The Deep Dive on Bit Depth
When shopping for a new display or projector, how do you determine which model has the best image quality, or even just the best quality for your limited budget? Unless you can compare two models in a side-by-side shootout, you'll have to base your purchase on trusted product reviews from sites such as this one, or the manufacturer's ads and brochures. Either way, you're going to encounter a variety of technical terms used to describe the potential image quality, including its ANSI Lumens, white light output, pixel resolution, contrast, color accuracy, and color bit-depth.
Ok, so you might have a hard time finding that last one. Color bit-depth is often hidden on the specs page or described in some obscure way. However, bit depth is becoming an increasingly important metric for comparing projectors that claim the ability to reproduce wide color gamut (WCG) and high dynamic range (HDR) content. In fact, it may actually tell you more about a projector's potential image quality than its contrast, pixel resolution, or even color accuracy ratings—all of which can be varied based on display modes or focusing accuracy.
What Is Bit Depth and Why Does It Matter?
Theoretically, a projector's bit-depth rating describes the highest number of tonal values and colors that it can reproduce in any given frame of content. As the bit depth rating increases (to a point, anyway), the number of colors and tonal values a projector can reproduce on screen increases exponentially, resulting in fewer jagged transitions and posterization effects (i.e., smoother blue skies), along with wider color gamuts and improved shadow and highlight details. The improvements are relatively easy to see as you increase bit depth from 1-bit to 8-bit per color, less intense between 8- and 10-bits, and difficult or impossible to notice between 10-, 11-, and 12-bits due to the limitations of the human eye.
How do you translate a projector's bit depth rating into the number of colors it can reproduce? Let's first take the example of a monochrome projector that forms a single grayscale image on the screen. Its numeric bit depth rating ("x"-bits per color) can be used to quickly calculate the projector's entire range of unique gray scale values, from its deepest black to its brightest white. All you need to do is apply the log function formula. For the grayscale calculation it's: 2x = number of gray values. The chart below (Figure 1) shows you the results of the math for both grayscale only or RGB color . For now, just have a look at the grayscale values; we'll discuss color later.
As seen in the illustration below (Figure 2), once an 8-bit grayscale or full color scale is achieved, you won't see the incremental benefits of 10-bits per color on your computer monitor or tablet, and probably not even on a true 10-bit display or projector driven by your computer or other internet-connected device. For starters, that would require a true 10-bit illustration (our illustrations are capped at 8-bits per color thanks to web color limitations).
Furthermore, you shouldn't be misled by the test patterns and even some movies available for download from the internet that claim to be 10-bit targets or 10-bit per color movies. Most are not what they claim! Unless you can download the test patterns as intact 16-bit TIFF format photos (all JPEGs are limited to 8-bits per color), you should quit while you're ahead. It's even harder to find animated 10-bit test targets and videos, as nearly all popular video formats available for download, including AVCHD and .MP4, are limited to 8-bits per color content. Even if you can actually find true 10-bit files available for download, you'll still need a computer with a 10-bit graphics card and 10-bit capable software. Otherwise, you'll wind up viewing a smooth 8-bit target or movie that shows no difference when viewed on an 8- or 10-bit display.
Using the ProjectorCentral 10-Bit HDR Grayscale Animation
Fortunately, there is a simple way for any serious video enthusiast to download and view 10-bit test patterns to help assess their display. All 4K UHD Blu-ray players have built-in 10-bit per color graphics capability for playing back 4K UHD Blu-ray movies—all of which are stored in 10-bits per color HEVC format video. Most of these 4K UHD Blu-ray players and a few 4K media players, including the Roku 4K HDR, have a USB input that enables them to play back animated 10-bit per color test targets that have been saved in 10-bit HEVC format.
If you'd like to see how your own projector handles 10-bit signals, you can download the 10-bit per color animated test target you see below (Figure 3), created by In-Depth Focus Labs, from ProjectorCentral.com. The spinning wheels display a 10-bit grayscale between video levels 0 and 20 on the left, and levels 20 through 100 on the right. Although it should appear as a grayscale image, it is actually a full color pattern containing metadata tags that should automatically turn on the HDR and WCG modes in any HDR10 compatible display.
To download the target to a Windows PC, you must RIGHT-CLICK on the link below, select "Save Link As" and save to your preferred location. The 10-bit HEVC file will download to that folder. On MacIntosh, right click and then select "Download Linked File" or "Download Linked File As."
Right Click to Download the Test Pattern Video File
To view the test pattern on your display, copy it to a USB flash drive and insert the drive into the USB media input on your UHD Blu-ray player. When you play the file from the disc player's built-in media player, it should be recognized by your display as a UHD resolution video with 10-bit bit depth, HDR, and BT.2020 color space.
As illustrated on the next page, obvious banding in the spinning wheels indicates that your display is playing back with less than full 10-bit bit depth.
Bring On The Color
Unlike a monochrome display, color monitors must form at least three grayscale images that represent the red, green, and blue data channels found in a standard SMPTE color signal. Most 3-chip projectors, whether using LCD, LCoS, or DLP imaging chips, start by using the data from each of the incoming R, G, and B data channels to form associated grayscale images. These are then illuminated by red, green, and blue lights (created by filtering a white light or using color LEDs or lasers) to form an overlapping full color image on screen (Figure 4).
Single-chip DLP projectors parcel out fractions of the R, G, and B data to form as many as seven grayscale images in rapid succession on the DLP micromirror device. Although none of these individual grayscale images contain the full number of tonalities found within the three individual RGB-based grayscale images, the totals should add up to the same in the end. White light from a bulb, colored LED, or a laser diode is then reflected off the DMD and passed through up to seven corresponding colors on a spinning wheel in order to form a full color image on screen.
In all of these color projector models, the total number of achievable colors winds up being the product of the grayscale values created, and are listed in the 8-bit row of the column labeled "Potential R,G, B Color Values" in Figure 1.
For example, here's the math in an 8-bit per color display that forms three grayscale images:
8-bits per color channel: 28 = 256 gray values.
Total colors: (256R) x (256G) x (256B) = 16.7 million colors
|Contents:||What Bit Depth Is and Why It Matters||What Bit Depth Looks Like||Understanding Bit Depth Specs When Shopping|