Comfort noise generator using modified Doblinger noise estimate
A background noise estimate based upon a modified Doblinger noise estimate is used for modulating the output of a pseudo-random phase spectrum generator to produce the comfort noise. The circuit for estimating noise includes a smoothing filter having a slower time constant for updating the noise estimate during noise than during speech. Comfort noise is smoothly inserted by basing the amount of comfort noise on the amount of noise suppression. A discrete inverse Fourier transform converts the comfort noise back to the time domain and overlapping windows eliminate artifacts that may have been produced during processing.
1. In a telephone having an audio processing circuit including an analysis circuit for dividing a audio signal into a plurality of frames, each frame containing a plurality of samples, a circuit for calculating an estimate of background noise, a circuit for generating comfort noise, and means for combining the comfort noise with a processed audio signal, the improvement comprising:
said circuit for calculating an estimate includes a smoothing filter having a long time constant when the estimate increases from frame to frame; and
said circuit for generating comfort noise includes
a circuit for calculating the gain of the comfort noise in accordance with said estimate;
a generator producing a pseudo-random phase spectrum; and
a multiplier for adjusting the gain of said spectrum to produce comfort noise that is spectrally matched to said background noise.
2. The telephone as set forth in claim 1 wherein said smoothing filter includes a first-order exponential averaging smoothing filter.
3. The telephone as set forth in claim 1 and further including a circuit for limiting spectral gain in said circuit for calculating a noise estimate.
4. The telephone as set forth in claim 3 and further including a speech detector, wherein the spectral gain limit is higher when speech is detected than when speech is not detected.
5. The telephone as set forth in claim 1 wherein said generator calculates transcendental functions.
6. The telephone as set forth in claim 1 wherein said generator calculates arithmetically.
7. The telephone as set forth in claim 1 wherein said circuit for calculating the gain of the comfort noise adjusts gain inversely proportional to a noise suppression factor.
8. The telephone as set forth in claim 1 wherein said comfort noise is generated in frequency domain and further including an inverse discrete Fourier transform for converting the comfort noise to time domain.
9. The telephone as set forth in claim 1 wherein said circuit for calculating an estimate includes a comparator for comparing the noise power estimate from one frame with the noise power estimate from another frame.
10. The telephone as set forth in claim 1 wherein said circuit for calculating an estimate includes a comparator for comparing the ratio of the noise power estimate from the current frame to the noise power estimate from the previous frame with a threshold.