MICBIAS buzz injection

Joerg Reisenweber joerg at openmoko.org
Thu Jan 15 02:51:53 CET 2009

We have an issue regarding HW-driven max output level of the A5..A7-mic vs max 
input sensitivity of Wolfson 8753. Anyway this is about some 20dB missmatch, 
and regarding the amount of noise/distortion the mic introduces by itself 
this won't spoil our overall signal quality balance. (you might get sth like 
a 10..14 bit resolution)
8753 has such a low noise floor even at A/D conversion (see datasheet) you 
won't recognize or notice the problem, as you still get much better than 60dB 
S/N. Just boost the digital signal to 0dB and everything will be fine. (I 
apologize for this botch, but it's not really a problem. Builtin mic isn't 
HiFi anyway, for bootleg record purposes please use a decent stereo mic 
connected via USB)

And yes, of course you can record from mic by sw. What do you think we 
designed this device for ;-) ?


Am Mi  14. Januar 2009 schrieb Erland Lewin:
> If you could record from the microphone by software on the Freerunner, that
> would be great.
> I would like to see confirmation that speaking at a conversational level to
> the microphone will give a nice signal level that fully uses the 16 bit
> range, and also show to some extent the degree of noise at the microphone. I
> did some recording tests a few months ago, and was never able to get a good
> signal level.
> Because of the degree of complexity of the audio path (both hardware &
> software), I think it would be nice if OpenMoko had such test software to
> verify that microphone recording works well, and that the level is ok as
> part of your test suite.
> Regards,
> Erland
> 2009/1/13 Werner Almesberger <werner at openmoko.org>
> > Joerg and I discussed today how to verify that the MICBIAS buzz rework
> > was done properly. Joerg had a very interesting idea: by toggling the
> > codec's MICBIAS output, we should be able to create noise that would
> > vary depending on whether the rework has been done. He asked me to
> > adapt one of my I2C-tweaking programs for this purpose.
> >
> > Here's a little program that hammers the Wolfson as fast as it can,
> > making MICBIAS oscillate at about 50Hz:
> > http://svn.openmoko.org/developers/werner/wolfhammer/
> >
> > The result looks like this, on a non-reworked A6, measured on the
> > codec side of R4305:
> > http://people.openmoko.org/werner/a6-nrw.png
> >
> > The amplitude seems pretty small (and I have plenty of ambiental
> > noise), so I'm not sure if this is good enough. To verify that
> > MICBIAS was indeed being toggled, I added a 1k resistor between the
> > measurement point to ground (i.e., parallel to R4305 and C4302),
> > which yielded this:
> > http://people.openmoko.org/werner/a6-nrw-1k.png
> >
> > Joerg, I hope this is useful.
> >
> > - Werner
> >
> > _______________________________________________
> > hardware mailing list
> > hardware at lists.openmoko.org
> > http://lists.openmoko.org/mailman/listinfo/hardware
> >

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