Bandwidth Extension of Speech in Telephone Systems
Telephone speech signals are usually very degraded in quality. Part of this degradation is due to the limited bandwidth used in the telephone systems. In most systems frequencies lower than 250 Hz are cut and bandwidth only extends to frequencies of 4 or 8 kHz. Using filtering and waveshaping low and high frequency response can be extended.
By low pass filtering the lowest octave and half-wave rectifying a waveform is created with a fundamental half of the original frequency. Due to the discontinuity in the waveform low pass filtering is needed to filter all harmonics. Using such a subharmonic synthesizer the essential frequency band between 125 - 250 Hz is recreated, adding weight to the signal.
To extend the high frequency bandwidth, we can isolate the top octave using high pass filtering and then generating harmonics of this. The generation of harmonics can be done through a simple full wave rectification, which is computationally cheap and not amplitude-dependent. As an alternative single-sideband modulation can be used, giving precise control over the number and amplitude of the harmonics. In theory envelope estimation can be used to extract the original high frequency envelope and regenerating high frequencies using a noise source. The sparse information available in the small bandwidth will probably be too limited to extract a proper envelope.
Read more about this topic: Bandwidth Extension
Famous quotes containing the words extension, speech, telephone and/or systems:
“The desert is a natural extension of the inner silence of the body. If humanity’s language, technology, and buildings are an extension of its constructive faculties, the desert alone is an extension of its capacity for absence, the ideal schema of humanity’s disappearance.”
—Jean Baudrillard (b. 1929)
“Language is legislation, speech is its code. We do not see the power which is in speech because we forget that all speech is a classification, and that all classifications are oppressive.”
—Roland Barthes (1915–1980)
“It’s a hard feeling when everyone’s in a hurry to talk to somebody else, but not to talk to you. Sometimes you get a feeling of need to talk to somebody. Somebody who wants to listen to you other than “Why didn’t you get me the right number?””
—Heather Lamb, U.S. telephone operator. As quoted in Working, book 2, by Studs Terkel (1973)
“People stress the violence. That’s the smallest part of it. Football is brutal only from a distance. In the middle of it there’s a calm, a tranquility. The players accept pain. There’s a sense of order even at the end of a running play with bodies stewn everywhere. When the systems interlock, there’s a satisfaction to the game that can’t be duplicated. There’s a harmony.”
—Don Delillo (b. 1926)