|Product Review |
by Jim Stanley
Applications: Broadcasting and all levels of production
Key Features: Can be used to match your cameras
Back in "the good old days" camera setups were a real challenge. Tubes and circuits drifted over time and any kind of rough treatment could require a complete re-alignment of the camera - and exacting and time consuming process.
Although modern television cameras are infinitely more stable and forgiving than their tube predecessors, there still exists the need for proper camera setup. Though cameras have improved, the way in which we align and match them has gone virtually unchanged for years. Registration charts are seldom seen anymore (modern CCDs being fixed permanently in place) but we still employ conventional resolution and logarithmic gray scale charts as an integral part of the alignment process. With an eye toward improving and refining traditional camera alignment procedures, DSC Laboratories developed a unique light box/test chart system known as the Ambi/Combi. DSC sells the Ambi box outright and has both lease and purchase options for the actual test charts. It's possible to put together a system for less than $2,500, a small fraction of the cost of the cameras you adjust using this interesting tool.
The Ambi illuminator differs from conventional light boxes in that the box holding the transparent test chart plates contains no light - the light source is externally mounted and fires into a mirror mounted on the rear of an open-ended box. Light is then transferred via a diffusion screen and gel holder (for color correction) to the test pattern. Although unusual, a significant advantage of this design is its ability to provide a chart illumination that is perfectly color-matched for any type of lighting the camera may encounter - quartz, HMI, fluorescent, even daylight. Light from a small lighting fixture (of the type you wish to align to) is directed into the mirror assembly, and the adjusted for even illumination of the test pattern.
For most applications, the chart holder and illuminating fixture would be mounted on a conventional wheeled light stand; however, wall, lighting grid, and even desk mounting is possible, as the light box assembly is very lightweight.
Rear-illuminated transparent test charts do have some significant advantages over their front-illuminated cardboard cousins. Surface flare, often generated when front-lighting a chart, can seriously impair correct black balance; this problem is eliminated when light shines through the chart from behind. Also, a lack of consistent lighting (many of us tend to use whatever source we can find to light the test chart) can lead to subtle (and someTrebuchet MSot so subtle) differences in colorimetry. This is eliminated by the consistent use of the same lighting source (in this case the light box) for each and every alignment.
Key to the quality of the Ambi/Combi system are the Combi precision test slides, which the manufacturer refers to not as test charts but rather as "Optical Signal Generators" (OSGs). Many video engineers may take exception at this point by noting that all test patterns generate an "optical signal" - and they would be correct. However, the Ambi/Combi system has several unusual test patterns which do in fact generate signals such as you would see from a conventional electronic test generator. This ability to pass conventional test signals through the entire camera path and use these for evaluation and alignment is at the heart of the Ambi/Combi system. The goal is to provide a precise and repeatable approach for objective measurement of video camera performance.
Ambi illuminators are available for both 4:3 and 16:9 format cameras (the Ambi Senior and Ambi Wide models, respectively). The system I received was the 16:9 Ambi Wide package; it consisted of a lightweight chart illuminator/holder and two optical-quality plastic charts. The two charts each included a combination of test patterns, and were labeled "Combi-1" and "Combi-2".
The Combi-1 generator plate includes;
- dual 11-step SMPTE grayscales;
- a "super white" chip which can be used for adjusting clip levels;
- black (7.5 IRE) and "super black" (0 IRE chips);
- calibrated color chips, matching standard vectorscope target colors;
- four flesh tone patches; Light Caucasian, Medium Caucasian, Medium Oriental and Medium Black.
The calibrated color chips of the Combi-1 chart are a key part of this system; the top six chips are super-accurate SMPTE reference chroma values. With a properly set-up camera, each color will drop right into the small 2% boxes on a standard vectorscope. The lower six chips reproduce the same colors at 50% saturation. These color test signals are perfect for critically matching cameras - with a quick glance any operator can tell if two or more cameras are matched properly or need to be adjusted. The flesh tone references are handy as well - compare these color chips with where your on-camera talent's flesh tones appear on your vectorscope and make a note of the position. You now have a reference you can use to fine tune these values during the setup process.
The Combi-2 chart includes;
- linear ramp;
- back focus (star chart);
- multiburst signals (to 10MHz);
- horizontal and vertical resolution patterns.
The Combi-2 chart provides an excellent set of tools for evaluating the overall technical performance of a camera. Calibration sheets, detailing accurate values as measured in the manufacturer's calibration lab, accompany all of the charts included with the Ambi/Combi system.
For my tests, I shot the Combi charts using a pair of Sony BVP-700 studio cameras. A quick check with the Combi-2 chart showed lens back focus to be correctly set and camera response to be linear. I used the resolution wedges to fine tune mechanical focus (with detail off and the aperture wide open, of course). With these checks completed, I installed the Combi-1 chart in the light box and moved on to "chipping" the camera. I began by adjusting camera black levels, using the appropriate chip reference on the chart (7.5 IRE in this case - there is no 0 IRE chip for component setup). Next, I opened the lens aperture and set the reference white step to 100 IRE. Clip levels were set next with the "super white" chip. The camera's gamma crossover point was set to the manufacturer's recommended setting and chroma variations were nulled out using the 11-step grayscale.
For the color alignment, I began by setting up my vectorscope as indicated in the instruction sheet included with the Combi-1 chart, and then adjusted the camera controls as necessary so that the top six color chips fell inside their respective vectorscope boxes. I paid particular attention to yellow and red in order to ensure optimum flesh tone reproduction. That's really all there was to it.
Now, to be fair, the cameras I used for this test are very expensive and very well taken care of, so only minor adjustments were required. But with each camera having been set to reproduce the Combi-1 color chart in exactly the same fashion, the output of the two cameras when aimed onto a studio set were virtually identical - a perfect match.
One of the places where it would seem to that the Ambi/Combi system would really shine (no pun intended) is in the post production arena. By recording the output of a camera set on the Combi-1 color reference chart at the beginning of a program (instead of recording more conventional color bars), post production personnel could precisely set luminance and chrominance values for each segment before editing. This will virtually ensure close matches between scenes.
My overall impression is that the Ambi illuminator and its accompanying Combi charts would make an excellent addition to anyone's studio or maintenance shop. While it may take a little getting used to, the idea of running "test signals" through the entire camera path from optics to output amplifier and utilizing these for alignment is a good one.
Once the Ambi/Combi system becomes a regular part of your operations or preventative maintenance program, you will almost certainly be rewarded with improved visual quality, better color matching and the ability to accurately reproduce this newly improved performance time after time - and isn't that what we all strive for?
DSC Laboratories Ambi/Combi System
- Uses external light for the backlight
- Provides lens-to-output camera test signals
- An efficient and practical camera set-up system.
(Reprinted with the permission of Pro Video Review)