While the death of the CRT is welcome news for the environment, professionals who have relied on the CRT for critical work like color correction are less enthusiastic about losing their trusted monitors. One reason is the inability of most flat-panel replacements to render true blacks and dark detail. For the Dolby PRM-4200 Professional Reference Monitor, we wanted black-level performance that would satisfy even the most discriminating eye. Achieving this goal required taking a fresh approach to the problem.
The primary cause for the lifted “milky” blacks that have made LCD displays infamous among color-critical users is the cold cathode fluorescent lamp (CCFL), which also limits the available color gamut. Static LED backlight units, meanwhile, offer improved color, but they do little to solve the lack of pure, deep blacks.
A viable CRT replacement
Our solution: dual modulation. The PRM-4200 features a unique backlight unit comprised of some 1500 RGB LED triads that directly illuminate the LCD panel. The output of each R, G and B element within each LED is dynamically modulated on a frame-by-frame basis to match the image content for each frame. When the image requires that a portion of the screen be dark, the corresponding LEDs dim. This means no light will pass through the LCD to the viewer. The result is absolute black levels — true blacks and superior dark details. In fact, the black-level performance of the Dolby monitor has been measured to surpass the black-level performance of any other display device on the market.
Once the full-color backlight image for a particular content frame is generated, this information is then used to create the LCD image. The modulation of the LCD panel occurs in real time and is based on a complex algorithm composed of a variety of matrix and 2D filtering operations.
The pairing of a modulated LCD with a dynamically modulated backlight unit has implications for overall image quality as well as black levels. Our dual-modulation process enables precise color accuracy at all luminance levels and exceptionally wide dynamic ranges. While other monitors claim color accuracy at a particular point in brightness, the color accuracy of the Dolby monitor is linear across the entire grey scale. Likewise, a very wide “dynamic envelope” and a very wide “gamut envelope” enable the monitor to emulate any monitor whose performance “fits” within gamuts up to DCI-P3 and luminance envelopes up to 600cd/m2.
The LCD/LED display technology built into the monitor also has economic benefits. It eliminates the costly re-tubing that CRT reference monitors need every 18 to 24 months. Similarly, the LCD/LED combination is less susceptible to burn in and other issues that shorten the lifespan of today's plasma technology and necessitate the frequent replacement of plasma monitors.
Indeed, we built the monitor with longevity in mind. Consider the increasing popularity of cameras such as the ARRI Alexa, RED Epic and Sony F35. Most monitors can display only a small portion of the high-resolution images produced by these cameras. Our monitor already supports native viewing of both 10- and 12-bit formats, and will be capable of displaying the advanced high-dynamic-range output of cameras, color grading systems, and other production and post-production devices moving forward.
By supporting 12-bit video input, the monitor is also future-proofed for higher bit-rate work flows like the new architecture for digital cinema mastering being developed by the Academy of Motion Pictures Arts & Sciences. It is the only Grade 1 monitor on the market that already supports the proposed infrastructure, which promises to become a standard for digital cinema mastering into the future.
With its patented dual-modulation design and a host of other innovative features, the PRM-4200 improves on the capabilities of the CRT and offers durability, versatility and compliance with emerging formats.
Bob Frye is product manager for the Dolby PRM-4200 Professional Reference Monitor.
