|DSC Charts -- The Ultimate Camera Matchmaker |
Trying To Remove Subjectivity In The Camera Set-Up Process - Now A Little Easier
By Mark Bell
For years, video technicians have tried to adjust cameras and properly reproduce reality as it appears in the studio. Part of any camera shading regimen is standardization, so all cameras on the same shoot reproduce images equally. In reality, the best reproduction that can be attained is sometimes defined by what the cameras agree on - if two cameras match by seeing red as if it were slightly pink, well, viewers will see those images equally, but slightly pink.
Video technicians are unique in that their product standardization is factual on one basis, and subjective on another. That double standard has historically depended on a lot of scope level accuracy as well as purely visual judgments. Bars and other test signals are phased and compared so they match, with little room for error. They are easily definable in their realm via strict waveform monitor level standards. Other parameters? Well, let's just say sometimes it matters who is in the room.
Few technicians ever get to set up the camera to any true consistent standard. For instance, during a thorough alignment, the "calibration" pulse on a camera is usually checked. This allows electronic adjustments, which translate to visual alignments of the calibration test signal. This test signal then uses electronic adjustments in the same path that the optical signal will pass - kind of a "front-end" electronics adjustment. After that procedure, a chip chart is used to set color levels by viewing a grey scale via the camera's optics, and excess color in the signal is removed by adjusting the back-end of the electronic path. We were factual with our bars and cal pulse, but became somewhat subjective on our chart as we shaded for best black-and-white reproduction.
The DSC Chart
In a perfect world, between bars being in phase, the cal pulse being set to the standard, then the greyscale being set to the tech's, the station's or studio's standard, cameras should match, right? Well, they don't match as much as they can in many cases, even when set up side by side in the controlled environment of a studio.
In today's real world many cameras aren't set up side by side in a studio. There are many field cameras maintained in different areas of the country, and they need to match for multiple camera situations after freelance crews travel hundreds of miles to the location. Many times crews travel hundreds of miles to the location. Many times crews discover that their cameras do not match and need adjustment. Perhaps some minor adjustments here and there, or the insertion of a "special" BVW-600 card can get them closer to the perfect match, but here we are being subjective again. How accurate can a purely visual comparison be? Once again, we've taken all of that potentially accurate "scope" set-up and translated it to a visual comparison. Shadows and darker areas will look different, color gains if different (and they probably will be) will cause other areas of comparative distortion. What looks good in one area might change when lighting is changed or a camera is moved.
There is an answer, and it is in the form of an optical-signal generator. Through an optical-signal generator, cameras can easily be set to match the same colors, levels and scope readings through their optics that we've used to match electronic test signals for years.
The DSC optical-signal generator (OSG) is a tool which allows for a greater number of items in a visual field to be analyzed through the waveform-vectorscope-monitor combination. It is a relatively simple device which allows a specific light level to be inserted through calibrated slides, just like in a conventional "light box".
The slides show special patterns which allow the camera to see images that reproduce test signal parameters we are used to. In this regard, the "optical" camera can match the electronic signal camera, and set up to a more consistently definable standard as viewed on a waveform monitor.
Why is this important?
The DSC chart takes us into a domain in which the consistency of adjustment of camera parameters occurs through alignment of the optics. In terms of one display, the exact level and slope of the cal pulse is featured on one part of the DSC chart, and when viewed on a waveform, allows adjustment of black, white and gamma as if generated from the preamps. In this manner, the "optical" camera becomes similar, if not linear, to its electronic self.
The color-step slide, another chart, is also a vehicle of analysis. When place in the lighted slide frame, it sets the stage for a color bar display to be calibrated on a vectorscope, and matched to the very well-known standard of encoder color bars. What the DSC chart's OSG has given us is a great visual representation of the color spectrum in a way we are used to seeing it. With the color reproduction defined by the camera's viewing the calibrated chart on a vectorscope, we are eliminating areas of distortion by defining exactly where and how intense we want our colors to be. It's easier to match amplitude and timing of color signals on a vectorscope as opposed to setting up via seeing "not-too-red, blue or green" on a greyscale.
The more sampling, and exact nature of color phase, the better the shading judgment and ability to exactly reproduce results. How many times have you seen color bars that were off? By optically reproducing a camera's ability to see color bars, then set to that standard, the easier it is to see errors, or great matches, quickly.
Obviously, matrix adjustments will be easier with the OSG. Switch on the matrix and set the specific intensity variation that is desired to "warm-up", or otherwise alter color output. With DSC's OSG, a matrix adjustment is no different than a color bar signal analysis; Gain, gain relationship, phase, phase relationship. Want more red? Go up a specific, visible, measure of gain. Two cameras looking at the same slide of an OSG at different times can be quickly adjusted to match. Variance from those, if any variance existed, would be very visible, just like differences in color-bar signal amplitude and phase would be visible. The charts also contain many different patterns for various analysis of other shading adjustments.
Who invented this device?
A gentleman by the name of David Corley, President of DSC Laboratories in Mississauga, Ontario, Canada, invented the OSG. Corley is an accomplished expert when it comes to such innovation, and among many honors, was recipient of the 1994 Fuji Gold Medal Award. The award honors the recipient by recognizing "outstanding engineering achievements in the design and development of new or enhanced techniques and/or equipment that have contributed to the advancement of photographic or electronic image origination". Corley served as Canadian Governor of SMPTE.
Going back into Corley's history, he was responsible for standards of telecine alignment in the '60s, at which time he patented a dichroic printer for the production of test slides and films. He later invented a vibrationless camera shutter, the color reproduction system, neutral grey scale test targets on color stock, and the color film evaluation system. One recent development has been a soft-edged masking technology and international A/V alignment grid which has become vital in today's popular multi-image industry.
Corley has looked at matching images as a challenge throughout the many technological developments over the past 40-plus years. Today we have robotic cameras, for instance, so Corley adapted his OSGs to easily mount on a lighting grid. While taking advantage of ambient light to create the back-lit illumination of the calibrated slides, a great characteristic of his "Ambi/Combi" OSG System, a camera person can perform a quick, comprehensive analysis of cameras, anywhere.
Is the DSC Labs OSG system perfect? Possibly, as the translation from optical to electronic in a camera is very true and easily reproducible once properly aligned. True and consistent color reproduction is certainly not a new thing for Corley, and consistent color alignments are not a new challenge in visual media. However, the "Ambi-Combi" has many characteristics which show it to be a well thought-out, easy-to-use, device.
Consistent shading is as old an issue as the problems it corrects. Corley's reflection of an incident in the past showed his early interest in the challenge of shading issues and standards charts. It also gave way to the humorous manner he brought along with him as he created the evolution of such aids.
"Way back when, while doing some black and white work, I had mentioned to some colleagues that I didn't like the way pictures looked." Corley stated. You have to keep in mind that we had no standard for light levels on film. They told me, "Well if you're so damn smart, why don't you make us a test pattern?" Corley was smart back then, and he did make some test patterns. He still is smart, and is still making patterns for better, more consistent vision. Few have known the man, but many of us have been watching the results of his work for years.
(Reprinted with the permission of Television Broadcast)