System for improving speech quality and intelligibility

• US 20060247922A1
• Filed: 04/20/2005
• Published: 11/02/2006
• Est. Priority Date: 04/20/2005
• Status: Active Grant

• Current Assignee: Malikie Innovations Limited (Key Patent Innovations Limited)
• Original Assignee: QNX Software Systems Company (Blackberry Limited)
• Inventors: Li, Xueman, Hetherington, Phillip
• Granted Patent: US 7,813,931 B2
• Time in Patent Office: Days
• US Class Current: 704/208
• CPC Class Codes: G10L 21/02 Speech enhancement, e.g. noise reduction or echo cancellation reducing echo effects in line transmission systems H04B3/20; echo suppression in hands-free telephones H04M9/08
• CPC Class Codes: G10L 21/0264 characterised by the type of parameter measurement, e.g. correlation techniques, zero crossing techniques or predictive techniques

Abstract

A system and method are provided for improving the quality and intelligibility of speech signals. The system and method apply frequency compression to the higher frequency components of speech signals while leaving lower frequency components substantially unchanged. This preserves higher frequency information related to consonants which is typically lost to filtering and bandpass constraints. This information is preserved without significantly altering the fundamental pitch of the speech signal so that when the speech signal is reproduced its overall tone qualities are preserved. The system and method further apply frequency expansion to speech signals. Like the compression, only the upper frequencies of a received speech signal are expanded. When the frequency expansion is applied to a speech signal that has been compressed according to the invention, the speech signal is substantially returned to its pre-compressed state. However, frequency compression according to the invention provides improved intelligibility even when the speech signal is not subsequently re-expanded. Likewise, speech signals may be expanded even though the original signal was not compressed, without significant degradation of the speech signal quality. Thus, a transmitter may include the system for applying high frequency compression without regard to whether a receiver will be capable of re-expanding the signal. Likewise, a receiver may expand a received speech signal without regard to whether the signal was previously compressed.

Claims

1. A method of improving intelligibility of a speech signal comprising:

identifying a frequency passband having a passband lower frequency limit and a passband upper frequency limit;

defining a threshold frequency within the passband;

receiving a speech signal having a frequency spectrum, a highest frequency component of which is greater than the passband upper frequency limit;

compressing a portion of the speech signal spectrum in a first frequency range between the threshold frequency and the highest frequency component of the speech signal into a frequency range between the threshold frequency and the passband upper frequency limit.

2. The method of improving the intelligibility of a speech signal of claim 1 further comprising:

transmitting the compressed speech signal;

receiving the compressed speech signal; and

audibly reproducing the compressed speech signal.

3. The method of improving intelligibility of a speech signal of claim 1 further comprising:

transmitting the compressed speech signal;

receiving the compressed speech signal; and

expanding the received compressed speech signal.

4. The method of improving intelligibility of a speech signal of claim 1 further comprising:

normalizing the peak power of compressed speech signal.

5. The method of improving intelligibility of a speech signal of claim 4 further comprising:

transmitting the compressed normalized speech signal;

receiving the compressed normalized speech signal; and

expanding the received compressed normalized signal.

6. The method of improving intelligibility of a speech signal of claim 5 further comprising re-normalizing the expanded received speech signal, and audibly reproducing the re-normalized expanded speech signal.

7. The method of improving intelligibility of a speech signal of claim 5 further comprising audibly reproducing the expanded received signal.

8. The method of improving intelligibility of a speech signal of claim 1 wherein compressing a portion of the speech signal spectrum comprises applying linear frequency compression above the threshold frequency.

9. The method of improving intelligibility of a speech signal of claim 1 wherein compressing a portion of the speech signal spectrum comprises applying non-linear frequency compression above the threshold frequency.

10. The method of improving intelligibility of a speech signal of claim 1 wherein compressing a portion of the speech signal spectrum comprises applying non-linear frequency compression throughout the spectrum of the speech signal wherein a compression function employed for performing the compression is selected such that minimal compression is applied in lower frequency and increasing compression is applied in higher frequency.

11. A method of improving intelligibility of a speech signal comprising:

receiving a passband limited signal having a lower frequency limit and an upper frequency limit;

defining a threshold frequency within the passband of the received speech signal;

defining an expanded signal upper frequency limit;

performing a frequency expansion on a portion of the received speech signal such that frequency components of the received speech signal in the frequency range between the threshold frequency and the upper frequency limit of the passband are expanded to fill the frequency range between the threshold frequency and the expanded signal upper frequency limit; and

audibly reproducing the expanded speech signal.

