I am recording EMF (electro magnetic frequencies) from a telephone pick-up coil through the Microphone input on a good external USB sound card and mixer. According to Audacity the sound card can deliver up to 384 KHz sampling rate, which I have chosen. But when I do a Plot Spectrum of my recording, the frequency cuts off around 23 KHz. That seems a bit strange.
I am sure the coils are picking up higher frequencies, and the sound card is actually delivering 384 KHz individual samples. So why is the max. frequency Audacity Plot Spectrum finds around 23 KHz?
(Actually I use 2 coils in series, because this cancels out some noise. The coils are 1 kOhm impedance, 280 kOhm resistance.)
I look forward to hear from you. Thank you.
I don’t think Audacity knows anything about the hardware. And the drivers will automatically up-sample (or down-sample) without telling you. i.e. You can play a 384kHz file on any-old cheap soundcard just like you can print a high-resolution photo on a lower resolution printer…
“The rumor is”… a lot of soundcards are fixed at 48kHz. Theoretically, that allows audio up to 24kHz but there is an (imperfect) anti-aliasing filter so 23kHz would be reasonable.
Your audio mixer may also be limited to the audio range.
Also, a high sample rate doesn’t guarantee that the soundcard can go beyond the audio range. The Nyquist limit (half the sample rate) is a hard-limit and you can’t go higher digitally, but soundcards and audio interfaces are made for “regular sound” and there is an analog section and analog filtering.
OK. Thank you for your reply.
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