Author Topic: Auto gain adjustment?  (Read 23598 times)

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Offline Things

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Auto gain adjustment?
« on: February 09, 2010, 01:57:17 am »
Hi, I have one o the new L shaped sound cards. I have it all connected up and working, but ...

I have "System input" set to 8 channels, and Analog output set to 6 speakers. Volume is up max, EQ is default etc. Nothing else is changed.

However, the scanned output is continuously vibrating (Mainly near the edges)

I am assuming this is the gain correction thing in the sound card, but I'm not sure what to do about it. If I set "System output" to 6 channels, I get a much smaller scan angle, but it seems to fix the vibrating thing.

Anyone got some things I can try to get rid of the annoying vibrating edges?

Thanks
Dan

Offline James

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Re: Auto gain adjustment?
« Reply #1 on: February 09, 2010, 02:57:15 am »
Are you absolutely sure you tapped the DAC correctly and you can play back sustained DC?

You might have an unregulated power system on the sound card itself. It can show 60Hz hum in the scan.

How clean is the USB voltage going to it?

Is this a laptop with a two wire plug or a tower with a three wire?

James.  :)
« Last Edit: February 09, 2010, 02:59:44 am by James »
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Offline Things

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Re: Auto gain adjustment?
« Reply #2 on: February 09, 2010, 03:06:28 am »
I have it connected as here: http://laserpointerforums.com/f47/sound-card-dac-tutorial-40569.html

I don't see anything that resembles 50hz, the vibration changes when I change the position o the paddle (In zoof's laser tennis, for example).

Not sure about how clean the USB power is, and this is on a laptop.

Thanks
Dan

Offline James

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Re: Auto gain adjustment?
« Reply #3 on: February 09, 2010, 03:09:56 am »
Oh lookie there! It's my favorite thread!  ;D

You're not properly DC coupled to the output of the DAC on at least one of the X or Y channels.

The picture moves because the decoupling capacitor wants everything to return to the center at zero volts.

James.  :)
« Last Edit: February 09, 2010, 03:19:48 am by James »
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Offline Things

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Re: Auto gain adjustment?
« Reply #4 on: February 09, 2010, 03:15:38 am »
So, should I leave the wires where they are but desolder all the decoupling capacitors?

Offline James

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Re: Auto gain adjustment?
« Reply #5 on: February 09, 2010, 03:18:06 am »
Can you measure continuity from your wires back to the individual pins on the DAC chip?

Follow the foil traces on the sound device pcb back to the chip.

Leaving the caps in place is fine. That way the output jacks work like they should, for speakers.

James.  :)
« Last Edit: February 09, 2010, 03:21:27 am by James »
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Offline Things

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Re: Auto gain adjustment?
« Reply #6 on: February 09, 2010, 03:28:42 am »
I can see where I soldered goes directly to the chip.
I can make a movie if you like :)
« Last Edit: February 09, 2010, 03:31:32 am by Things »

Offline Things

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Re: Auto gain adjustment?
« Reply #7 on: February 09, 2010, 03:41:07 am »
http://dl.dropbox.com/u/203420/scane.wmv

It seems to be OK when the paddle is at the bottom! Weird ...

Offline James

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Re: Auto gain adjustment?
« Reply #8 on: February 09, 2010, 05:19:06 am »
When I read this it was 3AM. Now it is after 5AM.

Of course it's digital auto-gain!

What an aweful thing to do to audio!

I remember now. I have not worked with the 6 channel version for a long time.

You need to crank up the gain on your color channels all the way on the correction amp.

Then play a wave of the white circle from LaserBoy. You should get full voltage on the color channels.

Now go into the mixer in the sound device driver and set the wave playback volume all the way up. Reduce the master volume to where your meter reads just above +5VDC. You will notice it moves in steps. Once you find the master volume setting that gives you a bit more than +5 volts on the colors, then trim the gains on the correction amp back down to exactly +5VDC and set your X&Y gains to be the size you want.

That should fix it!

James.  :)

PS. Maybe I can sleep now!  ^-^
« Last Edit: February 09, 2010, 05:23:32 am by James »
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Offline Things

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Re: Auto gain adjustment?
« Reply #9 on: February 09, 2010, 05:58:51 am »
Yeah, I tried setting it to 6 channel input mode and readjusted the correction amp and it works now!

Channel 4 on my correction amp seems to be dead though, maybe I accidently killed an opamp!

Oh well, I can just use 3 and 5 for the lasers :)

Thanks

Offline Xer0

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Re: Auto gain adjustment?
« Reply #10 on: February 10, 2010, 08:18:12 am »
With a µC the Gain Stage could adjust itselft automatically without any need for Trimmers...
you have to write it so that you push a button, play the white circle, and it measueres the gain outputs overa voltage divider and adjust them until they match... just an idea

Offline James

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Re: Auto gain adjustment?
« Reply #11 on: February 10, 2010, 01:40:49 pm »
Quote
Channel 4 on my correction amp seems to be dead though, maybe I accidently killed an opamp!

Have you tried swtching the green output channel of your sound card (4) to the last channel of the correction amp (6) and wiring the output of amp channel 6 to the green pin of your ILDA output connector?

That will give you X, Y, R, G, B and (not working).

It's pretty hard to blow up half of a TL072. Did you try swapping two of them on the board to see if the bad channel moves with the chip? Are you sure it's not the output of the DAC that is bad? Can you measure the correct resistance backwards to the signal input and the negative voltage reference? Do the trim pots measure correctly?

James.  :)
« Last Edit: February 10, 2010, 02:05:32 pm by James »
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