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Path: utzoo!watmath!watnot!watcgl!onfcanim!dave
From: dave@onfcanim.UUCP (Dave Martindale)
Newsgroups: net.graphics
Subject: Re: Film Recorders
Message-ID: <14755@onfcanim.UUCP>
Date: Tue, 19-Nov-85 15:27:13 EST
Article-I.D.: onfcanim.14755
Posted: Tue Nov 19 15:27:13 1985
Date-Received: Wed, 20-Nov-85 07:42:10 EST
References: <471@sdchema.sdchema.UUCP> <648@dicomed.UUCP> <14752@onfcanim.UUCP> <651@dicomed.UUCP>
Reply-To: dave@onfcanim.UUCP (Dave Martindale)
Organization: ONF, Montreal
Lines: 209

I hadn't intended my comments about Dicomed and Celco film recorders to
turn into a debate about their relative merits.  And I'm neither in a
position to compare them fairly (since we don't have a Dicomed and the
only one I've used is rather old but we have a new Celco) nor to
champion Celco (since I don't work for Celco, haven't read all their
sales literature, and don't have circuit diagrams).  So I'll just
provide some more information.

First, I should point out that the principal difference between the
Dicomed and the Celco, in terms of driving them, is that the Dicomed is
fundamentally a point-plotting display (with spacing of points that is
much finer than the CRT spot size) while the Celco is a pure raster
device - it draws scanline after scanline on the CRT face.  Thus the
Dicomed can draw random vectors that look very smooth on screen, while
to get vectors on a Celco you have to do antialiased scan conversion in
software.  So if you want to plot line drawings, the Dicomed has a
clear advantage.  But if you're going to plot images that have anything
other than lines or areas with strictly horizontal and vertical edges,
you have to do the scan conversion yourself anyway.  For our
application (animation), vectors are almost useless.

The other major difference is that the Dicomed changes the brightness of
a pixel on film by varying the time that the beam is unblanked, while
the Celco modulates the beam intensity to directly change the phosphor's
light output.  The Dicomed's system has the advantage of constant spot size,
but the brighter the image the longer it takes to plot.  The Celco's plotting
time is constant and, for reasonable images, apparently significantly
faster than the Dicomed.

For a best-case example of plotting speed, the Celco will plot a
1536x2048 pixel resolution full-intensity white frame in about 20
seconds real time, including the time spent rotating the filter wheel.
This is at the fastest pixel clock rate; there is enough light to use
this rate on 5247 colour negative film (rated at ISO 80 daylight) with
the lens stopped down one stop, when recording onto a standard 4-perf
35mm movie frame.  I'm curious how long the Dicomed would spend on the
same frame.

>CRT spot size is the one of the most important
>factors contributing to the actual "res": resolvable line pairs/mm on the
>film plane.  Other important factors which can compromise the spot size are
>the quality of the optics in the lens, the color filters (Dicomed filters
>are actually lenses to correct for chromatic aberrations), and mechanical
>stability.

I know - I was just pointing out that you shouldn't expect to get 8000
resolvable lines with either a Celco or Dicomed, but that 4000 was
achievable.

>>But the number of pixels it can resolve
>>on screen is also in the same range as the Celco's - again limited by
>>CRT spot size.
>
>This can be hotly debated.  The primary difference between Celco and Dicomed
>in this area is that Celco uses an intensity modulated beam, whereas Dicomed
>uses time modulation with constant intensity thus maintaining uniform spot
>size.  The Dicomed spot is under 0.8 mils, but Celco's is tough to measure.
>It is a distribution based on spot intensity.

Celco claims to have 7500 resolvable pixels across the
useful diameter of the 7 inch CRT (this would be across the image diagonal,
not one edge).  Doesn't the Dicomed use a 5 inch tube, requiring a smaller
spot size for the same resolution?  In any case, if you divide a .0008
spot size into a 5 inch tube, you come up with a number that agrees with
my original comment - both film recorders can plot 4000 resolvable lines,
neither can do 8000.

>>I believe the primary reason for
>>plotting 8000 lines was that if you plotted on very large sheet film
>>(8x10) you could start seeing spaces between the lines if you plotted
>>only 4000.  But you still couldn't resolve 8000 distinct lines, since
>>the CRT image isn't sharp enough.
>
>This is perhaps true on the Celco, but Dicomed does not have problems with
>inter-line gaps at 4000 lines.

Is this because of the Dicomed's pixel formatting?  As I understand it,
each pixel is plotted as several overlapping points.  This should give
you the ability to "size" the pixel so that adjacent lines overlap.
Or is it simply that the resolution of the system (CRT spot size plus
lens sharpness) is not high enough to show interline gaps?  :-)


Definitely positive features of the Celco:  Both film recorders have
circuitry of some sort to help set the "brightness" control to
compensate for CRT characteristics drift.  The Dicomed one involves a
manual adjustment, performed by a human reading a meter.  The Celco's
is an automatic adjustment triggered on command from the host, and thus
can be done every few frames during filming, improving exposure
repeatability.  The Celco mechanical assembly allows easy access to the
CRT face for focusing adjustments, and the heights of the camera and
lens stages are easily adjusted.  This makes changing film formats
reasonably painless.  I've never seen a Dicomed camera stand of
comparable design.  The Dicomed recorder I've seen has a tendency to
drop bits of the filter-wheel drive belt onto the CRT face; the Celco
filter wheel is enclosed within the lens assembly and gear driven.