12. The method of improving intelligibility of a speech signal according to claim 11 further comprising normalizing the peak power of the expanded signal.

13. The method of improving intelligibility of a speech signal according to claim 11 wherein the frequency expansion comprises a linear expansion beginning at the threshold frequency.

14. The method of improving intelligibility of a speech signal according to claim 11 wherein the frequency expansion comprises a non-linear expansion beginning at the threshold frequency.

15. The method of improving intelligibility of a speech signal according to claim 11 wherein the frequency expansion comprises a non-linear expansion across the entire spectrum of the received signal wherein an expansion function employed for implementing the expansion applies little or no expansion to lower frequency portions of the received signal, and applying increasing expansion to higher frequency portions of the received signal.

16. A system for improving the intelligibility of a transmitted speech signal, the system comprising:

a high frequency encoder adapted to compress high frequency components of a speech signal which are outside a passband of a communication channel into a frequency range within the passband of the communication channel, while leaving lower frequency components of the speech signal substantially unchanged; and

a transmitter for transmitting speech signals compressed by the high frequency encoder over the communication channel.

17. The system of claim 16 wherein the high frequency encoder comprises:

a time-domain-to-frequency-domain transform for transforming a time domain speech signal to a frequency domain signal;

a high frequency compressor for compressing the high frequency components of the frequency domain signal; and

a frequency-domain-to-time-domain transform for transforming the compressed speech signal output from the high frequency compressor into a time-domain signal.

19. The system of claim 16 further comprising:

a receiver for receiving speech signals over the communication channel; and

a bandwidth extender adapted to expand frequency components of received signals in an upper portion of the communication channel passband into a frequency range extending beyond an upper limit of the passband, while leaving frequency components of the received signal in a lower portion of the passband substantially unchanged.

20. The system of claim 19 wherein the bandwidth expander comprises:

an upsampler for increasing the sample rate of a received signal;

a time-domain-to-frequency-domain transform for transforming the upsampled signal into the frequency domain;

a spectral envelope extender including an frequency demapping matrix for mapping frequency components of the unsampled frequency domain signal from frequency bins in the unextended frequency range to larger frequency bins in the extended frequency range;

an excitation signal generator for generating harmonic and phase information from the upsampled frequency domain signal;

a combiner for combining the output of the spectral envelope extender and the excitation signal generator; and

a time-domain-to-frequency-domain transform for transforming the combined signal into the time-domain.

18. The system of claim 18 wherein the high frequency compressor comprises:

a high pass filter and a low pass filter for separating the high frequency components of the speech signal from the low frequency components of the speech signal;

a frequency mapping matrix for mapping the high frequency components of the speech signal from frequency bins in the uncompressed frequency domain to frequency bins in the compressed frequency range; and

a combiner for combining the compressed high frequency components of the speech signal with the low frequency components of the speech signal.

21. A high frequency encoder comprising:

an A/D converter for converting an analog speech signal to a digital time-domain speech signal;

a time-domain-to-frequency-domain transform for transforming the time-domain speech signal to a frequency-domain speech signal;

a high frequency compressor for spectrally transposing high frequency components of the frequency-domain speech signal to lower frequencies to for a compressed frequency-domain speech signal;

a frequency-domain-to-time-domain transform for transforming the compressed frequency domain speech signal into compressed time-domain speech signal; and

a down sampler for sampling the compressed time-domain signal at a sample rate appropriate for the highest frequency of the compressed time-domain speech signal.

22. The high frequency encoder of claim 21 wherein the high frequency compressor comprises a highpass filter for extracting high frequency components of the frequency domain speech signal and a frequency mapping matrix for mapping the high frequency components of the frequency domain speech signal to lower frequencies, to which the high frequency components are spectrally transposed.

23. The high frequency encoder of claim 21 wherein the high frequency compressor further comprises a low pass filter for extracting low frequency components of the frequency domain speech signal, and a combiner for combining the extracted low frequency components of the frequency domain speech signal with the high frequency components of the frequency-domain speech signal spectrally transposed to lower frequencies.

24. A method of improving intelligibility of a speech signal comprising:

identifying a frequency passband;

receiving a speech signal having a frequency spectrum, a highest frequency component of which is greater than an upper frequency limit of the passband;

applying non-linear frequency compression throughout the frequency spectrum of the speech signal by applying a frequency compression function in which minimal compression is applied to a lower frequency range of the speech signal spectrum and significantly greater compression is applied to an upper frequency range of the speech signal spectrum such that a compressed speech signal spectrum is within the passband.