jerry@pioneer.arpa (Jerry Scharf) (05/26/88)
This is the body of a message from Kok Chen, one of the printer
interface designers at Imagen. I take his word as gospel, so I asked
him if I could pass it on.
Jerry
--------------------
Some things that you missed about CX marking engine resolutions:
(1) because of the interlockingness of the horizontal and vertical
scan direction on a CX, any degradation in the vertical resolution
accuracy makes the aspect ratio even WORSE. I.e., if vertical
resolution is 302 dpi, it would turn the horizontal towards
298 dpi. I don't think gears wearing out has much to do with
it. More likely some DC servo stuff. But then, I don't go
measuring gear backlashes of CXes that have gone through 200K
copies :-)
(2) the pixel frequency that everyone (Laserwriter I, LaserJet I, Imagen 2308)
uses for the CX to produce 300 dpi really produces about 297.5 dpi in
the scan direction for the CX engines that I have laid eyes on. The CXes
seem to prefer to rotate the drum at a rate that yielded 302.1 or so dpi.
So, I don't think anyone does 300 dpi by 300 dpi with a CX at all.
(How did I measure fractional dpi? Ahhh... by printing a vernier
[remember those things they teach you in high school physics?] pattern
and comparing the output with a good metal rule ... yeah, think about it,
it works. Don't use a widely spaced vernier, you may be measuring the
scan non-linearity (sin(theta) vs. theta, modulo correction lens) instead,
yuk, yuk.)
The SXes that I have seen seems to hold the tolerance much better.
And the LBP-20's even better still. No, things are not getting better
with time. The LBP-10 was so darn close to 240 dpi (and the LBP-5 to
480 dpi) that you can hardly measure any deviation. Ahh! them were
nicely built machinery - lotsa metal parts. Can also inflict much
pain when dropped on a foot! Moreover, all the liquid toner will
splash all over you. :-)
Sorry, drum rotation rate is set at the factory (meaning Canon). In
fact, short of Tokyo doing it, I have no idea if anyone else knows how to
adjust that parameter. The 302 dpi cited is WELL within the tolerance
given in the Canon engine specs, by the way. So, I think this may be
a case of some printer users having a much higher expectation than the
manufacturer as to what "acceptable" means. One wonders what the
"threshold of pain" of users is and whether the threshold will go down
as users view printing systems as becoming more refined.
(3) I think the early Laserwriter I's derived it's CPU clock frequency from
the video clock frequency - not the other way around as you stated. Thus,
it gives exactly the "nominal" CX resolution.
(4) ease of changing dpi is almost the opposite of what you stated. Changing
the crystal frequency feeding the video clock will allow you to easily
change the resolution in the scan direction (until you violate timing
parameters of the controller design). The drum direction resolution,
however, is all but impossible to tweek.
Pardon typos, lapses of memory and the like. Anyway, some of this should stop
your speculations; or fuel more :-) :-)
- kc