Measurements on the speakers

[Jean-Marc Valin]

Hi,

I just made some impulse response and distortion measurements on the
speaker->microphone path. The measurements are shown in figures
http://people.xiph.org/~jm/olpc_distortion.png
http://people.xiph.org/~jm/olpc_distortion2.png

Both figues have the same data. The only difference is the frequency
axis: first figure has the frequency of the harmonic on the horizontal
scale, while the second one has the frequency of the fundamental that
caused it. e.g. for a 1 kHz tone, you need to read the harmonics at 2,
3, 4, 5 kHz in the first figure, while for they're all aligned at 1 kHz
in the second one. (second figure is easier to read)

The main things that strike me from these figures is that there are
resonances in the fundamental around 3-6 kHz and which cause quite a bit
of harmonic distortion. The distortion is also especially significant
because it happens at a "normal" volume. It's measured using a sine
sweep from 40 Hz to 24 kHz as explained in:
http://pcfarina.eng.unipr.it/Public/Papers/134-AES00.PDF
The sine waves were nearly full amplitude and the alsamixer volume for
Master and PCM were both set to 61.

In case anyone's interested, the raw data is available at
http://people.xiph.org/~jm/response and requires the paper above to
interpret. I'm still not sure what can (or can't) be done to improve the
situation. Any thoughts? Oh, and I was just wondering, what kind of
microphone is the BT2 using (omni, figure-of-eight, cardioid, ...)?

Cheers,

	Jean-Marc