System for improving speech quality and intelligibility with bandwidth compression/expansion
View Patent ↗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.
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 frequency passband that generally preserves a tone quality and pitch of a received speech signal;
receiving the speech signal, the 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 frequency 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; and
normalizing a peak power of the compressed portion of the speech signal by an amount that is based on an amount of compression in the frequency range between the threshold frequency and the passband upper frequency limit, where the act of normalizing comprises reducing the peak power by an amount proportional to an amount of compression in the 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:
transmitting the compressed normalized speech signal;
receiving the compressed normalized speech signal; and
expanding the received compressed normalized speech signal.
5. The method of improving intelligibility of a speech signal of claim 4 further comprising re-normalizing the expanded received compressed normalized speech signal, and audibly reproducing the re-normalized expanded speech signal.
6. The method of improving intelligibility of a speech signal of claim 4 further comprising audibly reproducing the expanded received compressed normalized speech signal.
7. The method of improving intelligibility of a speech signal of claim 1 where compressing a portion of the speech signal frequency spectrum comprises applying linear frequency compression above the threshold frequency.
8. The method of improving intelligibility of a speech signal of claim 1 where compressing a portion of the speech signal frequency spectrum comprises applying non-linear frequency compression above the threshold frequency.
9. The method of improving intelligibility of a speech signal of claim 1 where compressing a portion of the speech signal frequency spectrum comprises applying non-linear frequency compression throughout the spectrum of the speech signal where 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.
10. The method of improving intelligibility of a speech signal of claim 1 where the act of defining the threshold frequency comprises selecting the threshold frequency to be about 3000 Hz.
11. 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 for a compressed frequency-domain speech signal;
a frequency-domain-to-time-domain transform for transforming the compressed frequency-domain speech signal into a compressed time-domain speech signal; and
a down sampler for sampling the compressed time-domain signal at a sample rate appropriate for a highest frequency of the compressed time-domain speech signal;
where a peak power of the compressed frequency-domain speech signal or the compressed time-domain speech signal is normalized based on an amount of compression in the compressed frequency-domain speech signal, where the peak power of the compressed frequency-domain speech signal or the compressed time-domain speech signal is reduced by an amount proportional to an amount of compression in the high frequency components of the frequency-domain speech signal that were moved to lower frequencies.
12. The high frequency encoder of claim 11 where 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.
13. The high frequency encoder of claim 11 where 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.