Flat Panel TV and Projector Technology – Part 2
JVC D-ILA LCoS Projector DLA-RS640
The first post examined the technologies used in the current modern day flat panel displays. We continue with a review of those technologies and techniques as used in today’s modern residential home theater projectors.
Projectors
LCD
The first affordable residential projectors were based upon the passive LCD technology. These LCD panels perform in exactly the same way as they do in flat panel displays. The crystals twist according to the applied voltage allowing a varying amount of light to pass. The difference being that these panels are typically only 0.75” or less square, and there are three of them, one for red, blue and green light.
A Typical LCD Projector Panel
A single high-power bulb is used as the light source, the light from which passes through filters to create the red, green and blue light that strikes the three LCD panels. After passing through the panels the three light beams are applied typically to a set of mirrors and a prism in order to merge them back into a single beam before being passed through the lens system to the reflective screen.
Typical LCD Projector Light Path
LCoS
This is a variation on the LCD technology whereby the liquid crystal display, sometimes referred to as a microdisplay, is reflective rather than passive. Again, it is very small in size similar to passive LCD panels, being about three quarters of an inch square. Being mirror like in nature, like DLP technology, these panels are more efficient than the passive types and can provide both brighter images and much higher contrast ratios due to lower light leakage.
A Simplified Reflective LCoS Panel
A Simplified Passive LCD Panel
The primary users of LCoS panels are:
- JVC D-ILA – Digital direct Image Light Amplifier. Click here to see my review of the JVC DLA-RS640.
- Sony SXRD – Silicon X-tal Reflective Display
A Typical SXRD Panel
A Typical D-ILA Panel
Simplified LCoS Projector Light Path
As with the passive LCD projector a single light source is filtered to provide the three colors. Each color being reflected of an LCoS panel to be recombined in a prism before driving the remainder of the light path and lens.
DLP or DMD (Digital Micromirror Device)
DLP technology was originally only used in cinemas and high-end screening rooms due to the size of the chips, lenses and their cost. Being a reflective technology, it could provide exceptional levels of brightness but has always had a restricted contrast ratio due to the technique used to create a ‘black’ pixel.
A TI DLP DMD Panel
Concept of The Mirror Assembley
Electron Microscope View of Real DLP Mirrors
Due to the size of the original DLP panels large lenses were required which made the early projectors far too expensive and large for home HT use. Once the panels were reduced in size the resulting reduction in lens and construction costs allowed the DLP to become a popular HT projector.
An HDTV DLP panel is composed of 2,073,360 mirrors that are laid out on a semiconductor chip known as a Digital Micromirror Device (DMD). These mirrors can be electrically tilted by up to + – 17 degrees from their flat position, thereby reflecting the light away from the lens and screens light path, creating a ‘black’ pixel. How well this reflected light is absorbed has a profound effect on the pixels black level and the projectors contrast ratio.
The varying brightness, or greyscale, is created by toggling each mirror between its two positions, with the amount of time spent in each determining the brightness of the pixel.
Three chip DLP projectors are assembled in a similar manner to the LCos projectors, having a similar light path. However, these projectors are still very expensive and are rarely found in the average HT. The most common type of residential DLP projector being a single chip system that uses a rotating color wheel to create the three primary colors. Each being flashed up one frame at a time and allowing your eyes to ‘merge” them all together into one image.
Simplified Single DMD Light Path
A Typical Seven Segment Color Wheel
Depending upon the projectors design, color wheels come in a large range of both segment counts, from three to seven, and colors for the segments. The color wheel rotational speed is typically 7200RPM, but can be as high as 10,800 RPM.
Panel Resolution
Most modern projectors still use HDTV image processing panels that are 1920×1080 pixels. The only major exception to this is Sony who manufacture both 1920×1080 and a 4096×2160 (8.85 million pixels) 4K SXRD panel. JVC that has now released a D-ILA 4096×2160 panel for its top of the line 4K DLA-RS4500K projector. For home size screens, and at typical viewing distances, the additional resolution offered by 4K is not as beneficial as one might think. The most important parameters of the new 4K UHD standard is its significantly higher contrast ratio and extended color gamut. This has prompted many manufactures to use optical pixel doubling by shifting the 1920×1080 image up and sideways by half a pixel, thereby doubling the image resolution to 4.15 million pixels. This pseudo 4K is often considered to provide a perceived resolution, depending upon the manufactures processing, to be almost as good as a native 4K image with the attendant savings in manufacturing costs, and providing a less demanding pixel registration for the panels. See here for a detailed description of the 4K e-shift process by JVC.
