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The Deep Dive on Bit Depth

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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.

Bit-Depth Values
Figure 1: Bit-Depth Gray Tone and Color Values

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).

BitDepthCompared
Figure 2: Comparison of bit-depth gradations (note: illustration limited to 8-bits due to web 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.

ProjectorCentral 10-bit HDR Grayscale Pattern
Figure 3: 10-bit HDR grayscale animated test target (© In Depth Focus Labs 2018)

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).

ProjectorCentral 10-bit HDR Grayscale Pattern
Figure 4: Grayscale signal data for each primary color, when illuminated by light of that color, combine to form a full color image.

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

Next Page
What Bit Depth Looks Like
Contents: What Bit Depth Is and Why It Matters What Bit Depth Looks Like Understanding Bit Depth Specs When Shopping
Comments (10) Post a Comment
David Rivera Posted Dec 20, 2018 2:13 PM PST
Bring on the knowledge Michael. Your fantastic article, chuck full on great info, leads me to quote the late great Teddy Pendergrass: "The more I get the more I want"(Disco circa 1977). Quality information will lead to a wiser and more selective consumer base, which in turn promotes competition between manufacturers. Competition in projector development breeds better quality at better prices. Great work PC for keeping your readers informed and connected. You continue to build our trust and reliance on your site. To all your advertisers I say, invest your marketing dollars in Projector Central and you will reap the benefits.
Rob Sabin, Editor Posted Dec 20, 2018 3:03 PM PST
Thanks for the comments, David. I agree that Mike did an awesome job covering the bases for our readers on an important new topic. We'll look forward to bringing you his future contributions.
Michael J. McNamara Posted Dec 20, 2018 5:02 PM PST
David: Thanks for the compliments and appreciation. Hope I can continue to raise the bar a "bit" higher in future articles.
Harold Veatch Posted Dec 20, 2018 5:05 PM PST
Just curious. So practically speaking, how does this relate to an old projector like the Optoma HD80 which claims 10 bit color processing when it obviously doesn't support HDR.

Quotes from HD80 brochure. (Is this a bunch of bull?)

"Currently filmmakers record and process movies at greater colour depths than most consumer Home Cinema equipment can reproduce. Movie studios have had to reduce the colour depth of their films for home distribution so they are compatible with Home Cinema equipment. The pure 10 bit digital signal path of the HD80 paves the way for movie and gaming content to be displayed in a virtually lossless form producing a level of visual acuity and realism never seen before in the home"

"Commanding over two million individual pixels, luminance and vibrant colours blend fluidly with the ThemeScene ® HD80. At the heart of the projector is the latest 1080p DLP ® technology. A pure 10-bit signal path and processing architecture combine with an advanced colour wheel featuring NDG (Neutral Density Green) technology. NDG increases the visual colour resolution, creating a higher quality image that dramatically reduces low-level dithering artefacts."

"10 bit DNX Rich Colour Processing technology increases the number of colours that can be displayed from 16 million to over 1 billion by offering 4 times the colour information for each pixel."
Rob Sabin, Editor Posted Dec 20, 2018 7:19 PM PST
Harold, end-to-end 10-bit processing most certainly would have been forward thinking in the HD80, which was released in July 2007. But unless I'm mistaken there would have been no true 10-bit content to make use of it. 1080p Blu-rays are 8-bit, and 1080i broadcast did not support 10-bit content then or now. Perhaps photographic content might have been an option at that time.
Jason Posted Dec 20, 2018 11:46 PM PST
Thanks for the piece. Is it possible to add color depth into database as a feature search option? Thanks.
Rob Sabin, Editor Posted Dec 20, 2018 11:50 PM PST
Jason, the manufacturers don't always make this information immediately available in the spec sheets we use to create the database. But we are discussing how we might be able to incorporate this information. At this point, pretty much all new 4K displays ought to be able to do full 10 bit processing,though I suppose some budget models might not.
Mike Collins Posted Dec 21, 2018 3:31 AM PST
Loved this article. Always learning. Can you explain the difference between 4:4:4 vs. 4:4:2 vs 4:2:0 in general and how it relates to visual differences? That would be a great next article. Something on frame rate and interpolation (soap opera effect) would be wonderful as well, ie 60 FPS vs. 24 FPS, vs. 30 FPS.
Rob Sabin, Editor Posted Dec 21, 2018 8:35 AM PST
Mike, we actually do have a article explaining chroma subsampling in the works right now, and I agree that frame rates, frame interpolation techniques, and their affect on image quality would make another good one for consideration. Thanks.
Dadix Posted Jul 27, 2019 3:12 AM PST
So an Optoma HD20 has it or not a 10 bit color deph (because I want to buy one second hand)? How about Optoma 142x ? ( Because in description I see 10 bit but also Rec709 which is standard colors )

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