Well, that is weird. To be clear, the brochure for this printer describes the resolution as follows:
Copy: Up to 600 x 600 dpi; Print: Up to 1200 x 2400 dpi
So no asterisk, for what that's worth. Just the standard nomenclature: 1200 "by" 2400 dpi, which is what I understood the OP to mean. Perhaps it would be helpful to read what Xerox itself says about print resolution. From Print Resolution: Understanding 4-bit depth (updated 9/12):
Dots per inch
In specifications for laser printers and multifunction printers, print
resolution is listed as horizontal dpi by vertical dpi. Some common
examples are 600 x 600 dpi or 1200 x 1200 dpi. Higher dpi
numbers correspond to smaller dots when printing an image. In
general, devices with higher dpi can produce finer lines and more
detailed output. Horizontal dpi is determined primarily by the laser
used in the device, and the size of the dots the laser utilizes as it
scans side-to-side when preparing the image to be printed. Vertical
dpi is determined by how quickly the device moves paper through
the engine when printing. A device outputs pages slower in 1200 x
1200 dpi mode than in 1200 x 600 dpi mode.
Doesn't seem very weird. So the vertical resolution of 2400 dpi is determined by the speed of the paper moving through the engine when printing. If the C7020 is able to print up to 1200 x 2400 dpi, it should be able to move the paper through the engine slowly enough to print 1200 one-dot lines spaced one-dot apart inside a vertical inch, yes?
Xerox appears forthright in representing resolutions. Other companies use resolution enhancement technology to optimize output and make it comparable to, say, 1200 x 1200 dpi, but they won't tell you it's a smart implementation of 600 x 600 dpi. They'll just say 1200 x 1200 dpi. It's a marketing game, and you're left to wonder about the actual/physical/optical/1-bit depth resolution. Is it really 1200 x 1200 dpi? By contrast, Xerox will tell you if the stated resolution is "enhanced" or reflects the color depth, such as 600 x 600 x 8 dpi. I respect that. And the fact that they invented laser printing!
The C7020 has an unqualified resolution of "up to" 1200 x 2400 dpi. That's 1200 dots per inch horizontally, and 2400 dots per inch vertically, which is terrific for text and fine lines. I don't have an answer for the OP, but I suspect the problem isn't the resolution of the printer, but rather an overlooked settings or PDF issue. The OP does say the test lines are spaced 1 "pixel" apart, not 1 dot apart. Perhaps that's a clue.
These machines use a 1200dpi LED array. The other resolution comes from the Stepper Motor. Often the more steps used makes the printer slower, eats more toner (manufacturers love that part) and therefore make prints more saturated.
So Xerox could flash the 1200dpi LED's 1200, 2400, 4800, 9600 times per inch simply by increasing or decreasing the number of steps per inch. The size of the toner particles means that there's a point at which having too many static charge dots in an inch is completely pointless because you'll simply get lots of overlapped toner with no benefit.
Depending on what you're printing you may find that having a balanced resolution (i.e. 1200x1200dpi) is better than an unbalanced resolution (i.e. 1200x2400dpi).
The proof is in the pudding. Forget looking at what the resolution says or the bit depth and get print samples instead. That's the only way you'll truely know.
1200/2400 isn't what the specs say, 1200*2400 is what they say
In short, divide by 4 and print in Color
Color printer resolutions are weird. Here is how it breaks down..
You actually are always printing at 1200*2400 DPI, because that is what the printer does by hardware.
What you see in the driver is how the jobs are interpreted and ripped.
So the driver takes your job and renders it according to the resolution dropdown (600, 1200 etc) Obviously file size changes massively dependent on which you choose. 1200 is great for text (crisp), 600 is great for images (colors, gradients, saturation). Color issues can and do occur at 1200 (faded, de-saturated), not always, but often enough.
And here is where the real difference is noted. 1200DPI does not equal 1200*1200DPI, they are entirely different measurements.
1200DPI is the amount of dots used to make a digital image appear on screen total within one inch.
1200*2400 is very different, in the CMYK world, unlike the RGB, you divide resolution by the toners.
So with this CMYK device you have the 4 colors, so you divide 2400*1200 by 4.
So each color (CMYK) prints 600*300 DPI, when all 4 colors are laid down, there are 2400*1200 DPI.
Then you have to contend with bit depth as well, or will, not much using them (yet)
8 bits just means you can represent 256 shades of tone/darkness/grey with 8 bits:
one bit (0) is either 0 or 1, two shades= binary (bi=two)
two bits (00) is either 00, 01, 10 or 11= 4 shades
three bits (000) gets you to 8 shades
4 bits (0000) gets you 16 shades
5 bits (00000) gets you 32 shades
6 bits (000000) gets you 64 shades
7 bits (0000000) gets you 128 shades
8 bits (00000000) gets you 256 shades.
9 bits gets you 512 shades, and
theoretically, 10 bits gets you 1024 shades. (MegaGrey!)
The image quality of a 2400 x 2400 dpi @ 1 bit device is overall better than a 600 x 600 dpi @ 8 bit device because of the higher resolution for finer detail (fine lines/text, sharper halftone dots etc), not so much for its smoother tones.
None of this is Xerox, they are all industry standards.
I was referring to physical print resolution when I used the word "optical." These two terms are often used interchangeably, but since optical resolution is used much more often to refer to scanners, I can see how this can be confusing and will edit my post accordingly.
In any case, the "Physical print resolution limitation of 600 DPI" issue remains.
The VersaLink C7000/C7020 series of color laser printers are spec'd offering true physical 1200/2400 DPI resolution. However, if I test this by printing thin horizontal/vertical lines at 1200 DPI, the printer is flat out incapable. It is limited to 600 DPI in both the Enhanced and Photo printing modes, both of which should offer true 1200/2400 physical DPI.
Create a photoshop document that is letter size (with some margins), so let's say 8 in x 10 in of printable area. We want all color management in Photoshop to be turned off. Likewise, the printer settings should be set to perform no color correction/management, print enhancement, and any other corrections/features in the VersaLink (PS) driver as well. We will perform the test in the Enhanced printing mode. This should be a flat-out dump of CMYK values from Photoshop straight to the printer's engine via the PostScript driver, resulting in a print of our 8 in x 10 in document at 1200 DPI.
Now, what exactly is the test data? Well, we can create solid (pure) black horizontal lines (C=0%,M=0%,Y=0%,K=100%) that are 1 inch in length and exactly 1 pixel high, formed in a stair step offset pattern. In other words, something that resembles this (not drawn to scale or proportion, of course):
The lines above represent horizontal strokes that are one inch in width and are staggered apart 1 pixel (i.e., 1/1200 in) vertically. This test of the printer's vertical (optical) DPI will be to see if all four lines print or if just the even or odd lines print, indicating that the printer is printing at 600 DPI vertically and not 1200 DPI (along with some potential interpolation, despite the fact that all such corrections/enhancements in the printer are turned off).
When I do this test at 1200 DPI, only every other horizontal line appears in the printout.
When I perform the same test at 600 DPI, all fours lines are visible, indicating that the physical print resolution, at least vertically, so far, is actually limited to 600 DPI.
The same test can be done with vertical lines and will result in the same shortcomings of the printer (i.e., that the horizontal resolution is likewise limited to 600 DPI - only every other vertical line is visible in a 1200 DPI printout).
So, the big question is: If the printer actually has a maximum physical print resolution of 600 DPI, why do the specs in the spec sheet at Xerox' web site lie about its physical DPI capabilities?