Hi Eric,
It is a good question. The sad fact is that most people in the industry simply can not *properly* "handle" differential signals.
We have done our best to work with suppliers in the industry, including NEOS and Cambridge Technology. For those with whom we have worked, their products are perfectly suited to the industry. Unfortunately, these are not the only suppliers who are selling into this industry and, despite the fact that we openly say that we will help anyone from any company to improve their products for our industry, essentially at no cost to them, not everyone has sought our help.
As a result, you have some companies who's PCAOM driver will flat-out malfunction if you feed a negative voltage (of any kind, from any leg for any reason) into them. And you have others who are just not all that bright... The result is that equipment gets damaged -- either our QM2000 (as a result of putting voltages "back into" the QM2000), or other's equipment that can not handle negative voltages.
There is another factor which is that the output of our QM2000 represents what is called a Gyrator. This is a special circuit which behaves like a transformer. If you only hook up one leg (like unfortunately many not-too-bright people do), all of the voltage will go out the other leg. This leads to "phone calls" with people complaining that they aren't getting the right color signals. They aren't measuring it correctly either... but they aren't getting the right signals.
As a result, two things have happened. First, around 2 years ago, we changed the design of the QM2000 such that the negative COLOR outputs are already grounded for you. In the past, people clearly demonstrated that they could not handle it, and now there is simply no choice. We did it for them because... they just can't handle it. As a result, guess what. No more phone calls, no more emails, no more equipment being damaged. Second, for those with older QM2000s, we recommend that they ground the negative outputs so that nothing will be damaged. Since it is hard to tell if people have an older or newer QM2000, we simply tell everyone to ground the negative color outputs. Given the Gyrator output, what this does is force all signal out of the + color output. In other words, nominally (if the palette is trained to do so and all "brightness" related controls are set to 100%) you will get a 0 to +5V signal out of the color outputs. Now, even if people use a meter and connect only the + output, they will see what they expect.
Also there is a common misconception about balanced and un-balanced signals. I have written about this before, but for completeness I will include it here as well.
The reason you use a differential signal is so that any interference that is picked up along BOTH WIRES can be subtracted AT THE RECEIVING END, thereby restoring the original signal being transmitted. What this means is that the ENTIRE ONUS of balanced signal restoration is on the RECEIVER and NOT the transmitter. "Grounding" the negative output at the TRANSMIT side is a perfectly legitimate use of differential transmission because, interference will be picked up, along the line, equally on both wires, and removed by the receiver. The technical term for this technique is "pseudo differential" and it is used in most industrial systems. Note that this same technique is also used by many pro-sumer audio products that use a differential line.
Basically, we decided that what's good enough for everyone else is certainly good enough for Pangolin. Again, after the mod and when people follow our recommendations -- no phone calls, no emails, no complaints. And no more damaged equipment.
Internally we jokingly refer to this change as the "Jack Nicholson mod". I invite you to watch the movie "A few good men" staring Jack Nicholson, to discover why we call it that.
Bill
PS: Note that the entire discussion above relates only to color signals, and not to XY signals. The XY signals emanating from the QM2000 are still fully balanced differential signals. People don't seem to blow them up... On the other hand, scanner drivers require a + and - power supply, so there is never any problem with them receiving a signal that goes below ground. Plus, there are far fewer scanner and driver manufacturers out there when compared to diode driver suppliers and PCAOM suppliers.
