Rockbox Technical Forums

Support and General Use => Recording => Topic started by: greg_m on January 17, 2006, 02:23:30 PM

Title: Direct microphone recording
Post by: greg_m on January 17, 2006, 02:23:30 PM
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?
Title: Re: Direct microphone recording
Post by: trevmar on January 30, 2006, 01:27:09 AM
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 :(