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Color smoothing and delay filters for use with PCAOMs and Solid-State lasers

 

  Historical perspective
  Enter the PCAOM
  Early color filters
  Use with RGB solid-state lasers
  Characterizing the response of a laser
  "Ready for anything" filter
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  Lasershow Designer 2000

 
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Occasionally, 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 software.
     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. 

Historical perspective

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 throughput.
     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.

Enter the PCAOM

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 modification.
     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.

Early color filters used for laser displays

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.

Filters for use with RGB solid-state lasers

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.

Characterizing the response of the laser

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 diagram below:

     For 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:

     When the 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 pattern.

"Ready for anything" filter/amplifier

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 laser.


     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 minimized.)
     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. 

Plea to laser manufacturer's to incorporate this solution

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 laser displays.
     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...
 

Summary

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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  This page last updated: Wednesday, March 10, 2010 09:04 AM

 
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