Electrical audio signal processing systems and devices – Including amplitude or volume control – Automatic
Reexamination Certificate
1998-04-15
2001-07-24
Isen, Forester W. (Department: 2644)
Electrical audio signal processing systems and devices
Including amplitude or volume control
Automatic
C381S106000, C381S120000
Reexamination Certificate
active
06266423
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to the field of signal processors. More particularly, the present invention pertains to the field of electroacoustic transducers, such as microphones, and the amplification of their output signals. Even more specifically, the present invention pertains to the field of automatic control of the output signal level of electroacoustic transducers.
2. Description of the Prior Art
In the field of electronic audio devices, the use of microphones usually includes microphone pre-amplifiers which are used to boost up the low output voltage of the microphone to a higher operating level. A problem occurs when the dynamic range of the microphone output exceeds the dynamic range of the associated pre-amplifier, a case which commonly occurs. For example, if the sound level near the microphone should exceed a certain maximum level, the output voltage of the microphone may remain undistorted but will be high enough to overload the pre-amplifier causing distortion of the pre-amplifier output signal. A typical case where this problem occurs is in live sound reinforcement where a performer may sing or speak very loudly, causing distortion. It would be desirable to have means to automatically prevent pre-amplifier overload whenever the microphone output would exceed the maximum input level of the pre-amplifier.
In prior art, there are mainly two methods used to deal with this problem. First, an operator can ride gain on the pre-amplifier and manually readjust the pre-amplifier gain to avoid overload as necessary in reaction to ongoing events. The principal disadvantage of this method is the need for a skilled operator dedicated to the duty.
A second approach to deal with the problem is to configure the pre-amplifier into two stages. In this case it is desirable to have a first stage with a relatively low gain first stage, which allows more input headroom. The second stage adds the required gain boost to make up for the reduced gain of the first stage, but also includes an automatic gain control (AGC) limiter function with a preselected threshold level to prevent overdriving subsequent circuitry. Whenever the microphone output signal rises above the threshold level, the limiter's gain is automatically reduced proportionately. In this way the pre-amplifier is prevented from clipping the output signal. This method is effective until the input stage finally reaches an overload level above which the limiter function can no longer prevent any overload distortion. In the sound studio and live recording sessions, the first and second stages just described are usually comprised of separate and independent units. The stages usually involve a separate microphone pre-amplifier stage, set to a relatively low gain and a separate adjustable limiter stage having the necessary make-up gain. A few microphone pre-amplifiers are commercially available which integrate the two stages. The principal disadvantage of this two-stage approach is a significant noise floor penalty, that is the thermal noise is amplified along with the signal, roughly on the order of the net input headroom improvement which is gained. It would be desirable to have means to extend the input dynamic range of a microphone pre-amplifier, through the use of some sort of limiter function, which does not cause a noise floor penalty.
At this point it may be useful to mention that it seems possible to construct a microphone pre-amplifier having electronically variable front-end gam which could be operated with an AGC limiter function, thus avoiding the need for a second stage limiter and the consequential noise floor penalty. However, this type of “automatically controllable” microphone pre-amplifier which may retain the high quality attributes of conventional top-grade pre-amplifiers has not yet been produced and seems unlikely to be practical with presently known technology.
SUMMARY OF THE INVENTION
The present invention solves the problem mentioned above without the need to automatically control the pre-amplifier gain, and thus eliminates any need for a pre-amplifier electronic gain control feature.
According to the present invention a microphone is provided with an output signal at its output terminals which are connected directly to the input of an audio pre-amplifier. A controllable variable impedance device is coupled to the output terminals of the microphone in order to load its output terminals with an impedance load. The variable device acts to combine its impedance with the output impedance of the microphone to form a shunt load impedance. The variable impedance device is in turn controlled by a threshold level detector which is coupled to the input of the pre-amplifier. By varying the shunt load impedance the microphone's output signal will automatically be maintained at a preselected level as it passes into the pre-amplifier's input circuit. In operation the microphone will transform various sounds, having different frequencies and amplitude, into a directly proportional electronic signal at its output terminals. This signal is immediately detected by the threshold level detector that will generate a control signal once the microphone's output signal has exceeded a preselected limit or threshold. The control signal is then coupled to a variable impedance device, which creates a variable impedance shunt load impedance across the microphone's output terminals. In effect, a negative feedback circuit is created that will respond to rises in the level of the microphone's output signal by producing an offsetting decrease in the total impedance across the microphone's terminals. In this way the output of the microphone and the input to the pre-amplifier will be automatically maintained within certain limits. Of course, it is a matter of design choice whether the feedback signal to the input of the detector comes from the input of the pre-amplifier or the output of the pre-amplifier.
In alternative embodiments, an impedance matching transformer may be inserted in the circuit at various points. It may be used as a step-up or step-down transformer or simply to match the various input impedances of different components.
It would be useful and desirable to have an automatic limiting device to avert and prevent the input overloading of a microphone pre-amplifier over a significant dynamic range above its normal overload threshold. The device should act smoothly as a limiter. Ideally, the device should also be relatively simple and low in cost, and should work satisfactorily with a wide variety of conventional microphones and microphone pre-amplifiers without significant modifications to either the microphone or the pre-amplifier. It would also be desirable for the automatically limiting device to introduce no appreciable noise penalties.
It is one objective of the present invention to provide a means of automatically averting the input overload of a microphone pre-amplifier over a significant margin of dynamic range above its normal input overload threshold.
It is another objective of the present invention to provide a means which functions smoothly as a limiter without creating sudden, discretely switched steps of gain or attenuation.
It is yet another objective of the present invention to function effectively with conventional microphones and microphone pre-amplifiers and be relatively simple and inexpensive to implement.
Although the present invention is herein described in relation to microphones and pre-amplifiers of the types commonly found in the audio field, it is anticipated by the inventor that the teachings of the invention should well apply to many other fields where various types of electroacoustic transducers are used, for example such as in Sonar and other ultrasonic equipment.
The novelty, usefulness and scope of the present invention will become more apparent through the following detailed description taken together with the drawings and the appended claims.
REFERENCES:
patent: 4112384 (1978-09-01), Buchberger
pate
Aphex Systems Ltd.
Chen Tony D.
Fong Jerry
Isen Forester W.
Pendleton Brian Tyrone
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