Texas Instruments have released a new DLP engine based upon a DMD substrate that has a native resolution of 2716×1528 (4.15 million pixels). To provide full 4K resolution the mirrors are ‘wobulated’’ in order to create two images from each mirror, a total of 8.3 million pixels. It is arguable whether this is native 4K or not. The technique does provide outstanding image resolution with all the benefits of a single chip DLP image sharpness. However, its current implementation provides very poor native contrast, typically 1000:1, and it is still a single chip solution with its attendant rainbow and color wheel issues. Several manufacturers, to include BenQ, Optoma, JVC and Epson are now using this chip with both bulb and LED light sources.
Projector Light Sources
The light source in a projector, in some ways, is more important than any other part of the technology. It defines the images brightness and its color gamut. A great deal of research has, and is, going on in order to improve both of these parameters in order to fully support the extended color gamut and contrast requirements of 4K UHD. For now, the bulb is still the most popular light source, but with the ongoing developments of both high power LED’s and lasers, together with their reduction in costs will, I am sure, soon supersede the bulb.
There are several techniques for the creation of the light source in a projector;
- Bulbs:
- Metal Halide – Mercury/metal halide electric arc
- UHP – Ultra High Performance
- Xenon arc lamps
- HID – High Intensity Discharge
A Typical UHP Projector Bulb
- LEDs – High power LED’s. The colors and filters are selected to optimize the light paths.
Examples of Projector LED Light Sources
- Lasers – Light Amplification by Stimulated Emission of Radiation
Put simply, semiconductor laser light sources are like LED’s but are made from different materials to silicon and generate a very precise frequency and color, in a coherent and collimated light beam.
Simplified LASER Construction
Laser Techniques
Lasers are used in several configurations being combined with LED’s and various phosphors to create the red, green and blue light beams. The techniques are summarized as follows:
- RGB Laser or RGB LED – Three separate lasers (or LED’s) are used, one for each primary color.
- Laser/Phosphor – Comprised of one or two blue lasers, this technique splits one blue beam and uses various phosphor filters to create the red, yellow and green light. These light beams, together with the blue laser light pass through the LCD, or are reflected by, the LCoS/DLP panels, before being combined to drive the remainder of the light engine.
- Laser/LED – In this DLP arrangement, the red light is created using a red LED. The blue light laser has its beam split, one half striking a rotating phosphor color wheel to create the green light. All three colors are then sequentially reflected of the DLP chip before driving the remainder of the engines light path.
Bulbs were the original light sources for all home projectors, still ruling the roost today. Careful design goes into the types of gases that these bulbs contain to get a color spectrum that has sufficient energy at the red, green and blue wavelengths. Red generally posing the biggest demand on the bulbs spectrum. They are fairly cost effective, with operational lives up to 5000 hours. But at up to $400 to replace, they are not inexpensive.
The advent of high power LED’s and miniaturization of lasers has permitted the replacement of the bulb. While they provide significantly enhanced color gamut’s and comparable brightness they cannot be replaced by the user. As of this posting I have no data on the cost to replace these light sources. However, based upon the fact that they can last up to 30,000 hours it should be unlikely that the average user will ever need to replace one unless there is a failure of some type.
Projector Advantages and Disadvantages
- Brightness – All home projectors have limited peak brightness due to the ‘single’ light source. Remember that the single light source may need to spread its lumens out over screens with diagonals of more than 300”.
- DLP
- LCD
- LCoS – D-ILA/SXRD
- Sharpest Image – Very dependent upon the quality of the lens and panel alignment.
- DLP – single chip
- D-ILA/SXRD/3 chip DLP/LCD
- Best Native Resolution
- LCoS – SXRD (JVC now has a 4K native DILA chip) -8,847,360 individual pixels.
