MICBIAS buzz injection

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MICBIAS buzz injection

Werner Almesberger
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

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Re: MICBIAS buzz injection

Werner Almesberger
I wrote:
> http://people.openmoko.org/werner/a6-nrw.png

This one is better, with a clean trigger and then a lot of averaging:
http://people.openmoko.org/werner/a6-nrw-avg.png

It's exactly at 50Hz, but the shape of the waveform clearly shows
that I'm not just picking up mains.

- Werner

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Re: MICBIAS buzz injection

Erland Lewin
In reply to this post by Werner Almesberger
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 <[hidden email]>
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

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Re: MICBIAS buzz injection

Joerg Reisenweber
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 ;-) ?

cheers
jOERG



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 <[hidden email]>
>
> > 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
> > [hidden email]
> > http://lists.openmoko.org/mailman/listinfo/hardware
> >
>


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