[maemo-developers] Noisy signal recording audio

[From: Jeffrey Malone]

Hey all,

So I'm trying to work with magnetic stripes on the N900 over the audio
device -- recording a swipe through the headset jack, and then
processing the signal.
Unfortunately, the N900 appears to have some terrible noise when it
comes down to this precision recording.  To illustrate my point, here
are some waveform plots:

On the N900 recorded with:
arecord -f S16_LE -c 1 -r 48000
http://tehinterweb.com/wf-n900.png

On an iPod touch:
Recorded using a voice memo application as 16bit mono LE at 44.1khz
http://tehinterweb.com/wf-ipod.png

In order to decode the stream, I need to identify each peak in the
audio stream accurately.  In the iPod stream, it's painfully obvious
-- but in the N900 recording, it's a real mess.

Does anyone have some helpful hints on how to reduce the noise while
recording on the N900?


As a note, I've tried using the PulseAudio simple API, QAudioInput and
the basic ALSA arecord app.  All produce the same amount of noise.
With the theory the GSM modem could interfere with the audio line, I
even tried this in offline mode with no discernible improvement.

I'm still trying to work with the stream, and am having some limited
success in filtering through it, but even a best case scenario seems
like it would have an unfortunate read failure rate.

Advice on cleaning this up, or even a better explanation as to the
cause would be greatly appreciated!

Thanks!

Jeffrey