System and apparatus for speech communication and speech recognition

• US 20040193411A1
• Filed: 02/26/2004
• Published: 09/30/2004
• Est. Priority Date: 09/12/2001
• Status: Active Grant

• Current Assignee: Bitwave Pte Ltd.
• Original Assignee: Bitwave Pte Ltd.
• Inventors: Loh, Kok Heng, Hui, Siew Kok, Lau, Yean Ming
• Granted Patent: US 7,346,175 B2
• Time in Patent Office: Days
• US Class Current: 704/233
• CPC Class Codes: G10L 15/20 Speech recognition techniques specially adapted for robustness in adverse environments, e.g. in noise, of stress induced speech G10L21/02 takes precedence
• CPC Class Codes: G10L 2021/02161 Number of inputs available containing the signal or the noise to be suppressed
• CPC Class Codes: G10L 2021/02168 the estimation exclusively taking place during speech pauses
• CPC Class Codes: G10L 21/0208 Noise filtering
• CPC Class Codes: H04M 1/271 controlled by voice recognition
• CPC Class Codes: H04M 1/6033 for providing handsfree use or a loudspeaker mode in telephone sets arrangements for preventing acoustic echo H04M9/08

Abstract

A headset system is proposed including a headset unit to be worn by a user and having two or more microphones, and a base unit in wireless communication with the headset. Signals received from the microphones are processed using a first adaptive filter to enhance a target signal, and then divided and supplied to a second adaptive filter arranged to reduce interference signals and a third filter arranged to reduce noise. The outputs of the second and third filters are combined, and are be subject to further a processing in the frequency domain. The results are transmitted to a speech recognition engine.

Claims

1. A headset system including a base unit and a headset unit to be worn by a user and having a plurality of microphones, the headset unit and base unit being in mutual wireless communication, and at least one of the base unit and the headset unit having digital signal processing means arranged to perform signal processing in the time domain on audio signals generated by the microphones, the digital signal processing means including at least one adaptive filter to enhance a wanted signal in the audio signals and at least one adaptive filter to reduce an unwanted signal in the audio signals.

2. A headset system according to claim 1 in which the base unit includes a cradle for holding the headset unit.

3. A headset system according to claim 1 in which the headset unit is associated with a loudspeaker operable by the headset unit for generating audio signals to the user.

4. A headset system according to claim 1 in which the digital signal processing means includes:

a first adaptive filter arranged to enhance a target signal in the digital signals, and

a second adaptive filter and to a third adaptive filter each receiving the output of the first adaptive filter,

the second filter being arranged to suppress unwanted interference signals, and the third filter being arranged to suppress noise signals.

5. A headset system according to claim 4 which the digital processing means is adapted to combine the outputs of the second and third adaptive filters, convert to the frequency domain and perform further processing in the frequency domain.

6. A headset system according to claim 5 in which an output S_i(t) of the second filter and an output S_n(t) of the third filter are linearly combined using weighting factors to derive two interference signals, a first of the interference signals Ic being subtracted from the output of the first filter, and a second of the interference signals Is being converted into the frequency domain.

7. A headset system according to claim 4 in which the second and third filter are not adapted if it is determined that a target signal is present.

8. A headset system according to claim 4 in which the second filter is not updated if it is determined that an interference signal is not present.

9. A headset system according to claim 7 comprising the step of at intervals determining signal energy, and deriving at least one noise threshold from a plurality of values of the signal energy, said determination including determining whether a further signal energy is above the noise threshold.

10. A headset system according to claim 9 in which the derivation of said noise threshold includes using the plurality of signal energy to derive a histogram representing the statistical frequencies of signal energy values in each of a number of bands, and deriving the noise threshold from a signal energy value Emax associated with the band having the highest histogram value.

11. A headset system according to claim 4 to 10 in which the digital signal processing means comprises a fourth adaptive filter for determining the direction of arrival of the target signal.

12. A headset system according to claim 11 in which the weights of the fourth adaptive filter are updated including repeatedly performing an update process which attenuates each existing weight value by a forgetting factor α

.

13. A headset system according to claim 11, in which the digital signal processing means is adapted to determine a ratio P_k indicating the ratio of the highest central weight value A of the fourth adaptive filter to the sum of A and the highest peripheral weight value B, the digital signal processing means only adapting the first filter if the ratio P_k is above a given value T_Pk1.

14. A headset system according to claim 13 in which, following an adaptation of the first filter, the digital signal processing means calculates a new value P_k2 of the ratio, determines whether the value of P_k2 is below the previous maximum value of P_k2 and below a threshold T_Pk, and if so restores at least one of the first, second and third filters to its previous state.

15. A headset system according to claim 13 when dependent on claim 8 in which the determination that an interference signal is not present includes a determination that the value of said ratio is below a threshold T_Pk2.

16. A headset system according to claim 4 in which the weights of the second filter are adapted by a weight updating factor μ

which varies inversely with an error output e_c1 of the second filter.

17. A headset system according to claim 5 in which the combined signals are transformed into two frequency domain signals which are a desired signal S_f and an interference signal I_f, S_f and I_f are transformed into respective modified spectra P_s and P_i, the modified spectra are warped into respective Bark spectra Bs and Bi.

18. A headset system according to claim 17 in which, prior to said warping, frequency scanning is applied to the modified spectra Ps and Pi, and peaks which are found to be common to both are attenuated in Pi.

19. A headset system according to claim 17 in which a ratio is derived of the sum of the values of B_s over the Bark critical bands up to the voice band upper cutoff, and the sum of the values of B_s over the Bark critical bands at and above the unvoiced bank lower cutoff.

20. A headset system according to claim 16 in which the ratio is above a given threshold, the values of B_s above the unvoiced band lower threshold are amplified.

21. A headset system according to claim 1 further including

a speech recognition engine receiving the output of the digital signal processing means.

22. A headset system according to claim 18 in which the speech recognition engine receives from the digital signal processing means information indicating any one or more of:

a) a direction of a target signal Td,

b) a signal Energy Er1,

c) a noise threshold used by the digital signal processing means,

d) an estimated SINR (target signal to interference ratio) and SNR (target signal to noise ratio),

e) a signal A′

indicating the presence of target speech,

f) a spectrum of processed speech signal Sout,

g) potential speech start and end points, and

h) an interference signal spectrum, If (step 570).

23. A headset system according to claim 1 in which the headset unit comprises two arms for location proximate the mouth of the user and for positioning to either side of the user'"'"'s head.

24. A headset system according to claim 23 in which the headset is suitable for positioning supported by the user'"'"'s shoulders with the arms embracing the user'"'"'s neck.

25. A headset system according to claim 23 in which at least one microphone is provided on a free end of each of the arms.

26. A headset unit for use in the headset system of claim 1.

27. A method of processing signals received from an array of sensors comprising the steps of sampling and digitising the received signals and processing the digitally converted signals, the processing including:

filtering the digital signals using a first adaptive filter arranged to enhance a target signal in the digital signals,

transmitting the output of the first adaptive filter to a second adaptive filter and to a third adaptive filter, the second filter being arranged to suppress unwanted interference signals, and the third filter being arranged to suppress noise signals; and

combining the outputs of the second and third filters.

28. Signal processing apparatus arranged to carry out a method according to claim 27.

29. A microphone headset comprising first and second microphones disposed at respective ends of a support, the support being adapted to be worn around the neck or head of a user.