Methods and arrangements in a telecommunications system

• US 6,424,942 B1
• Filed: 10/25/1999
• Issued: 07/23/2002
• Est. Priority Date: 10/26/1998
• Status: Expired due to Term

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Johansson, Ingemar, Mustel, Peter
• Primary Examiner(s): Dorvil, Richemond
• Application Number:
  US09/425,799
• Time in Patent Office: 1,002 Days
• Field of Search: 704/233, 704/219, 704/230, 704/226, 704/216, 704/217, 704/218, 704/225, 704/227, 704/228, 704/263, 704/262
• US Class Current: 704/233
• CPC Class Codes: G10L 19/012 Comfort noise or silence coding

Abstract

A method and arrangement for telecommunication comprises that it is detected (120) whether an incoming signal is speech or background noise, and encoding (100, 110) and transmitting parameters characterising the incoming signal. In or before (103) in the encoding of the background noise, parameters are produced, which represent background noise having increased low frequency components. Thus, the incoming signal can be subjected (103) to a frequency tilting operation. The degree of increasing the low frequency components is determined by the maximum long term correlation of the incoming signal. This method and arrangement provides a better generation of comfort noise, when the input signal comprises low frequency sinusoids, such as engine noise from cars and trams.

Claims

1. A method for processing an incoming signal to generate comfort noise, said method comprising the steps of:

encoding the incoming signal to generate said comfort noise;

before encoding, tilting a spectrum of the signal so that low frequency components are amplified when the incoming signal contains harmonic noise, in order to enhance the generated comfort noise; and

scaling the incoming signal with a gain factor to ensure that a perceived level remains constant despite the tilt operation.

2. The method of claim 1, further comprising the step of computing a maximum long term correlation of the incoming signal for use in determining the degree of amplification of the low frequency components.

3. The method of claim 1, further comprising the step of computing a spectral tilt factor for use when computing coefficients representing the comfort noise, said spectral tilt factor is dependant on an open loop LTP maximum long term correlation of the signal.

4. The method of claim 3, further comprising the steps of quantizing and transmitting the spectral tilt factor “

a”

by;

calculating the open loop long term predictor (LTP) maximum long term correlation;

filtering the signal using the spectral tilt factor “

a”

in a z-domain as given by $T<br><br>\left(z\right)=\frac{G}{1+{\mathrm{az}}^{-1}},$

wherein T(z) is a tilt filter and G is a gain factor;

producing short term predictor (STP)-coefficients from the filtered signal; and

transmitting the STP-coefficients.

5. The method of claim 4, wherein the STP-coefficients are quantized before the transmission to the receiver.

6. The method of claim 1, wherein said step of tilting further includes the steps of:

calculating a set of coefficients b₁, . . . , b_N for a synthesis filter (H(z));

calculating a coefficient “

a”

for a tilt filter (T(z));

calculating N+1 coefficients b″

₁, . . . , b″

_N+1 of a resulting filter having the form $1+\underset{n=1}{\overset{N+1}{\sum <br><br>}}<br><br>b_n^{\mathrm{″<br><br>}}<br><br>z^{-n};<br><br>$

reducing an order of the resulting filter to produce N coefficients b′

₁, b′

_N; and

quantizing and transmitting the reduced number of coefficients b₁′

-b_N′

.

7. A method for telecommunication comprising the steps of:

detecting whether an incoming signal is speech or background noise;

subjecting the incoming signal to a tilting operation to generate comfort noise, said tilting operation including;

computing parameters “

a” and

“

G”

from a function {a, G}=F(C);

determining a long term correlation using the incoming signal;

computing a coefficient a′

using a maximum long term correlation (C);

smoothing the coefficient a′

;

tilting the incoming signal so that low frequencies are amplified when the background noise contains harmonic noise, and

scaling the signal with the gain “

G”

to ensure that a perceived level remains constant despite the tilt operation;

encoding and transmitting the background noise and

encoding and transmitting the speech.

8. The method of claim 7, wherein before the encoding of the background noise the incoming signal is subjected to a tilting operation in order to increase the low frequency components.

9. The method of claim 8, wherein a degree of reducing the low frequency components is determined by a maximum long term correlation of the incoming signal.

10. The method of claim 8, wherein the tilting operation implements the function $T$

(z)=G1+az-1,

where “

a”

is a tilt factor, which is calculated depending on a maximum long term correlation, T(z) is a tilt filter, and G is a gain factor.

11. The method of claim 10, wherein “

a”

is slowly varying between 0 and −

1 inclusive.

