Method and apparatus for enhancing loudness of an audio signal
Human hearing perceives loudness based on critical bands corresponding to different frequency ranges. As a sound's frequency spectrum increases beyond a critical band into a previously unexcited critical band, the perception is that the sound has increased in loudness. To take advantage of this principle, a filter is applied to a speech signal so as to expand the formant bandwidths of formants in the speech sample.
1. A method for increasing the perceived loudness of a speech signal, comprising:
receiving a vocoded speech signal;
recreating the speech signal from the vocoded speech signal, the speech signal having a plurality of formants and an energy, each formant having a natural bandwidth; and
filtering the speech signal to expand a bandwidth of each of the plurality of formants beyond their natural bandwidth without increasing the energy of the speech signal.
2. A method for increasing the perceived loudness of a speech signal as defined in claim 1 , wherein the speech signal is warped so as to expand formant bandwidths in a manner dependent on a frequency of the formant.
3. A method for increasing the perceived loudness of a speech signal as defined in claim 1 , wherein the filter is selectively applied when the speech signal has significant vowelic content.
4. A method for increasing the perceived loudness of a speech signal as defined in claim 3 , wherein the vowelic content is indicated by a spectral flatness measure of the speech signal.
5. An apparatus for increasing the loudness of a speech signal, comprising:
a demodulator for receiving a radio frequency signal and providing a vocoded speech signal from the radio frequency signal;
a vocoder coupled to the demodulator for recreating the speech signal from the vocoded speech signal, the speech signal, the speech signal having a plurality of formants and an energy, each formant having a natural bandwidth; and
a post filter coupled to the vocoder for filtering the speech signal to expand a bandwidth of each of the plurality of formants beyond their natural bandwidth without increasing the energy of the speech signal.