Author Topic: Correction Amp  (Read 39499 times)

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Offline James

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Re: Correction Amp
« Reply #15 on: November 08, 2009, 08:51:06 pm »
Here! Now you can shoot it on the wall!  ;D

Uhhhh.... the laser, that is.....  ;)

James.  :)

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« Last Edit: January 07, 2010, 06:17:47 pm by James »
LaserBoy is Sofa King Cool!
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Offline meandean

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Re: Correction Amp
« Reply #16 on: November 08, 2009, 11:02:13 pm »
  Common audio cards use so-called single bit D/A converters; a single positive reference voltage is turned on and off in an organized manner at speeds well into the megahertz. Because the output is on only half of the time, the actual voltage output averages to a value that is about half of the reference voltage when the card is idle. As the digital signal goes positive, the ref voltage is turned on for a longer time, and less time for a negative signal. The bottom line here, is that an audio card produces a DC idling voltage with no signal applied, and the average DC value moves up and down from that median value (but is always positive) as the digital signal is applied- all you need for audio purposes is a DC blocking capacitor in the signal path so that you can get AC into your speakers. For a laser app, you need DC response with the ability to swing the voltage equally positive and negative, so the capacitors have to be jumpered out. The correction amp is just a 2-input summing amplfier; an adjustable DC voltage is applied at equal but opposite polarity and is summed together to cancel out the soundcard's idling voltage, and provides the ability to adjust overall gain independently. 
"Patience is for the dead."

Offline Fanny Pack

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Re: Correction Amp
« Reply #17 on: November 09, 2009, 07:09:01 am »
Soundcard DACs are pulse width modulated?  I didn't notice that on my scope.  And I don't notice it in the laser output.  Are you sure about this?

Offline drlava

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Re: Correction Amp
« Reply #18 on: November 09, 2009, 10:35:09 am »
yes, at a very high speed.  then they are filtered to DC internally so you wouldn't see it on your scope.

Offline johannesgj

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Re: Correction Amp
« Reply #19 on: November 09, 2009, 07:37:41 pm »
thanks again for your fast replys.
we have started the 'getting suplys' phase now.
i will need some time do digest the knowledge so until we hit the build-phase i might be a little off the forums, just so my occasional missing reply isn't misunderstood :)
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noob with a course

ibeams.org

Offline meandean

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Re: Correction Amp
« Reply #20 on: November 10, 2009, 12:46:59 am »
 Single bit converters have been around since the early 90's, but have taken years to perfect (many still stink). Earlier technologies (parallel converters) required the manufacture of extremely tight tolerance resistor voltage divider networks- the quality of a specific D/A converter chip could vary from lot to lot. Single bit converters place much more of the burden of quality on firmware mathematics, rather than hardware component  tolerences, so that each unit off of the assembly line performs nearly the same for cheap.

 The scary part is that single bit A/D conversion has been around just as long, but don't ask me how that works (I suspect that it involves some kind of rf modulation process) Drlava, maybe you could elaborate... Here's another one- from what I understand, Sony's SACD audio disk format is about the storage of a pure rf frequency bitstream totally devoid of the concept of individually clocked samples as we know them! Single bit in, single bit out.
"Patience is for the dead."

 

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