Path: utzoo!attcan!uunet!husc6!uwvax!oddjob!ncar!ames!pioneer!jerry
From: jerry@pioneer.arpa (Jerry Scharf)
Newsgroups: comp.lang.postscript
Subject: Re: Postscript scaling off
Message-ID: <9101@ames.arc.nasa.gov>
Date: 21 May 88 00:11:41 GMT
Sender: usenet@ames.arc.nasa.gov
Reply-To: jerry@nike.UUCP (Jerry Scharf)
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Organization: NASA Ames Research Center, Moffett Field, CA.
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Never letting a little knowledge stop me, I'll pass on what I know
about laser printer resolutions. If some of the actual interface
designers would like to comment, that would be even better (Chen, are
you out there.)

As with everything else, different laser printers work differently.
The two types I am familiar with are the Canon CX type, and the Xerox
XP10 type. On the CX, the resolution in the paper motion (or slow)
direction is set by the engine in the form of a signal for start of
scan. The mirror is spinning at a fixed rate, and the projection is
compensated by a cylindrical lens to make all the pixels the same
width. The exact resolution in the scan (or fast) direction is
controlled by the rate at which the bits are clocked out to the
printer by the controller.  This rate is some form of divided down
crystal clock. It turns out that Imagen couldn't buy (I don't think
they make) a crystal that would divide down to give an exact 300 dpi
in the scan rate. My memory of the laserwriter was that they used the
same crystal for both cpu clock and video output clocking, so they
also have small resolution problems. The bottom line on CXs that that
they are approximately 300x300 dpi. Once you factor in the gear wear
and paper slippage in the paper direction, again they can only be
approximate. The XP10 type systems provide clocks for both pixel and
scan line timing, so I don't know the exact accuracy details, but I
would assume them to be the same because of the constraints being
similar.

As for tuning up the accuracy of the printers, the crystal set speed
makes it difficult to do anything about it for the CX scan direction.
My guess is that it is as hard or harder to make it accurate in the
paper direction.  Remember that in many cases, you are dealing with a
marking engine that probably very few dollars to construct (pre yen
inflation), so the final exactness of the resolution is not that of
your Linotype or Hell typesetter.

Jerry Scharf
NASA-Ames