High-frequency bandwidth extension in the time domain
A system extends the high-frequency spectrum of a narrowband audio signal in the time domain. The system extends the harmonics of vowels by introducing a non linearity in a narrow band signal. Extended consonants are generated by a random-noise generator. The system differentiates the vowels from the consonants by exploiting predetermined features of a speech signal.
1. A system that extends the high-frequency spectrum of a narrowband audio signal in the time domain:
an interface configured to receive a narrowband audio signal;
a squaring circuit that squares a segment of the narrowband audio signal to extend harmonics of vowels by introducing a non linearity in the received narrowband audio signal in the time domain;
a random noise generator that generates consonants by introducing random-noise in the received narrowband audio signal in the time domain;
a plurality of filters that pass a portion of the frequencies on the non-linearity and the random noise;
a first amplifier that adjusts an envelope of the filtered portion of the random noise to an estimate of a high pass filtered version of the received narrowband audio signal; and
a second amplifier that adjusts an envelope of the filtered portion of the non-linearity to a level of an envelope of the high pass filtered version of the received narrowband audio signal.
2. The system of claim 1 , where the first amplifier adjusts the envelope of the filtered portion of the random noise to a variance of unity.
3. The system of claim 2 , where the envelope of the filtered potion of the random noise is adjusted to a variance of unity by a gain factor of an absolute value of the high pass filtered version of the received narrowband audio signal smoothed with a leaky integrator filter.
4. The system of claim 1 , further comprising a plurality of mixers that select a portion of an output from the first amplifier and a portion of an output from the second amplifier.
5. The system of claim 4 , further comprising a summing circuit that sums the portion of the output from the first amplifier and the portion of the output from the second amplifier to generate an extended portion of a high frequency signal.
6. The system of claim 5 , further comprising a second summing circuit that sums the extended portion of the high frequency signal with the received narrowband audio signal to generate a bandwidth extended signal.
7. The system of claim 6 , further comprising an adaptive filter configured to dampen a background noise detected in the bandwidth extended signal.
8. The system of claim 7 , where the adaptive filter comprises an estimating circuit that estimates a high frequency signal to noise ratio of a high pass filtered version of the received narrowband audio signal, and a scalar coefficients estimating circuit.
9. The system of claim 7 , further comprising an adaptive shaping filter configured to vary the spectral shape of the output of the adaptive filter configured to dampen a background noise detected in the bandwidth extended signal.
10. The system of claim 9 , where the adaptive shaping filter is configured to change a spectrum shape of the output of the adaptive filter configured to dampen a background noise detected in the bandwidth extended signal when a processed signal represents a consonant.
11. A method of extending a high-frequency spectrum of a narrowband signal, comprising:
receiving a narrowband signal at an interface;
evaluating a portion of the narrowband signal to determine a speech characteristic in that portion of the narrowband signal;
generating a high-frequency time domain spectrum based on the determined speech characteristic in the evaluated portion of the narrowband signal; and
combining the generated high-frequency time domain spectrum with the narrowband signal to create an extended signal,
where the high-frequency time domain spectrum comprises squaring the evaluated portion of the narrowband signal when the speech characteristic in the evaluated portion of the narrowband signal represents a vowel.
12. The method of claim 11 , further comprising adaptively passing selective frequencies of the extended signal to suppress a portion of a background noise in the extended signal.
13. The method of claim 12 , further comprising shape adjusting the extended signal.
14. The method of claim 11 , further comprising adjusting a magnitude of the high-frequency time domain spectrum before combining the high-frequency time domain spectrum with the narrowband signal.
15. A method of extending a high-frequency spectrum of a narrowband signal, comprising:
receiving a narrowband signal at an interface;
evaluating a portion of the narrowband signal to determine a speech characteristic in that portion of the narrowband signal;
generating a high-frequency time domain spectrum based on the determined speech characteristic in the evaluated portion of the narrowband signal; and
combining the generated high-frequency time domain spectrum with the narrowband signal to create an extended signal,
where the high-frequency time domain spectrum comprises a random generated signal when the speech characteristic in the evaluated portion of the narrowband signal represents a consonant.
16. The method of claim 15 , further comprising adaptively passing selective frequencies of the extended signal to suppress a portion of a background noise in the extended signal.
17. The method of claim 16 , further comprising shape adjusting the extended signal.
18. The method of claim 15 , further comprising adjusting a magnitude of the high-frequency time domain spectrum before combining the high-frequency time domain spectrum with the narrowband signal.