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Tech
Focus:
A
look at new technologies
shaping the world of laser displays.
CG
Tools for Laser Artists
How
I Learned to Love Fiber Optics
Fiber
Optics: Technical Primer
Evaluating
New Catweazle Scanners
Low-cost
German
Scanners Perform Well
By
Greg Makhov,
Lighting Systems Design, Inc. lsdi@lsdi.com
I recently had a chance
to evaluate the Catweazle LC II scanners manufactured by MediaLas
of Hechingen, Germany. MediaLas promotes the Catweazle (named
after a European cartoon character) as a high-speed, low-cost
scanning system capable of reproducing the ILDA test pattern
at speeds as fast as 30,000 points per second (30K pps). Bill
Arkin of Holo-Spectra, a MediaLas dealer in the U.S., provided
an X-Y pair with mount, scanner amp, and diode laser.
I was impressed with the compact package this presented. The
Catweazle scanners are nearly cubical enclosures, with the mirror
surface located at one edge of the cube. They come with a special
mount to hold the two scanners in the correct orthogonal orientation.
Each scanner “cube” measures about 1.25 x 1 inch and
has a short cable with a DB-9 connector that plugs into the amplifier.
When using the supplied laser diode, I noticed that the elliptical
shape of the beam was larger than the rather small mirrors on
these scanners, resulting in some light loss. This should not
be a concern with narrower beams from ion lasers. MediaLas recommends
that laser power not exceed 2 watts when using the Catweazles.
Single
Circuit Board
The amplifier and built-in power supply are packaged on one compact
circuit card measuring about 8.75 x 3 x 2 inches. Input power
is switchable from 110 to 220 vac. The power components have
a large heatsink, but fan cooling may be desirable as the heatsink
becomes hot during sustained operation. A 12 vdc output on the
card is provided for fan operation. Input and output signals
come from a 10-pin header style connector, with differential
inputs and a position output. Polarity inversion is accomplished
with computer-type jumpers.
When evaluating scanner performance, I use a variety of test
patterns at different scan angles. Use of a single pattern may
overlook some performance features of the scanning system.
For this evaluation I used an Amiga computer with Pangolin’s
LD 400 as a signal source. The scanners were placed on an optical
breadboard with a calibrated target, allowing quick and accurate
determination of scan angle (in degrees). The basic test approach
involved displaying a test pattern at a particular point output
rate, and then increasing the scan angle until distortion was
observed.
I used four different patterns to evaluate the scanners, the
results of which are shown in the accompanying table. I did not
test any pattern at speeds greater than 30K pps, as the scanners
could not accurately reproduce the ILDA test pattern at speeds
beyond this level. The table shows both the scanning speeds and
the scan angle at which the image was reproduced (scanners typically
perform better at smaller scan angles, sacrificing image size
for speed and detail).
The first pattern I used was a quadrature square wave. This consists
of two square waves positioned at 90 degrees to each other. The
pattern is useful for determining large step response and critical
damping. Some manufacturers sacrifice damping to gain increased
speed, but this was not the case with the Catweazles. They reproduced
the pattern with good results at a 20 degree scan angle at 30K
pps. I next used the ILDA test pattern, which is primarily a
test of small-step response (when the scanners make relatively
short “jumps” between adjacent points). At the high
end of the speed scale, scanning 30k pps, the Catweazles were
able to reproduce this pattern at a respectable 6 degree scan
angle. At the other end of the speed scale, the size of the pattern
grew to 38 degrees when the scanners were slowed down to 12K
pps.
A 9 x 9 grid pattern is a very useful test for approximating
images used in an actual show. I frequently use it to fine tune
scanners after the ILDA Test Pattern is already dialed in. The
Catweazles accurately reproduced this pattern at 30K pps with
a 7 degree scan angle. At a slower 18K pps speed, the pattern
size could be increased to as large as 20 degrees. My final test
involved the Laser Media test pattern. This pattern was originally
designed for relatively slow scanner speeds of 12K pps, but is
still widely in use. One of the interesting aspects of the Laser
Media pattern is that it has two diagonal lines that are retraced
in each direction. This is a useful feature in evaluating the
relative speed of the two scanners. If one scanner is running
faster than the other, the retrace lines will not be superimposed,
but will follow two opposing curves.
I took the Laser Media pattern with a scan angle of around 40
degrees and started a slow Z-axis rotation. This creates a condition
where the diagonal retrace line approaches the scanner axis (vertical
or horizontal) and one scanner makes a small step, while the
other makes a large step. Essentially, this shows us the relation
of small step response relative to the large step response.
Large
vs. Small Steps
With the Catweazle scanners, this test found a substantial discrepancy
in the small and large step response at 40 degrees, creating
a large separation of the diagonal retraces. By reducing the
scan angle to 20 degrees, this effect is minimized (large step
response is decreased in size). At about 10 degrees, the discrepancy
virtually disappears.
The Catweazle II scanner system offers a very economical scanning
engine with reasonable capabilities. When I played back a show
animation module I found the image quality to be quite good.
To my eye, the quality was not as good as the imagery produced
by GSI G120D or CTI 6800 scanners. However, users with little
experience in laser graphics may not notice much of a difference.
I found the Catweazles to be capable of 30K speed, although perhaps
at a sacrifice of better overall performance, especially when
it comes to balancing small-step and large-step response. The
system seems stable, and the electronics are professionally designed.
The compactness of the overall package suggests point-of-sale
systems or portable systems. It may not be truly equivalent to
CTI 6800 or GSI G120D scanners, but the price/performance ratio
makes it a real bargain.
Greg Makhov,lsdi@lsdi.com
MediaLas, http://www.showlaser.com |
Catweazle scanners
measure 1 x 1.25 inches.
The four test
patterns:
ILDA test pattern
Laser Media Test Pattern
9 x 9 Grid
Quadrature square wave
Note: patterns are for illustration purposes. They do not represent
actual scanner response.
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