This page last updated:
Wednesday, March 10, 2010 09:04 AM
Color smoothing and delay filters for use with PCAOMs and
people will write to us and ask how they can get smoother, and more
flowing color transitions from the QM2000 laser controller. Since
the QM2000 was designed with the ability to create photorealistic
TV-like raster imagery, the color outputs from the QM2000 are
designed to be extremely fast. However, in an effort to provide the
capability of producing raster imagery along with flowing vector
imagery, we also incorporated a "faded line endings" capability in
While this feature provides a method of creating
smoother, flowing vector artwork, this is only available in the PRO
version of LD2000 and, nevertheless, it is a feature which is often
overlooked. In addition, since this is a software-only technique,
steps are still sometimes perceived in the projected imagery, and to
some people looking for the smoothest and most flowing vector
imagery reminiscent of the late '70s and early '80s, these perceived
steps may be annoying.
For those clients who do not have the PRO version of
LD2000, and for those would place smoothness of vector imagery as a
priority over the ability to create photorealistic raster imagery,
an external color filter may be employed to provide the smoothing
effect. In the context of more and more people using solid-state
lasers, this same color filter may also offer additional benefits.
Before describing the exact filter circuit, a brief
history lesson may provide some additional insight.
In the mid 1970s, when laser lightshows were
first getting started, most companies used galvanometer scanners,
not only for the X-Y beam positioning, but also for blanking and
color modulation. Sophisticated optical networks with mirrors and
prisms were used along with the galvanometer scanners to perform
blanking and color modulation.
The galvanometer scanner was somewhat of a natural
choice for this task because lightshow companies already had
experience with them while using them for X-Y beam positioning in
non-blanking projectors. Also, the alternative -- an Acousto-Optic
Modulator -- was more expensive and (at that time) had lower light
Galvanometer scanners were a known and understood
quantity, which also had another perceived benefit: If you use the
same device for both X-Y scanning and blanking/color modulation,
the speed of X-Y scanning and blanking is exactly the same.
Therefore, point data being output by computers (and more
commonly, the analog output of abstract generators) fed to X-Y
scanners could be fed in-sync with the blanking/ color signals.
Since sophisticated software had not been developed at that time,
this "in-sync" phenomenon was perceived to be a huge benefit.
As a result of galvanometer scanners being used for
both the task of X-Y beam positioning, and blanking/color
modulation, companies came to rely on this "in-sync" phenomenon
when creating laser frames and show content. Huge libraries were
developed of hand-digitized images as well as tapes with abstract
content -- all relying on the blanking/color modulation to be
in-sync with the X-Y scanning signals. But therein lies a
potential problem. With huge libraries being built up that rely on
the exact synchronization and specific device speeds, people were
unwittingly locking themselves out of future developments which
provided greater benefits, but did not preserve the same timing
and speed relationship of an all-galvanometer-scanner projector.
In 1991 and 1992, Pangolin worked with Manhar
Shah of MVM Electronics (the person credited as having invented
the PCAOM) as well as Bob Belfatto and Eddie Young of NEOS
Technologies, to create the Poly-Chromatic Ocousto-Optic Modulator
(PCAOM). The PCAOM is a single device that can select and modulate
individual wavelengths in a multi-wavelength laser beam.
The PCAOM has many benefits, with perhaps the two prime
benefits being simplicity and cost. As discussed above, up until
that time, most people used a sophisticated optical network of
prisms, mirrors and galvanometer scanners for blanking and color
control. This sophisticated network had numerous parts and
adjustments. The PCAOM promised to replace all of that with a
small box that cost about as much as a single galvanometer scanner
and driver, while simultaneously providing greater light
throughput and flexibility.
The PCAOM is surely one of the greatest inventions to
come into the laser lightshow industry. However, with the onset of
this new technology brought a dilemma for some people. The
response time of the PCAOM is hundreds of times faster than that
of scanners which were previously used for blanking and color
modulation. As a result, the huge library of images that had been
built up which relied on the "in-sync" phenomenon were not
immediately usable with the PCAOM.
Since Pangolin had been working with the two earliest
developers of the PCAOM, we recognized this potential
incompatibility early on, so we incorporated a "color shift"
capability into our software. Using the "color shift" capability,
the color signals could be delayed digitally, inside the software,
so that all existing imagery could continue to be used without
This "color shift" capability was of great benefit to
Pangolin users at the time, but back in 1992, Pangolin did not
have the large market-share that we have today. There were other
software vendors on the market that never incorporated this "color
shift" capability, and still other people who were performing
mostly abstract imagery. Because of this, people looked at other
alternatives to allow them to use their existing software and
library along with the new PCAOM.
