Amplifiers – Hum or noise or distortion bucking introduced into signal...
Patent
1986-06-20
1988-03-22
LaRoche, Eugene R.
Amplifiers
Hum or noise or distortion bucking introduced into signal...
328165, H03F 126
Patent
active
047331933
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an arrangement for eliminating signal hum, and more specifically to an arrangement which includes a first signal input through which the signal is fed into the arrangement, a second signal input for feeding into the arrangement a hum signal of the same frequency as the hum frequency, a delay circuit having an input which is connected to the first signal input and an output which is connected to a subtraction circuit, which has a second input connected to the first signal input and an output connected to the output of the device, and a synchronizing circuit which is connected between the second signal input and a control input on the delay circuit.
BACKGROUND PRIOR ART
Disturbances in the form of quasi-stationary tones having an harmonic spectrum, such as hum for example, often create problems. An example of one area in which such problems arise is the field of electronic music. Many electro-mechanical musical instruments, inter alia electric guitars, have magnetic pick-ups which readily capture the magnetic hum-field present in the vicinity of mains supply cables and mains transformers. Some musical instruments have poor mains components, which also results in humming tones from loudspeakers. The cables in the loudspeaker systems of auditoriums are also liable to cause hum in the mains network.
Another area in which such problems arise is that of medical technology, in which extremely weak electric signals from the human body are measured, such as in electrocardiograms for example. Disturbances in the form of mains hum also occur when transmitting electric signals over long distances; in systems which incorporate sensitive transducers; and applications which utilize extremely low voltages or current strengths.
Experience has shown that there are many different ways of attempting to eliminate hum disturbances of the aforesaid kind.
One method in this respect is to remove the source of disturbance, or to rotate the source so as to minimize the hum transmission coupling.
The electrical system may be screened and/or earthed more effectively.
When the signals are transmitted through cables, balanced cables may be used.
Hum which is caused by poor rectifiers in the instrument can be eliminated by re-designing the mains components of the instrument.
The strongest harmonic components of the hum can be cutoff with the aid of high-pass filters
Despite these many methods, however, it can still be difficult to eliminate hum to a satisfactory extent. For example, in the case of a guitarist who constantly moves around the stage, it is practically impossible to eliminate hum transmission effectively. In the case of loudspeakers in auditoriums, the cost of changing the wiring system can be very high. Balanced cables will only reduce hum when used together with balanced signal sources and balanced receiver inputs, which impairs flexibility. The cost of re-designing the mains components of an instrument can also be extremely high. In the case of high-pass filtration, the useful signal is also affected and when the hum is rich in harmonics, high order harmonic components are not eliminated.
The object of the present invention is to provide an arrangement with which these and other drawbacks can be overcome.
DISCLOSURE OF THE INVENTION
The delay circuit of the arrangement according to the invention has positive feedback, and means are provided at some location in the feedback loop for limiting the amplitude, the first derivative and optionally also higher order derivatives of the feedback signal.
The arrangement is also constructed so that the strongest hum which can conceivably occur on the signal input of the arrangement is filtered precisely without distortion, and has a relatively wide bandwidth, so that a useful signal can readily overmodulate the same, although the distortion which occurs will be very slight, due to the limitation in the feedback. When the useful signal ceases, the arrangement will rapidly build-up to the curve form of the hum, due to the wide band
REFERENCES:
patent: 4165492 (1979-08-01), Motola et al.
patent: 4328591 (1982-05-01), Baghdady
patent: 4379994 (1983-04-01), Bauman
LaRoche Eugene R.
Mottola Steven J.
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