Speech enhancement device
A speech enhancement system for the reduction of background noise comprises a time-to-frequency transformation unit to transform frames of time-domain samples of audio signals to the frequency domain, background noise reduction means to perform noise reduction in the frequency domain, and a frequency-to-time transformation unit to transform the noise reduced signals back to the time-domain. In the background noise reduction means for each frequency component a predicted background magnitude is calculated in response to the measured input magnitude from the time-to-frequency transformation unit and to the previously calculated background magnitude, whereupon for each of said frequency components the signal-to-noise ratio is calculated in response to the predicted background magnitude and to said measured input magnitude and the filter magnitude for said measured input magnitude in response to the signal-to-noise ratio. The speech enhancement device may be applied in speech coding systems, particularly P 2 CM coding systems.
1. Speech enhancement device for the reduction of background noise, comprising a time-to-frequency transformation unit to transform frames of time-domain samples of audio signals to the frequency domain, background noise reduction means to perform noise reduction in the frequency domain, and a frequency-to-time transformation unit to transform the noise reduced audio signals from the frequency domain to the time-domain, characterized in that the background noise reduction means comprise a background level update block to calculate, for each frequency component in a current frame of the audio signals, a predicted background magnitude B[k] in response to the measured input magnitude S[k] from the time-to-frequency transformation unit and in response to the previously calculated background magnitude B −1 [k], a signal-to-noise ratio block to calculate, for each of said frequency components, the signal-to-noise ratio SNR[k] in response to the predicted background magnitude B[k] and in response to said measured input magnitude S[k] and a filter update block to calculate, for each of said frequency components, the filter magnitude F[k] for said measured input magnitude S[k] in response to the signal-to-noise ratio SNR[k]; wherein the previously predicted background magnitude is updated according to the relation: B[k]=max{min{B′[k], B″[k]}, B min }, with B min the minimum allowed background level, while B′[k]=B −1 [k]. U[k] and B″[k]=(B′[k].D[k])+(|S[k]|.C.(1−D[k])), in which U[k] and D[k] are frequency dependent scaling factors and C a constant.
2. Speech enhancement device according to claim 1 , characterized in that the signal-to-noise ratio block comprises means to calculate the signal-to-noise ratio SNR[k] in response to the predicted background magnitude B[k] and to the measured input magnitude S[k] according to the relation: SNR[k]=|S[k]|/B[k].
3. Speech enhancement device according to claim 1 , characterized in that the filter update block comprises first means to calculate an internal filter value F′[k] and second means to derive therefrom the filter magnitude for the measured input magnitude, the first means comprising a memory unit to obtain a previously calculated internal filter magnitude F′ −1 [k] and processing means to update the previously calculated internal filter magnitude.
4. Speech enhancement device according to claim 3 , characterized in that the second means comprise comparator means for scaling and clipping the filter magnitude according to the relation: F[k]=max{min{H.F′[k], 1}, F min }, where H is a constant, F min a minimal filter value and F′[k] the internal filter value.
5. Speech encoder for a speech coding system, particularly for a P 2 CM audio coding system, provided with a speech enhancement device according to claim 1 .
6. Speech coding system, particularly a P 2 CM audio coding system, provided with a speech encoder having a speech enhancement device according to claim 1 .
7. P 2 CM audio coding system with a P 2 CM encoder comprising a pre-processor including spectral amplitude warping means and an ADPCM encoder, characterized in that the pre-processor is provided with a speech enhancement device according to claim 1 , the speech enhancement device having background noise reduction means, integrated in the spectral amplitude warping means of the pre-processor.