Amplifiers – With semiconductor amplifying device – Including gain control means
Patent
1988-04-11
1990-01-02
Mullins, James B.
Amplifiers
With semiconductor amplifying device
Including gain control means
330149, 330129, 381 94, H03G 332
Patent
active
048916056
DESCRIPTION:
BRIEF SUMMARY
This invention relates to adaptive gain control amplifiers for automatic enhancement of desired signals delivered to a receiver or receivers which are susceptible to external interference of a kind which may corrupt the received signal. More specifically the invention relates to an adaptive gain control amplifier and to a method of adaptive gain control which has particular utility in audio systems such as public address systems, telephones, automotive and domestic hi-fi and the like where there is a need to adjust the output volume of a loudspeaker automatically in response to variations in background noise. It will however, become readily apparent to those skilled in the art that there are many other applications of the invention such as for example in adaptive radio transmitter control and automatic contrast control in illumination.
Most known electronic circuits for signal enhancement are designed to minimize the signal degradation at the source, during electronic processing or during storage or playback (for example, "Dolby" (Registered Trade Mark) or "DNR" (Registered Trade Mark)). However, these techniques are not capable of signal enhancement directly at the receiver as is the case with this invention. Thus, the action of the adaptive gain control amplifier is complementary to that offered by the abovementioned circuits.
Most of the circuits designed specifically for acoustical systems such as public address, intercoms, telephones, automotive and domestic audio, where the desired signal enhancement is required to occur at the ear of the listener are based on two techniques. The first technique requires the corrupted signal to be decomposed into its desired and interference componets. Simpler systems use apriori separation based on assumed or empirically determined spectral (or temporal) characteristics of the signal or interference. Examples of this technique are found in automotive audio modules such as Hitachi ASLC (Automatic Sound Level Control) and Alpine 3015 (Preamplifier, 7 Band Computer Graphic Equalizer). These are specifically designed for automotive audio use and rely on measured road noise spectral characteristics, which renders them restrictive, arbitrary and generally low performance. Ideally, dynamic or adaptive decomposition of the corrupted signal into its desired and interference components would improve the performance of these systems. However, in most cases this decomposition or analytical separation is very complex and costly to implement.
This is because in acoustical applications sufficiently precise separation of the corrupted signal into its desired and interference components is rendered complicated or impossible because of diffraction and multi-path effects which preclude simple spatial separation as well as temporal and spatial distortions induced by transducer and medium non-linearities, time delays, reverberations, and other reflection, refraction and absorption phenomena resulting from material properties and discontinuities thereof. Imperfect separation renders some known enhancement system susceptible to instabilities (such as howling in public address systems) or reduces their effectiveness. Some known systems (for example Alpine) avert the first consequence by imposing severe restrictions on gain control thereby reducing it, for example, to a single discrete step adjustment. Another technique (noise cancellation), uses a controlled filter to subtract out the said interference, employing a least mean square algorithm to derive the control signal. This technique is normally used for signal enhancement at the source, but can be adapted for use on receivers. The amount of signal enhancement obtainable by this method is the highest known at this time. However, this perceived enhancement varies drastically depending on the listener's location, and can even result in actual degradation at some locations. In addition, the implementation of the algorithm and filter is expensive and the system is rather slow and requires a "learning period".
Accordingly, it is an object of
REFERENCES:
patent: 3814856 (1974-04-01), Dugan
patent: 3934084 (1976-01-01), Munson et al.
patent: 4079335 (1978-03-01), Doerig
patent: 4322579 (1982-03-01), Kleis et al.
patent: 4644292 (1987-02-01), Kunugh et al.
patent: 4677389 (1987-06-01), Op De Beek et al.
"Automatic Control of Speaker Output Compensates for Noisy Background", Electronics, Nov. 1972 edition, pp. 118 to 121.
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