Voice band expander and expansion method, and voice communication apparatus

• US 20090240489A1
• Filed: 03/05/2009
• Published: 09/24/2009
• Est. Priority Date: 03/19/2008
• Status: Active Grant

• Current Assignee: OKI Electric Industry Company Limited
• Original Assignee: OKI Electric Industry Company Limited
• Inventors: Aoyagi, Hiromi
• Granted Patent: US 8,396,703 B2
• Time in Patent Office: Days
• US Class Current: 704/205
• CPC Class Codes: G10L 21/038 using band spreading techniques

Abstract

A band-limited voice signal is processed to reduce its spectral envelope or harmonic structure, or both. The resulting reduced signal is moved into a frequency band above the upper limit frequency of the band-limited voice signal, and then combined with the band-limited voice signal to form a band expanded signal with improved quality and comprehensibility, free of unnatural high-frequency resonances and unnaturally strong high-frequency harmonics.

Claims

1. A voice band expander for expanding a frequency band of an input voice signal with a frequency spectrum limited to frequencies below an upper limit, the voice band expander comprising:

a reduced signal generator for generating, from the input voice signal, a reduced signal with a modified frequency spectrum in which at least one of a frequency spectral envelope and a harmonic structure of the input voice signal is reduced;

a band expanding signal generator for generating, from the reduced signal, a band expanding signal having a frequency spectrum in a band higher than the upper limit of the limited band of the input voice signal; and

a band expanded signal generator for combining the input voice signal and the band expanding signal and thereby forming a band expanded signal with an expanded frequency band.

2. The voice band expander of claim 1, wherein the reduced signal generator reduces the frequency spectral envelope of the input voice signal by suppressing formants.

3. The voice band expander of claim 1, wherein the reduced signal generator reduces the frequency spectral envelope of the input voice signal, the reduced signal generator further comprising:

a linear predictive coding (LPC) analyzer for carrying out an LPC analysis of the input voice signal; and

an LPC filter for reducing the frequency spectral envelope of the input voice signal by using LPC coefficients obtained by the LPC analyzer.

4. The voice band expander of claim 3, wherein the LPC filter multiplies the frequency spectrum of the input voice signal by a transfer function H_LPC(z) in a z-transfer domain, the transfer function H_LPC(z) being of the form

$H_{\mathrm{LPC}}$

( z ) = 1 - ∑

i 



α

i ·

a i ·

z - i

where i is an index integer, a_i is an LPC coefficient, α

is a positive constant not exceeding unity, and z is a complex variable.

5. The voice band expander of claim 1, wherein the reduced signal generator reduces the harmonic structure of the input voice signal, the reduced signal generator further comprising:

a pitch analyzer for determining a pitch and pitch intensity of the input voice signal; and

a pitch filter for reducing the harmonic structure of the input voice signal according to the pitch and pitch intensity obtained by the pitch analyzer.

6. The voice band expander of claim 5, wherein the pitch filter multiplies the input voice signal by a transfer function H_P(z) in a z-transfer domain, the transfer function H_P(z) being of the form

H_P(z)=1−

β

·

b·

z<sup>−

L</sup>

where β

is a positive constant not exceeding unity, b is the pitch intensity, L is a pitch period, and z is a complex variable.

7. The voice band expander of claim 1, wherein the reduced signal generator reduces both the frequency spectral envelope and harmonic structure of the input voice signal.

8. The voice band expander of claim 1, wherein the band expanding signal generator shifts the frequency spectrum of the reduced signal into the band higher than the upper limit of the limited band.

9. The voice band expander of claim 1, wherein the band expanding signal generator folds the frequency spectrum of the reduced signal over into the band higher than the upper limit of the limited band.

18. A voice communication apparatus receiving a band-limited voice signal, comprising the voice band expander of claim 1 for expanding the band of the received voice signal.

10. A method of expanding a frequency band of an input voice signal with a frequency spectrum limited to frequencies below an upper limit, the method comprising:

generating, from the input voice signal, a reduced signal with a reduced frequency spectrum in which at least one of a frequency spectral envelope and a harmonic structure of the input voice signal is reduced;

generating, from the reduced signal, a band expanding signal having a frequency spectrum in a band higher than the upper limit of the limited band of the input voice signal; and

combining the input voice signal and the band expanding signal and thereby forming a band expanded signal with an expanded frequency band.

11. The method of claim 10, wherein generating a reduced signal further comprises reducing the frequency spectral envelope of the input voice signal by suppressing formants.

12. The method of claim 10, wherein generating a reduced signal further comprises:

carrying out a linear predictive coding (LPC) analysis of the input voice signal; and

reducing the frequency spectral envelope of the input voice signal by using LPC coefficients obtained by the LPC analysis.

13. The method of claim 12, wherein reducing the frequency spectral envelope further comprises multiplying the input voice signal by a transfer function H_LPC(z) in a z-transform domain, the transfer function H_LPC(z) being of the form

$H_{\mathrm{LPC}}$

( z ) = 1 - ∑

i 



α

i ·

a i ·

z - i

where i is an index integer, a_i is an LPC coefficient, α

is a positive constant not exceeding unity, and z is a complex variable.

14. The method of claim 10, wherein generating the reduced signal further comprises:

determining a pitch and pitch intensity of the input voice signal; and

reducing the harmonic structure of the input voice signal according to the pitch and pitch intensity.

15. The method of claim 14, wherein reducing the harmonic structure further comprises multiplying the input voice signal by a transfer function H_P(z) in a z-transform domain, the transfer function H_P(z) being of the form

H_P(z)=1−

β

·

b·

z<sup>−

L</sup>

where β

is a positive constant not exceeding unity, b is the pitch intensity, L is a pitch period, and z is a complex variable.

16. The method of claim 10, wherein generating the reduced signal comprises reducing both the frequency spectral envelope and the harmonic structure of the input voice signal.

17. A tangible machine-readable medium storing a voice band expansion program to be executed by a computer to expand a frequency band of an input voice signal with a frequency spectrum limited to frequencies below an upper limit, the voice band expansion program including:

instructions for generating, from the input voice signal, a reduced signal with a reduced frequency spectrum in which at least one of a frequency spectral envelope and a harmonic structure of the input voice signal is reduced;

instructions for generating, from the reduced signal, a band expanding signal having a frequency spectrum in a band higher than the upper limit of the limited band of the input voice signal; and

instructions for combining the input voice signal and the band expanding signal and thereby forming a band expanded signal with an expanded frequency band.