In 1993, ILDA's Technical Committee began considering the creation
of a standardized analog circuit that ILDA would recommend to be
used along with the PCAOM. The idea was to use an analog low-pass
filter to slow down the color signals being input to the PCAOM --
the "slowness" being approximately equivalent to that of
galvanometer scanners. ILDA's then Technical Committee Chairman,
Casey Stack, proposed the circuit seen below.
Pangolin's William Benner proposed a simpler and more stable
circuit seen below.
Note that the circuit diagrams show only
a single channel. Three such circuits would be used for an RGB
projector -- one for red, one for green. and one for blue. Also
note that the circuit is adjustable. This allows for precise
matching of the color response to that of the X-Y scanning, and it
also provides for channel-to-channel response matching, which is
important to make sure there are no undesirable color "tails" as
lines go from white to black. The filter circuits shown here were
used by a variety of companies during the mid 1990s.
For better or for worst, when the ILDA Technical
Committee came up with the ILDA Standard Projector specification
in 1996, it was decided to leave this filter out, and that the
color modulation system within the projector should be able to
respond at maximum speed. Any low pass filtering that was desired
would therefore have to be done before the projector. In any
event, most Pangolin clients simply relied on the "color shift"
capability built into our software, and never used such a filter.
While most of our clients do not use any kind
of color filter and simply use the "color shift" and "faded line
endings" features of our software, a need has arisen for which a
color filter may once again be handy.
Recently, while attending an event at the offices of a
client in Germany, it came to my attention that the response times
of the individual red, green and blue lasers were not the same.
Lines in the projected graphics that went from white to black
would have a colored "tail". I stayed over an extra day to
characterize the performance of each of the red, green and blue
lasers in order to prescribe a solution for this problem.
Coincidentally, shortly after I returned from the
German trip, a large laser company in Orlando also called me for a
similar problem. They had acquired an RGB solid state laser from a
company in The Netherlands. The red laser was very fast, but the
green and blue lasers were slower. To make matters worst, the
response time to turn the laser on was different from the response
time to turn off.
Since the industry had successfully employed analog
filters in the past to compensate for speed differences in the
devices, and to match up the red, green and blue response times,
it seemed like a promising technique to try once again. However,
with some lasers having a faster off time than on time, a more
sophisticated filter is needed -- one which allows to adjust the
rise time and fall time separately.
Below you will see the filter that was used to solve
the problem seen by the Orlando-based company. Once again, only a
single channel is shown. This is a similar Sallen-Key filter that
was recommended and used in the past, but with an additional diode
to help speed up the rising edge. The filter's overall speed/delay
can be adjusted, as well as that of the rising edge. If you need
to have the reverse effect -- a faster falling edge, you can
simply reverse the polarity of the diode.
Note that the delaying
action is a side effect of the low-pass filter. The low pass
filter has a group-delay characteristic that will tend to provide
a delaying action, in addition to the filtering. The delaying
action is handy to match up the separate red, green and blue
channels, but the response action of the filter is also important
because basically the fastest lasers must be slowed down to become
timing and response-compatible with the slowest lasers.
Before beginning to solve a problem,
the problem must first be "characterized". That is -- all
relevant parts of the problem should (hopefully) be
understood. The best way to characterize the response time
of a laser is to use a dual-trace oscilloscope in
conjunction with a function generator (to produce
modulation signals) and an optical detector (to observe
the resulting light output). If the laser is a DPSS laser,
Pangolin also recommends you use a prism in between the
laser and the detector, since DPSS lasers often output
multiple wavelengths (such as 880nM, 1064nm and 532nm all
at the same time). In most cases, we have seen each
wavelength have its own rise time and fall time. See the
those who do not have a scope or other test
equipment, a specialized test pattern will do. You
could use the Standard ILDA Test Pattern and look
very closely at the top portion -- the part that
shows rise-time and fall time and overall delay.
Otherwise a specialized test pattern can be used.
Back when ILDA was contemplating the standardization
of an analog delay filter, one member proposed an
update to the ILDA Test Pattern with separate
sections to aid in the adjustment of Red, Green, and
Blue, while simultaneously providing the "white"
portion of the Rise and Fall times from the Standard
ILDA Test Pattern. The proposed newer ILDA test
pattern is shown below:
ILDA Technical Committee decided that the color
system in ILDA Standard Projector should operate at
its maximum speed, interest in alternative test
patterns waned. Therefore the pattern that you see
above was never officially adopted by ILDA. However,
it is still available to anyone interested. Please
contact Pangolin if you would like to use this
Up until now, Pangolin has
prescribed filter circuits to individual laser
manufacturers. The idea is that once these laser
manufacturers understand how to solve this problem,
they would incorporate the solution directly into
the laser diode driver itself, and thus, all laser
customers would benefit from that point forward.
