[sdiy] Auto Center waveforms

[Jerry Gray-Eskue]

Yes it is Audio but has the potential to go into the LFO range. The DC level
shift is pronounced and audible when hopping a few octaves. The whole thing
has to do with producing traditional analog signals in an unconventional
way.

I am close to shelving the approach due to this problem.

0.1 seconds doesn't sound like much but consider that it is 10 cycles of an
audible 100hz tone. I actually have tried this method but the DC variation
is to drastic to control using this method. The integrator generating the
saw tooth magnifies any duty cycle or offset imbalance. Normally this is
self correcting using the upper and lower thresholds, but as I said this is
not the conventional approach and that mechanism is missing. I have this
part under control to avoid saturation of the integrator, but I still have a
significant issue with DC offsets in the output.

The whole thing got me thinking that there has to be a elegant way to remove
DC offsets that is not AC coupled. This would be useful in general for LFOs
and Ramp based VCOs asymmetrical wave shapers and etc.

Yes there is a lot to be said for just using a DC blocking cap, as well as
the integrated DC offset removal, but these do tend to work the best when
the DC component is fairly constant.



-----Original Message-----
From: Tom Wiltshire [mailto:tom at electricdruid.net]
Sent: Tuesday, July 14, 2009 11:41 AM
To: Jerry Gray-Eskue
Cc: Synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Auto Center waveforms


Sorry to repeat Eric's question, but what *is* wrong with a DC
blocking cap?

I also don't really see a problem with Bill Forbes' suggestion of
integrating over a number of wavecycles to get the DC level. If you
integrate (say) 10 cycles, you'll have a delay of 0.1 sec for 100Hz
before the DC level settles down. 100Hz is pretty low, so consider
that close to the worst case - and a tenth of a second isn't very
long. Or am I wrong in assuming these are audio waveforms? Is this an
LFO?

Why does it matter so much? What have you got following this circuit
that is so sensitive to DC offsets?

T.




On 14 Jul 2009, at 16:56, Jerry Gray-Eskue wrote:

>
> I know there are some guys out there with a heavy math background
> (including
> at least one PHD)
>
> How about this approach, I use a Differentiating Amplifier to
> calculate the
> Derivative of the input signal and take that output (the
> Derivative) into an
> Integrating Amplifier to recreate the signal without the DC component.
>
> It is in the simplest form just 2 op amps 2 caps and 2 resistors
> and runs
> Real Time with no significant delays.
>
> Is it mathematically sound?
>
>
>
>
> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl
> [mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Jerry
> Gray-Eskue
> Sent: Tuesday, July 14, 2009 8:46 AM
> To: Synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] Auto Center waveforms
>
>
>
> I have been thinking about centering arbitrary wave shapes on 0 volts.
>
> Wave shapers and some VCO waveforms tend to create waveforms with a dc
> offset and it would be nice to have a circuit that would "balance"
> these so
> that the sum of the + voltage = the sum of the - voltage.
>
> To do this it would seem that if you took the sum of one waveform
> cycle
> using a simple integrator you would have a voltage that represents
> the DC
> offset. Using the proper scaling you could then subtract this
> voltage from
> the waveform to remove the offset. A pair of voltage hold circuits
> would
> store the voltage of the previous cycle and remove the offset from the
> current cycle. The integrator (s) would reset each cycle after the
> voltage
> hold circuits were updated.
>
> The problem that I am having with this scheme is that it appears
> the period
> of the waveform must be used to scale the resulting voltage. Now it is
> simple enough to use a constant current into a second integrator to
> derive a
> voltage representing the time period, and it would appear that
> using this
> voltage and a multiplier in the feed back of an op amp will allow
> division
> of the total voltage / period voltage.
>
> The issue with this approach is cost and complexity. It seems like
> there may
> be a "Slick" way to do this without resorting to expensive ICs and
> still
> give good offset removal performance.
>
> Any thoughts, or ideas on a better method ?
>
> - Jerry
>
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