12. The method of claim 7, further comprising:

a long term predictor (LTP) analysis to produce a maximum long term correlation and a STP-analysis are made on the incoming signal; and

for background noise, parameters obtained in the STP-analysis are modified in accordance with a value of the maximum long term correlation.

13. The method of claim 12, wherein a tilt parameter is calculated from the maximum long term correlation and the STP-parameters and the tilt parameter are combined to form a new set of STP-parameters using a convolution operation of a filter corresponding to the tilt parameter and a filter corresponding to the STP-parameters.

14. The method of claim 7, wherein a′

=−

min(1,7c²;

0,

     9), a=0, 8a+0, 2a′

, G=1+0,7a.

15. The method of claim 7, wherein a′

=−

min(1, max(0,C−

0,3)/0,2)*0,7, a=0, 8a+0,2a′

, G=1+0,7a.

16. A system for processing an incoming signal to generate comfort noise, said system comprising:

a device for tilting a spectrum of the signal so that low frequencies are amplified when the incoming signal contains harmonic noise, in order to enhance the generated comfort noise; and

means for scaling the incoming signal with a gain factor to ensure that a perceived level remains constant despite the tilt operation.

17. The system of claim 16, wherein said device further including means for computing at least one coefficient to enable production of the comfort noise at a receiver end of a transmission line, said at least one coefficient is dependent on an amount of tonal characteristics of the signal.

18. The system of claim 16, wherein said device further including means for computing a spectral tilt factor for use when computing coefficients representing the comfort noise, said spectral tilt factor is dependent on an open loop LTP maximum long term correlation of the signal.

19. The system of claim 18, wherein said device is further operable to quantize and transmit the spectral tilt factor by using:

a speech coder for calculating the open loop LTP maximum long term correlation;

a tilt filter T(z) for filtering the signal in a z-domain which is $T<br><br>\left(z\right)=\frac{G}{1+{\mathrm{az}}^{-1}},$

wherein a is the tilt factor and G is a gain factor;

a decoder for determining STP-coefficients of a synthesis filter, said decoder having a form $H<br><br>\left(z\right)=\frac{1}{1+\underset{n-1}{\overset{N}{\sum <br><br>}}<br><br>b_n<br><br>z^{-n}};<br><br>$

and

a receiver operable to receive the coefficients b₁−

b_n from the decoder.

20. The system of claim 19, wherein said device further capable of quantizing the coefficients b₁-b_n before transmission to the receiver.

21. A telecommunication system comprising:

a detector for detecting whether an incoming signal is speech or background noise;

means for computing parameters “

a” and

“

G”

from a function {a, G}=F(C);

means for performing a long term correlation using the incoming signal;

means for computing a coefficient a′

using a maximum long term correlation (C);

means for smoothing the coefficient a′

;

means for tilting the income signal so that low frequencies are amplified when the background noise contains harmonic noise;

means for scaling the signal with the gain “

G”

to ensure that a perceived level remains constant despite the tilt operation; and

an encoder for encoding and transmitting the background noise for encoding and transmitting the speech.

22. The telecommunication system of claim 21, further comprising means for tilting the incoming signal to increase the low frequency components before the encoding of the background noise.

23. The telecommunication system of claim 22, further comprising means for determining a degree of reducing the low frequency components from a maximum long term correlation of the incoming signal.

24. The telecommunication system of claim 22, wherein the tilting means implements the function $T$

(z)=G1+az-1,

where “

a”

is a tilt factor, which is calculated depending on the maximum long term correlation.

25. The telecommunication system of claim 24, wherein “

a”

is slowly varying between 0 and −

1 inclusive.

26. The telecommunication system of claim 21, further comprising:

means for performing a long term predictor (LTP) analysis to produce a maximum long term correlation and for performing a STP-analysis on the incoming signal; and

means, which, for background noise, modifies the parameters obtained in the STP-analysis in accordance with a value of the maximum long term correlation.

27. The telecommunication system of claim 26, further comprising:

means for calculating a tilt parameter from the maximum long term correlation; and

means for combining the STP-parameters and the tilt parameter to form a new set of STP-parameters using a convolution operation of a filter corresponding to the tilt parameter and a filter corresponding to the STP-parameters.

28. The telecommunication system of claim 21, wherein a′

=−

min(1,7c², 0,

     9), a=0, 8a+0, 2a′

, G=1+0,7a.

29. The telecommunication system of claim 21, wherein a′

=−

min(1, max(0,C−

0,3)/0,2)*0,7, a=0