Rockbox Technical Forums
Support and General Use => Recording => Topic started by: greg_m on January 17, 2006, 02:23:30 PM
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There's been some talk in the past about recording on the Ondio directly from microphones, without any preamp. I tried this using two panasonic FET mic capsules, plus a battery box with a 9v battery and some appropriate series resistors, feeding the mic level signal into the Ondio's line input. The resulting recording was very muddy sounding, badly lacking in high frequencies.
I then did some further investigation and measurement. I generated a sweep tone from 1Hz to 15,000Hz, using Cool Edit Pro. Then I connected the output from my sound card to the Ondio's Line Input, and recorded the sweep (as an MP3 file). Finally, I opened that MP3 file in Cool Edit and observed the frequency response.
When I connected the sound card output directly to the Ondio Line Input, the recording was reasonably flat up to 15kHz. BUT... there's bad news.
I next tried connecting a series resistor (of various sizes) between the sound card output and the Ondio Line Input. I did this to see if there's any change in frequency response when the source impedance is something greater than the nominal 32ohm output from the sound card.
Even with a 1000 ohm series resistor, the high frequencies start to roll off well before 15kHz. With a 10,000 ohm resistor. the response at 15kHz is down about -30dB relative to 100 Hz!!! Above roughly 1kHz, there's a 6dB/octave rolloff!
So my conclusion is that the Ondio Line Input has a very low impedance, at least at high frequencies. (Probably there's a capacitor in parallel with the input, to get rid of high frequency noise generated by the the digital circuitry.)
The problem is that electret FET condensor mics have a relatively high source impedance. So they are not a good match for the low-z input of the Ondio.
I think the only real solution would be to use a buffer amplifier, with a high input impedance and a low output impedance, between the mic and the Ondio.
Has anyone else tried direct mic-input recordings, and had the same issue with muddy audio? Any other related thoughts?
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I use the Archos "Stereo" microphone. That is all that is printed on it. "Stereo." A high-output tie-pin type, it sounds fine, and the level is OK. I use my Sony ECM-K7 micro shotgun for more difficult (mono) recordings. All these are electret capsules and they work fine.
ps: you need to ground the alumimium cases of the two high-output microphone capsules to get rid of RF interference. I have no idea why Archos didn't do this :(