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Hello All,
I'm new to this forum, and am in search of a 3 pole active Low-Pass filter, apparently.
I am a videographer using Sony ECM 44b and 55B microphones and a Shure M367 mixer. I occasionally have to set up in conference rooms that are equipped with ultra-sonic motion detectors (used to turn out the lights when no one is there.) The signal emitted by these detectors (at 25 kHz) is picked up by my microphones and creating havoc with my recording. First; it looks like a very loud scream on the VU meter, and trips the limiter, muting the audio quality. It also is converted into a high-pitched whine by one of my recording decks. The main problem, though, is the VU meter/limiter.
I need to eliminate the signal between the microphone and the mixer. I tried a commercially available passive filter, but even two of them in line did not reduce the signal enough. The manufacturer suggested I need a 3 pole active low-pass filter, which they do not make.
I am willing to build such a filter myself, and I even have found some designs or plans that might do the trick - but I can't understand them at all. They are always written way above my level, and use diagrams written in code that I can't decipher.
So; I need help understanding and picking a design, or creating one, and reducing said design to diagrams and instructions I can understand and build.
Can anybody here help me with my project? Or point me to some information I can understand?
Any help would be much appreciated.
-Hjourzt
Follow Ups:
I thought ultrasonic things ran at higher frequencies like 38 to 40 kHz. 25kHz might be audible by younger women.
Maybe you can borrow an active crossover and try that first. Or order one from Marchand Electronics; they can configure the board for any desired frequency.
But, why not just try recording at 32ks/s rather than 44.1, 48k or higher. The recorder should have a brick wall filter for those higher frequencies.
On the other hand, maybe the problem occurs right in the microphone. Try different mics, maybe dynamic types with more limited bandwidth.
Hello Hjourzt,
have you had progress on your issue? I am curious, in recent projects that I have done (data) on commercial sites the lighting controls you were talking about are becoming popular.
There are DMM's to be had for about 100.00 bucks which will count frequency up to 500k - if you own the equipment you are using you could conceivably go back to a site and make a measurement.
I for one would be curious - got out of ENG years ago and do not have an easy way to replicate your situation.
When flying commercial you want the aircraft to be maintained and operated by sound standards.
I am curious - having done my fair share of ENG work, I have never encountered the problem you describe. How do you arrive at the 25K figure?
Is this a statement from the manufacturer of said motion detectors?
In any case you should look into other causes for these phenomena, I remain skeptical that the detectors are the culprit. I fail to see how the gear you describe - and which I also have extensively used - manages to somehow convert these frequencies into audible range at playback.
Try out the www.gearslutz.com forum - these folks will point you in the right direction for your filter.
When flying commercial you want the aircraft to be maintained and operated by sound standards.
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