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LDR attenuators - Better balance with DC bias
LDR attenuators - Better balance with DC bias
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Old 1st April 2012, 03:24 PM   #31
gootee is offline gootee  United States
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I was hoping that making them out of band would mean that no filter was required. I think that it's likely that the only way that many people would accept such a system would be if there was nothing added in the signal path.
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Old 1st April 2012, 03:31 PM   #32
gootee is offline gootee  United States
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Just off the top of my head (but hopefully close-enough to some potential reality):

I don't think that it would be very difficult to design and implement such a measurement and feedback control system. But of course there could be some gotchas that I haven't thought about.

After the details were evaluated, decided, and worked out for "continuous" or "only-at-change" interfacing to the LDRs, and what impedance range(s)/scheme to use, the actual measurement-and-control implementation should be fairly straightforward, possibly with an instrumentation amp across each LDR, a filter to discard everything except the measurement signal, an RMS-to-DC converter chip like the LTC1968 or a discrete equivalent circuit (if using out-of-band tones instead of DC), and an opamp as a differential integrator (possibly with a low-pass post-filter) to servo the LED current based on comparison of the setpoint to the measured resistance. The series and shunt setpoints could be derived from the output of a lightspeed-style pot, or, in some other way.

For better matching accuracy, it might be better to have only ONE measurement-and-feedback-control circuit, driving a digital pot or other memory scheme for each LED current, and time-multiplex (switch) it to connect to, and measure and set, each of the LED/LDR devices, in turn.

That way there wouldn't need to be any worries about the matching of the measurement and control circuits themselves. And as a bonus the parts count and cost should be significantly lower. But response speed and user perception might become a real concern.

There would probably be design issues to solve (or minimize) involving dynamic range and noise (and stability) at each stage in the measurement and feedback control chain. But I can't see any obvious show-stoppers, yet.

There might also be other implementation schemes that could be better.

Last edited by gootee; 1st April 2012 at 03:41 PM.
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Old 1st April 2012, 03:37 PM   #33
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
Please keep feeding control strategies, no matter how weird they initially seem.

We can sift through them and find those that have potential to implement simply and the few genii among us can contemplate the complex solutions.
We never know which might be "best", until each strategy has been implemented and developed to it's limiting condition.

There may turn out to be 23 "best"
regards Andrew T.
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Old 1st April 2012, 05:13 PM   #34
BFNY is offline BFNY  United States
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Join Date: May 2004
Location: Saratoga NY
Originally Posted by gootee View Post
I tend to agree that using LDRs almost seems like it would be more trouble than it's worth.

The main problem they are supposed to solve is simply the same one that all stereo attenuation schemes are supposed to solve: A stereo attenuator is needed. A couple of other possible problems that they might be able to mitigate are mentioned below.

One thing that LDRs should be able to do better than other passive attenuators is automatically adjust (or automatically be calibrated) for nearly-perfect matching of the two channels, at every attenuation level. But I don't know if that is a big-enough problem with other passive attenuators to warrant using LDRs, with the added complexity of auto-cal or auto-adjust.

And the same thing (automatic matching or auto-cal'd matching) could probably be accomplished a little more-easily with a dual pot and one LDR. Or, with more parts but less design effort, a stepped attenuator could be carefully matched.
I was thinking about stepped attenuators yesterday. What about a stepped attenuator as the control for driving the LED/LDRs. The high level idea is you use a 12 step switch, with say 4dB steps for 48dB control. And then run the balance calibration on it, and add pots, or series resistors, as needed to get perfect volume balance at all steps. This would be more efficient than just making it with pots on every step.
Then have a secondary fine control, for say 6 dB total, to give control between steps.
Doing this with a board for resistors/pots would make it easier than trying to do it point to point on the switch.
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