresponding to an angular frequency of the carrier wave signal and an amplitude of a frequency bin other than the center frequency bin, and detects a noise frequency bin having an amplitude whose ratio to the amplitude of the center frequency bin is higher than a predetermined value;
a suppressor that, in the discrete Fourier series expanded from the baseband signal, multiplies a value corresponding to the noise frequency bin by a third factor, and multiplies a value corresponding to a frequency bin having an amplitude whose ratio to the amplitude of the center frequency bin is not higher than the predetermined value by a fourth factor larger than the third factor; and
an inverse discrete Fourier transform executor that performs inverse discrete Fourier transform on the discrete Fourier series processed by the suppressor and obtains a discrete-time signal.
11. The noise suppression device according to claim 10 , wherein
the noise frequency bin comprises a plurality of noise frequency bins, and
the third factor is capable of taking an individual value for each noise frequency bin.
12. The noise suppression device according to claim 10 , wherein
the frequency bin other than the noise frequency bin comprises a plurality of frequency bins, and
the fourth factor is capable of taking an individual value for each frequency bin other than the noise frequency bin.
13. The noise suppression device according to claim 10 , wherein the center frequency bin is a frequency bin having the greatest amplitude near a DC component in the amplitude spectrum.
14. A reception device, comprising:
an antenna that receives an AM broadcast wave signal output from a broadcast station;
an amplifier that amplifies the AM broadcast wave signal received from the antenna;
an analog-to-digital converter that converts the amplified AM broadcast wave signal into a digital signal;
a quadrature demodulator that quadrature-demodulates the digital signal to generate a baseband signal;
a noise suppression device according to claim 10 that suppresses a noise included in the baseband signal to generate a discrete-time signal; and
a demodulator that demodulates the discrete-time signal into a sound signal.
15. A noise suppression method for suppressing a noise included in a received signal having an upper sideband signal and a lower sideband signal located symmetrically on a frequency axis with respect to a first angular frequency, the noise suppression method comprising:
generating a baseband signal by mixing the received signal with a complex sine wave having a predetermined angular frequency;
expanding the baseband signal into a discrete Fourier series;
calculating an amplitude spectrum of frequency bins of the baseband signal expanded into the discrete Fourier series;
detecting, as an asymmetric frequency bin, a frequency bin corresponding to an asymmetric component in the amplitude spectrum by evaluating symmetry of the amplitude spectrum with respect to a center frequency bin corresponding to the first angular frequency;
in the discrete Fourier series expanded from the baseband signal, multiplying a value corresponding to the asymmetric frequency bin by a first factor, and multiplying a value corresponding to a frequency bin other than the asymmetric frequency bin by a second factor larger than the first factor; and
performing inverse discrete Fourier transform on the discrete Fourier series in which the asymmetric component is suppressed and obtaining a discrete-time signal.
16. The noise suppression method according to claim 15 , further comprising:
comparing an amplitude of a center frequency bin corresponding to an angular frequency of a carrier wave signal included in the received signal and an amplitude of a frequency bin other than the center frequency bin, and detecting a noise frequency bin having an amplitude whose ratio to the amplitude of the center frequency bin is higher than a predetermined value;
in the discrete Fourier series expanded from the baseband signal, multiplying a value corresponding to the noise frequency bin by a third factor, and multiplying a value corresponding to a frequency bin having an amplitude whose ratio to the amplitude of the center frequency bin is not higher than the predetermined value by a fourth factor larger than the third factor; and
interpolating the discrete Fourier series in which a component corresponding to the noise frequency bin is suppressed into the discrete Fourier series in which the asymmetric component is suppressed, and calculating a new discrete Fourier series, wherein the discrete-time signal is obtained by performing inverse discrete Fourier transform on the new discrete Fourier series.
17. The noise suppression method according to claim 16 , wherein
the noise frequency bin comprises a plurality of noise frequency bins, and
the third factor is capable of taking an individual value for each noise frequency bin.
18. The noise suppression method according to claim 16 , wherein
the frequency bin other than the noise frequency bin comprises a plurality of frequency bins, and
the fourth factor is capable of taking an individual value for each frequency bin other than the noise frequency bin.
19. The noise suppression method according to claim 15 , wherein
the asymmetric frequency bin comprises a plurality of asymmetric frequency bins, and
the first factor is capable of taking an individual value for each asymmetric frequency bin.
20. The noise suppression method according to claim 15 , wherein
the frequency bin other than the asymmetric frequency bin comprises a plurality of frequency bins, and
the second factor is capable of taking an individual value for each frequency bin other than the asymmetric frequency bin.
21. The noise suppression method according to claim 15 , wherein the detecting of the asymmetric frequency bin comprises detecting the asymmetric frequency bin by evaluating the symmetry of the amplitude spectrum based on comparison between an amplitude of one frequency bin and the greatest amplitude among amplitudes of sequential frequency bins including a frequency bin located symmetrically to the one frequency bin with respect to the center frequency bin.
22. The noise suppression method according to claim 15 , wherein the center frequency bin is a frequency bin having the greatest amplitude near a DC component in the amplitude spectrum.
23. A reception method, comprising:
receiving an AM broadcast wave signal output from a broadcast station;
amplifying the AM broadcast wave signal;
converting the amplified AM broadcast wave signal into a digital signal;
generating a baseband signal by mixing the digital signal with a complex sine wave having a predetermined angular frequency;
suppressing a noise included in the baseband signal to generate a discrete-time signal with the noise suppression method according to claim 15 ; and
demodulating the discrete-time signal into a sound signal.
24. A noise suppression method for suppressing a noise included in a received signal having a carrier wave signal and a pair of sideband signals, the noise suppression method comprising:
generating a baseband signal by mixing the received signal with a complex sine wave having a predetermined angular frequency;
expanding the baseband signal into a discrete Fourier series;
calculating an amplitude spectrum of frequency bins of the baseband signal expanded into the discrete Fourier series;
comparing an amplitude of a center frequency bin corresponding to an angular frequency of the carrier wave signal and an amplitude of a frequency bin other than the center frequency bin, and detecting a noise frequency bin having an amplitude whose ratio to the amplitude of the center frequency bin is higher than a predetermined value;
in the discrete Fourier series expanded from the baseband signal, multiplying a value corresponding to the noise frequency bin by a third factor, and multiplying a value corresponding to a frequency bin having an amplitude whose ratio to the amplitude of the center frequency bin is not higher than the predetermined value by a fourth factor larger than the third factor; and
performing inverse discrete Fourier transform on the discrete Fourier series in which a component corresponding to the noise frequency bin is suppressed and obtaining a discrete-time signal.
25. The noise suppression method according to claim 24 , wherein
the noise frequency bin comprises a plurality of noise frequency bins, and
the third factor is capable of taking an individual value for each noise frequency bin.
26. The noise suppression method according to claim 24 , wherein
the frequency bin other than the noise frequency bin comprises a plurality of frequency bins, and
the fourth factor is capable of taking an individual value for each frequency bin other than the noise frequency bin.
27. The noise suppression method according to claim 24 , wherein the center frequency bin is a frequency bin having the greatest amplitude near a DC component in the amplitude spectrum.
28. A reception method, comprising:
receiving an AM broadcast wave signal output from a broadcast station;
amplifying the AM broadcast wave signal;
converting the amplified AM broadcast wave signal into a digital signal;
generating a baseband signal by mixing the digital signal with a complex sine wave having a predetermined angular frequency;
suppressing a noise included in the baseband signal to generate a discrete-time signal with the noise suppression method according to claim 24 ; and
demodulating the discrete-time signal into a sound signal.