Frequency Modulated Sound Card Capture

Posted on Feb 4, 2014

The problem with sound cards is that they bandpass their input signals. So, if you are wanting to record any signals below 20Hz or so, you are out of luck, ordinarily. There are two solutions to this problem that I have found. The first, is that if you are lucky, you can modify your sound card to remove the high-pass filter, which is removing the

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signals below 20Hz. The first option has a very nice tutorial written by Scott Molloy ( ). Unfortunately, the sound cards that I had access to could not be modified to remove the high-pass filtering (they are actually done in an IC now). The second option is to make a Frequency Modulated circuit, this idea was first published by David Prutchi and Michael Norris in August 19, 2002`s edition of Electronic Design ( ). This tutorial is based upon their design. The difference is that I have simpified the design and made it dual channel. The total cost should be $15 to $20. The circuit expects a +/-10 volt input signal. The circuit should generate a carrier frequency of approximately 4. 4Khz and a change of frequency of up to 3Khz. A greater range could be used to reduce the signal to noise and increase the upper frequency of the input signal (approximately 200Hz with this circuit). The carrier frequency is mainly determined by resisters R2 and R5 and capacitors C2 and C4. The range of frequency modulation is mainly determined by resisters R3 and R6. With a 10 volt input range having R2 = R3 and R5 = R6 seems to work well. Due to variability in the XR-2206 and the capacitors used for C2 and C4, the actual carrior frequency will probably be off. If the actual carrior frequency is substantially different between the two, you can try different resistors. In my circuit I ended up using 50K resistors for the lower circuit (R5, R6)...

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