An electronic filter for any 60 hz interference.
The most common type of interference heard in recordings, public address systems, and cordless phones comes from faulty grounding or radio interference. Sometimes a hum and sometimes a buzz, it repeats 60 times a second because it is caused by the power grid or some device in it. Simple filters will remove a smooth 60 hz hum but pass a spike that repeats 60 times per second as a buzz. The constant frequency of 60 hz in the power grid means the interference also repeats with a frequency of 60 hz.
The buzz filter adds a signal to the input which is the inverse of anything repeating at 60 hz. it consists of 1) Computer memory sufficient to store at least one second of sound. 2) Enough CPU to perform the calculations described below in real time. 3) Digital - audio chips for IO.
Allocate 10 strings of memories each sufficient to hold 1/60 second of sound. Create an index pointer to the strings. Sum 10 consecutive 1/60 sec segments to initialize total string.
Wait for next 1/60 second cycle start.
Index to next string in memory array.
Recall string from indexed memory.
Subtract it from total string.
Recall input string from buffer.
Store it at indexed memory.
Add it to total string.
Divide total string by 10.
Invert total string.
Store into output buffer.
Return to first step.
The output buffer is the source of a signal which is the inverse of any repeated 60 hz sound. If this is added to the input signal through an amplifier the resulting output will be free of any signal which repeats 60 times per second.
Due to the averaging process white sound is added to the output. The amplitude of the white sound is proportional to the amplitude of the sound signal so it will not be heard in silent parts. The amplitude of the white noise can also be reduced by sampling a larger number of 1/60 second sound segments. Ten segments were used in the example and this gives a rise-decay time of 0.17 second. If 100 segments were sampled there would be a rise-decay time of 1.7 seconds.
The retail device might be as small and powered by the sound signal with audio in and out jacks the only visible features. Sealed chips in plastic with no moving parts are durable and cheap.
Buzz filter Technical notes:
Today the functions of a buzz filter require at least a desktop computer. Computers will certainly become small and fast enough to bring the buzz filter down to a reasonable price. More intelligent noise reduction computers are presently installed in cars to cancel repetitive noises made by the car by making inverted sounds through the car stereo speakers.
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