Also, service from Celco has been (so far) impressive.  One of the
company vice-presidents, who is an engineer, came here to do the
installation.  Both he, and the regular service person, know the
system well enough to do troubleshooting at the chip level.
The experiences of the Dicomed owner I know of have been considerably
less favourable.

Now, some of my impressions may be based on a long-obsolete version of
the Dicomed.  But every year I wander past the Dicomed exhibit at
SIGGRAPH, certainly interested in taking a look inside the film
recorder, and find that the film recorder itself is closed up, off in
a corner somewhere, while all of the booth's attention is being focused
on Dicomed's slide-making turnkey system, in which I have no interest.

>>The amazing thing is how well they are built.  Our Celco seems to use
>>mostly military-spec parts.  The weakest link in both the Celco we
>>have and a Dicomed I've seen is the camera.
>
>What weaknesses do you find in the camera.  Marron Carrel makes some pretty
>good stuff !!

In the following, keep in mind that my application is animation, where
very accurate registration of film and long-term durability are important.
For slides, almost any camera would do.

The Dicomed I've seen is an old one, and the camera (made by
Electro-Optical Systems, or a name similar to that) was an absolute
joke for animation use.  It had no sprockets at all, moving the film
with a rubber capstan.  There were no registration pins, and the only
side-to-side guidance the film had was a roller with edge flanges.
The film would weave from side to side about 4 thousandths of an inch.
Good stability is somewhere around 4 ten-thousandths of an inch.
There was no aperture plate, just a hole that the film travelled over,
and the CRT blooming caused by a bright area in one frame would show up
in the adjacent frame.  It may have been OK for some slides, but nothing
more.

The Marron-Carrel cameras on present Dicomed and Celco film recorders
are a considerable improvement.  They are pin-registered to provide
much better frame-to-frame registration accuracy.  But when you compare
it with the Oxberry animation-stand camera we have sitting around here,
the differences are apparent.

The registration pins seem not to be as well-machined as they are on
professional equipment.  We've had some problems with oversize pins
that generate small pieces of emulsion chaff when the film is pulled
off the pins - this chaff accumulates inside the camera.

Also, the position of the pins relative to the film frame seems to be
whatever was convenient during design.  In professional cameras, the
position of the pins relative to the frame is standardized so that the
*same* film perforations will be used to register the frame later
during optical printing.  (This may not matter if you only intend to
project the film without further optical effects).

In the Oxberry, the film is removed from the registration pins by a
special carrier in the transport that touches the film only by its
edges, in the sprocket hole areas.  The Marron-Carrel uses a thin strip
of steel that goes all the way across the width of the film, with the
potential (at least) of touching and scratching the emulsion in the
image-forming area.  The metal parts that contact the film in the
Oxberry are all highly polished and most are chromed, and all appear to
be made of steel.  This provides smooth surfaces to avoid scratching
the film, and long wear.  The MC makes no use of chrome, makes the
aperture plate and pressure plate of aluminum, and generally does not
have as fine a polish on surfaces.

The Oxberry camera has a number of "buckle switches" in it.  They are
long pins that are placed just out of the film path in various places
in the camera.  If the film breaks or jams, it will soon touch one of
these switches, and shut down the camera.  This is a Good Feature for long
unattended shooting sessions.  The MC will just keep on merrily running,
potentially making a real mess of the camera innards.

This is made more important by the way the magazines are loaded.  When
a film magazine is put on the Oxberry, it automatically couples to the
film spool drive motors.  On the MC, you have to remember to run a belt
from the magazine motor to the magazine's spool.  If you forget the belt,
you guarantee a camera jam.

The Oxberry uses standard Mitchell film magazines.  The MC uses magazines
of their own design which are intended for 46mm film.  For 35mm use, they
supply spacers, but there is a metal plate supporting the film pack on
only one side.  The film is free to weave as it winds on the core, and if
you take a magazine off the camera and lay it on its back, the film will
"spill" off the core.  The Mitchell magazines have none of these problems.
(For people used to shooting slides: The normal film load in an animation
camera is 400 feet on a core; there is no spool to provide side support).

To be fair, the MC does have some advantages too.  It seems to use
sealed bearings (probably ball bearings) everywhere, so it should never
need disassembly and lubrication.  The Oxberry is much more complicated
mechanically, and uses many sleeve bearings and sliding surfaces, so
it does need to be serviced every once in a while.

The MC's viewing system uses a reflex mirror that slides into place,
allowing a reticle to be projected onto the tube face.  This is *much*
more convenient for setting up the film recorder than the rackover
viewing system in the Oxberry.

This has gotten long enough already.  I do *not* want this to turn into
a long argument, just provide information useful to other people
considering spending large sums of money on a good film recorder.  Some
of the information presented here was learned the hard way, and I hope
it will be valuable to others.

	Dave Martindale

Anti-disclaimer:
Everything I say is my own opinion unless I claim to represent someone
else.  Why should anyone expect anything posted to usenet to reflect
the "official policy" of any organization on any subject?