Electrical audio signal processing systems and devices – Including frequency control
Patent
1981-08-26
1984-06-26
Pellinen, A. D.
Electrical audio signal processing systems and devices
Including frequency control
381106, H04B 166
Patent
active
044570147
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to audio processing and modulation systems and, more particularly, to an improved signal processing system intended for maximizing the efficiency of line system transmission or transferring especially of audio frequency signals, as in telephony, for providing high modulation levels and for enhancing signal intelligibility and clarity while avoiding loss of audio dynamics.
In both long and short distance line signal transmission and reception of modulated signals, such as utilized in telephone and other transmission line systems utilizing any combination of various signal transmission modes, such as twisted pair, RF link, laser, fiber optics, and so forth, a major problem has always been to obtain a high level of dynamic amplitude while not only containing the full spectrum of speech harmonics but also keeping the audio bandwidth of the transmitted line signals as narrow as possible. Many modes of line transmission and types of audio or other signal modulation have been used where these matters are of great concern. Principal forms of modulation presently in use are AM, SSB, and FM. AM and FM utilize a constant carrier principal while SSB has a direct audio into power out relationship and is not a constant carrier mode of transmission when the carrier is suppressed, as is quite common.
In constant carrier FM mode, the dynamic content of the signal and the audio bandwidth of the signal are directly proportional to the amount of deviation allowed to be imposed on the carrier. E.g., in narrow band FM transmission line systems, an audio bandwidth of 3 kcs. may be imposed. As a result, dynamic harmonics of voice characteristics are restricted and lost.
In the constant carrier mode of amplitude modulation, dynamics are expressed in a direct relationship with the amplitude of the constant carrier and audio frequency bandwidth being theorectically limited only by the line frequency bandwidth available. In commercial line communication systems utilizing the amplitude modulation mode of transmission, the audio bandwidth usually is restricted to eliminate spurious AM modulated pulses resulting from ignition and other electrical noise being induced on the line. An AM mode now commonly used takes the form of pulse amplitude modulation (PAM), as in teletype and data transmission systems.
The more recent mode of SSB modulation is by far the most efficient form of transmitting an audio modulated high frequency line signal. The major drawback of this mode of modulation is the extremely restricted audio bandwidth imposed resulting in extreme losses of dynamic intelligibility. However, these losses are often traded off against the high efficiency of single side band line systems.
Relative to radio transmission, attributes of human speech of concern are dynamic amplitude and harmonic relationship. The latter is extremely important in identification intelligibility.
Dynamic amplitude can be defined as the varying level of audio received by a modulation stage in any mode of modulation. The human voice is made up by a complex structure of harmonics, the main bands of harmonics falling within a 3 kHz bandwidth. A speech band-pass frequency range commonly selected is 300 Hz to 3000 Hz, and all other harmonics are generally suppressed. However, these out-of-band harmonics define voice character and, thus, intelligibility. But the suppressed harmonics fall in such a wide spectrum that if the entire speech harmonic make-up were to be transmitted, a line transmission bandwidth of some 15 kHz would be required. With modern narrow band voice line transmission systems, this would become impossible.
Consequently, speech processing has been utilized heretofore. For example, the use of band-pass frequency filters in speech line transmission systems is very common. Although speech band-pass frequencies often vary, the pass band rarely exceeds 3 kHz. This type of processing is used chiefly with narrow band audio line transmission systems. It has the advantages of being not
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patent: 3729693 (1973-04-01), Dolby
patent: 3795876 (1974-03-01), Takahashi et al.
patent: 4025723 (1977-05-01), Blackledge
patent: 4061874 (1977-12-01), Fricke et al.
patent: 4249042 (1981-02-01), Orban
Metme Communications
Myers Randall P.
Pellinen A. D.
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