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'Noise in audio circuit'
2000\05\03@112459 by Bill Pierce

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<x-flowed>I'm working on a project that uses an ISD2560 sound recorder with an SX as
the controller. The circuit is working but there is background noise
generated by oscillations that occur when I read 2 pots that control the
frequency of the playback. If I add delays between the code that reads the
pot I get a lower pitch but I can't get it to go away completely without
making the response time to slow. I tried putting filter caps of different
sizes but no success. Any suggestions would be great, I'm a software guy and
this hardware stuff kills me sometimes.

Thanks

Bill
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2000\05\03@125616 by James Paul

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Bill,

Is there any way to rewrite the POT READING ROUTINE to sample at a
faster rate, and thus move the noise to higher than hearing range?


                                           Regards,

                                             Jim



On Wed, 03 May 2000, Bill Pierce wrote:

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2000\05\03@165335 by Mark Willis

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How do those oscillations occur, is the thing I'm curious about at this
point?  Instead of trying to stifle the effects, sometimes it's far
easier to solve the problem by solving it's cause.

You do have decoupling caps (say 10uF and 0.1uF in parallel) on the
Vss/Vdd at both the ISD2560 and the SX, right?  And perhaps a small
series resistor inline with the SX's Vdd, to reduce coupling of digital
noise to the ISD?  One-point grounding between the two, as well?

 Mark

Bill Pierce wrote:
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2000\05\03@194807 by Bill Pierce

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<x-flowed>I have 10uf and 0.1uf in parallel at the regulator but not at both the SX
and the ISD2560. I'll try that as well as the resistor tonite. The
oscillations happen becuase I am reading the pots at a pretty high frequency
and each time the pot is read the pin goes from high to low in a nice curve
because the cap is discharging. I'm not sure if changing the pin from input
to output is helping either.

I did try lowering the frequency that I read the pot and the noise did lower
in frequency as expected but did not go away. Now I just have a very low hum
instead of a high pitched noise. Its kinda cool to play with both pots and
change the frequency to play tunes. Unfortunately I don't think that my
customers would like that too much.

Thanks
Bill

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2000\05\03@200239 by l.allen
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> I have 10uf and 0.1uf in parallel at the regulator but not at both the SX
> and the ISD2560. I'll try that as well as the resistor tonite. The
> oscillations happen becuase I am reading the pots at a pretty high frequency
> and each time the pot is read the pin goes from high to low in a nice curve
> because the cap is discharging. I'm not sure if changing the pin from input
> to output is helping either.
>
The resistance of the pots/source is too high for the a/d
input.
The best way is really to buffer the analog with an opamp
where the output goes to the A/D and wires back to the
inverting input.

If this is not an option try a lower value pot and/or a cap
on the wiper to A/D wire, say 100nF. This may help stop
the sagging/oscillations but could be a problem if this pin
is shared with an output.

I have sometimes used 1uF caps to stop sag from a
relatively high impedence, very application dependent
though.

_____________________________

Lance Allen
Technical Officer
Uni of Auckland
Psych Dept
New Zealand

http://www.psych.auckland.ac.nz

_____________________________

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