If you either built a kit or bought a pre-assembled board that has one of those wonderful DC/DC converters on it, then you can magically turn the single ended USB voltage into a differential voltage supply. If you measure your USB voltage it is supposed to be +5VDC, but often it's not quite that. It's usually a bit less. You need a true differential voltage supply to power the op-amps. That is, you need ground and a positive and negative voltage that are both about the same difference from ground. And the supply voltages need to be greater than the maximum voltage swing you expect to get as signals out of the op-amps.
That being said, you also need to take into account that a sound card was not originally designed to produce laser signals. It just happens to work well for the job.
The origin of laser vector art uses 8 bit unsigned values to designate the levels of the red, green and blue. DACs that are made specifically for laser display only need to convert these 8 bit numbers (all positive numerical values) to corresponding voltages from 0 to +5VDC. But if you use an audio DAC, these 8 bit unsigned values need to be converted to 16 bit signed (positive) values. This is done by shifting the 8 bits up 7 bits and making sure the highest bit in the 16 bit number is zero. This is the sign bit and zero designates a positive value. To make matters a bit more confusing, the correction amp is an inverter. So every channel of digital numerical information needs to be inverted or negated before it goes through the DAC so that, when inverted in the correction amp, it comes out with the proper voltage polarity.
There is a standard that all laser (color) modulation signals go from 0 to +5VDC, so you can play a wave that is all white and set the gains with a meter to exactly +5VDC.
There really is no standard as to the peak-to-peak voltage for scanner control signals. Scanner amps have an input sensitivity trim pot and they are typically set to give you a predictable swing in radians per input voltage. But it really comes down to what you want. If you are interested in graphics display on a screen then it makes sense to set your X and Y gains such that your projections fit well on the screen. It also makes sense to adjust them carefully to match each other so that when you project a circle, you get a circle and not an ellipse. It makes more sense to set these gains visually, since a meter won't tell you anything about the scanner amp's input sensitivity or if there is any difference between your X and Y amps.
Making a wave of the big white circle (frame 10 from the default frame set) in LaserBoy is a good test wave. Not only is it all white and therefore drives all the color channels to maximum, but it also gives you a nice signal for the scanners to set their maximum scan angle and match the gains to make a perfect circle. Since it is a circle, you will be able to see if your are clipping. If so, the circle will have flattened sides. Some audio DAC drivers have "digital gain". If your driver is set to have a digital gain of more than 1, it will clip when you play a wave that uses the full swing of a 16 bit signed digital stream. If you see this, you need to bring down the master volume in the audio device driver (software).
If you find that your DAC does have digital gain and you need to decrease the master volume in software to get a circle without flattened sides, then you will need to go back and adjust your color signal gains again, because decreasing the master volume in software will decrease the color signals too.
James.
