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High Pass Filter Design?

Arnak

Apr 1, 2011
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Hi Folks,

I intend to add a high pass filter to remove 200hz and below to a directional
dish that I am constructing for bird song recording.

I will be using 2 x panasonic WM61A mics.

I can see that an RC combination would do that but how do you calculate the
optimum values for the resistor / capacitor combination?

Making the assumption that you would want to reduce signal loss as much as
possible how do you work out the best values please?

Martin
 

duke37

Jan 9, 2011
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Noswaith dda
The rolloff frequency will be R.C., say 500k and 100nF. The actual values will depend on the output impedace of the microphones and the input impedance of the amplifiers. The rolloff will be very gentle with just a RC filter and it may be better to make a pair of active high pass filters using op-amps, the more complex, the flatter response can be and the steeper cutoff.
 

Arnak

Apr 1, 2011
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High pass filter

Hi Duke,

Thanks for that info that will be enough to get me started.:D

I will consider an op amp filter as the next stage.

Just out of interest, what formula do you use to determine the component values?

Martin
 

duke37

Jan 9, 2011
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Hi Martin,
I'm no expert but I have a book "Electronic Filter Design" by Aurther B. Williams. This gives normalised values for various filters, the most tolerant would be the Butterworth filter and I could calculate the values for, say, a three pole 200Hz high pass active filter (no inductors) if you wish. It will take a little while to get my brain into gear!
 
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Arnak

Apr 1, 2011
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High Pass Filter

Hi Duke,

No that's OK thanks, I was just interested in how the values were arrived at.

I have checked the net and found plenty of info on filters but not on how the results are calculated for a given application.

A book on filters seems to be the way to go.

Thanks,

Martin
 

duke37

Jan 9, 2011
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I have looked at a 3 pole Butterworth filter. I have used all capacitors at 100n and selected resistors to the nearest preferred value. Will try to attach the diagram. I have 'tried' it with 5SPICE.
 

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Arnak

Apr 1, 2011
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High Pass Filter.

Hi Duke,

Brilliant, thanks very much for that.:D

What op amp do you recommend with that circuit?

Martin
 

duke37

Jan 9, 2011
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Hi Martin,
I don't know the best op-amp, I am still working in the age of the 741. I expect there are better ones around now.
 

Arnak

Apr 1, 2011
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High Pass Filter.

Hi Duke,

741's will do fine for me thanks.:)

Martin
 

poor mystic

Apr 8, 2011
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Hi Arnak
For an alternative approach, what about recording the entire bandwidth of the microphone and subsequently filtering the sound using Audacity? The advantages of this approach are that the system is simpler and thus more reliable, and that the microphone does not get noised up by the filter electronics.
 

Arnak

Apr 1, 2011
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High Pass Filter.

Hi Poor Mystic,

That is certainly one way to approach the problem, I don't have enough technical knowledge to say whether filtering before or after is the best way forward.:confused:

I'll have to leave that to the experts although I agree it it must be a valid suggestion thanks:)

Arnak
 

poor mystic

Apr 8, 2011
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Hi Arnak :)
I'll give my reasons for my opinion:

"I think that the quietest affordable professional mic pre-amp should be set to provide a suitable level to a digital recording system. e.g. the soundcard on your laptop.
The signal can be examined and conditioned as required at a later time."

This method has the following advantages over analogue filtering:
1) The unfiltered version of the signal can be archived.
2) Much better filter characteristics can be achieved with software that costs nothing (like Scilab or Audacity) than can be achieved with hardware.
3) Hardware filters are inevitably noisy because of the contributions of shot noise from the transistors, thermal noise from the passive components, power supply noise, and environmental noise. These noise sources have no counterpart in digital filters.
4) The effort involved in building an analogue filter is fairly large.

Iit seems clear to me than digital filtering of the cleanest possible signal must be the better answer.
 
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