The text above gives a brief description of how to
characterize and solve delay-related problems within
the laser diode driver itself. But what if you are a
customer who already has an existing projector, and
you need the ability to adjust the filter/delay
action yourself? Below you will see the circuit
diagram of a filter/amplifier that provides a lot of
adjustability. In this way, this circuit is "ready
for anything". No matter if the laser has faster
rise time, faster fall time, modulation-related
instabilities, etc. this circuit will be of benefit.
Note that only a single channel is shown below.
Three such circuits would be needed for an RGB
Before adjusting the circuit, start LD2000 and go
to the Palette Setup Dialog box. Indicate that you
don't want to use the Wizard and that you want to do
manual adjustments. Then select the menu
"Settings/Standard screen and laser colors". This
will allow the QM2000 to output a 5V maximum signal
for full brightness, and also provide a linear
brightness response. Next, click OK and then load
the ILDA Test Pattern.
To adjust the circuit, begin by setting the Gain
control (P1) to minimum. Switch S1, S2, and S3 can
be set to bypass the slew-rate limiter and Sallen-Key
filter. Next, adjust the Offset control (P2) so that
the laser is just extinguished. The idea is to set
the "lasing threshold" such that it is close to zero
volts. (Many current lasers have a lasing threshold
of about 1.5 volts or higher, which can lead to
instability during modulation and other problems.)
Next, increase the gain control (P1) until you get
maximum brightness out of that laser channel. If you
don't have an oscilloscope, this part can be done
"by eye". Repeat the steps of adjusting the Offset
control (P2) and Gain control (P1) for the other
color channels (i.e. for green and blue).
Once the Gain and Offset have been adjusted for the
Red, Green and Blue, the next part is to observe the
delay characteristics for each laser, and then to
adjust the filter to match all of the delay
characteristics. (Actually, we have found that by
merely setting the Offset and Gain, many of the
problems seen with RGB solid state lasers are
If you have the Newer test pattern shown above, you can
observe the response of the red, green, and blue
channels separately by simply looking at the upper
portion of the test pattern. If you do not have this
test pattern, you will have to pay very close
attention to the upper portion of the ILDA Test
Pattern and perhaps turn on each individual laser
separately (i.e. turn off the green and blue lasers
while adjusting the red laser).
Looking closely at the various elements of the test
pattern, you will notice that the test pattern has
pairs of lines -- one on top, which starts at the
left and seems to end in the center, and another
line slightly below it, which seems to start in the
center and ends on the right. The top line that
starts on the left and appears to end in the center
is for adjusting the fall time, while the one on the
bottom that appears to start in the center and end
on the right is for adjusting the rise time. When
adjusted properly, the test pattern should like like
it does above.
When testing the circuit above, switch S3 can be set to
the position that enables the Sallen-Key filter.
Adjust control P6 to set the rise time, and control
P5 to set the fall time for each color channel. If
you do not have enough range using only the Sallen-Key
filter, you can set switch S1/S2 to enable the slew
rate limiter and adjust control P4 to set the rise
time and control P3 to set the fall time. Contact
Pangolin if you have any difficulty adjusting the
circuit, or if you would rather us prescribe a
dedicated circuit for your particular laser.
Since Pangolin first delved into
solving the problem with RGB solid-state lasers with
different response times, we have prescribed simple
analog solutions for one large laser manufacturer in
Asia as well as several companies in the US. We
suggest that all manufacturers of RGB solid-state
lasers adopt a method to make sure all laser
response times are the same. This provides the best
looking images for the industry and also the maximum
number of happy customers, both for lasers and for
Just as Pangolin has worked with scanner manufacturers,
PCAOM manufacturers and Laser manufacturers many
times in the past, we continue to offer our
assistance to anyone interested in providing
products to this industry. We offer this assistance
FOR FREE, because we want to see the greatest number
of truly great products available to laser lightshow
customers. After all, these same laser lightshow
customers are also laser software customers...
This has been a
review of the history of color smoothing and delay
filters used in the laser show industry. They were
used by some in the earlier years of laser lightshows,
and then pretty much not used at all in the more
recent years. With the introduction of RGB solid-state
lasers that do not have the same response time for
each laser, color smoothing and delay filters may once
again find a use -- that is, until lasers and diode
drivers evolve in such a way that all laser response
times can be made the same.
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