(Some of) the microphones in B2 are really lousy: B3 should improve the
situation on the mikes quite a bit.
- Jim
On Fri, 2007-05-11 at 11:19 +1000, Jean-Marc Valin wrote:
> 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
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--
Jim Gettys
One Laptop Per Child
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