As the speech signal has some redundancy, some loss of information during transmitting is considered acceptable to provide the sufficient intelligibility. However, the allowed value of the transmitted signal loss depends significantly on ambient conditions. Because of different noise, distortions and reverberation interference, the total loss of information may be so considerable that it may lead to the impossibility of speech understanding, in other words, to decreasing of speech intelligibility level. Thus, the communication systems which provide the sufficient speech intelligibility in normal conditions may be inapplicable in case of the high-level ambient noise.
Overtones
It is commonly believed that the band of the main speech signal energy is quite narrow: about 500 Hz - 2 kHz. That is why the standard telephony systems are intended for the full operating frequency band within 300 Hz – 3,5 kHz. This band is considered sufficient for voice transmitting under normal conditions.In fact, the human voice contains a wider frequency spectrum.