- DLP -puedo 4K image using pixel doubling to 8,300,096 pixels
- D-ILA/LCD – puedo 4K image using pixel doubling to 4,147,200 pixels
- Best Contrast and Black Level – To meet the 4K UHD specifications very high contrast is required. As projectors have limited brightness this can only be achieved by creating outstandingly low black levels.
- D-ILA
- SXRD
- LCD
- DLP
- Widest Color Gamut – The light source determines the available color gamut that can be shown. Modern bulbs can barely provide the wide color gamut require to support DCI-P3 let alone REC 2020, together with the desired brightness. Manufactures have now turned to Lasers and LEDs to provide the increased color gamut’s. However, they are still limited to just being able to hit DCI-P3 and result in significant cost penalties.
- D-ILA/SXRD/DLP 3 chip/LCD
- DLP single chip
- Bulb/LED/LASER life – To hit the required 4K UHD performance most projectors run their light sources at full power, and often deploy additional filtering to get the extended color gamut. This results in bulb brightness that is significantly down after about 2500 hours of use. LED and laser light sources can significantly improve this life span with hours up to 30,000.
- Rainbows – This effect is where a viewer’s rapid movement of their eyes produces an RGB rainbow around objects. It is only applicable to single chip DLP projectors. With modern color wheels and speeds the effect is generally minimal and is highly dependent upon the susceptibility of the viewer.
- Lowest Fan Noise – Varies between models and brightness modes. A level of 25dB or less at 6 feet is considered to be reasonable in my experience.
- Burn in – This is not an issue for any home theater projector.
- Viewing angle – This is not relevant to the projector as it is a function of the gain and type of the screen.
The Future
Flat Panel Displays
For the immediate future those of us that want the best 4K UHD performance with the widest color gamut, OLED displays are really the way to go. They have a little less light output than LED/LCD’s, are currently more expensive and we do not yet know just how long they will maintain their brightness. I am sure that over the next few years the remaining small disadvantages will be overcome to create an outstanding display. For those of us who really need the brightness then the recent 4K UHD LCD and QLED displays take a lot of beating, providing a contrast and color gamut that is sufficient to support excellent 4K UHD images. These modern displays also have the advantage that they can readily support UHD and both Dolby Vision and HDR10+, assuming it is part of their technical specification.
Projectors
Unfortunately getting projectors to perform like OLED or QLED TV’s is some ways of. They still have a number technical challenges to overcome including:
- Panel alignment and pixel convergence unless they are single chip DLP. Manufacturers now must resort to electronic manipulation of the signal to achieve this. Extending this technology to the ubiquitous 8K seems a pointless exercise in my book. As of this post only Sony has affordable native 4K panels with JVC selling just one native 4K projector for $45,000! Most 4K projectors are pseudo 4K, achieving half 4K resolution by pixel shifting the image to double its 2K image. On large screens at typical viewing distances this apparent loss of resolution is barely noticeable when compared directly with a native 4K projector.
- Brightness is a much more complex issue to resolve as projectors cover a large range of screen sizes. LED and Laser light sources will improve this over time.
- Very high native contrast. LCoS is now providing excellent native contrast ratios, higher than 40,000:1, very suitable for HDR. The new pseudo 4K DLP projectors barely exceed 1000:1 which despite their brightness is still very poor. Dynamic iris control can significantly help improve the images contrast ratio, but often their impact can be seen on the image.
- Restricted wide color gamut barely reaches DCI-P3 from any light source, UHP, LED or laser. As for REC 2020, that is a long way of using todays technologies.
- Support for Dolby Vision or HDR10+. These standards are not yet suitable for projectors, so it is to be seen how they will be handled, if at all, in the forthcoming new models.
- Suitable OETF (gamma) curves to compliment the projectors image processing. Projector calibration is generally required to optimize this parameter.
- All projectors really need calibrating in home to get the best 4K UHD performance from them.
- The most popular light sources, UHP bulbs, still only last about 2500 hours, especially as they must run at full power to meet the gamut and brightness requirements of 4K UHD, and are expensive to replace, typically $250-$400.
- Laser and LED light sources that can last 20-30,000 hours are the future, and can meet DCI-P3, but are still expensive for the average consumer.
I have no doubt that over the coming years most of the above issue will be resolved. In my experience, a well calibrated 4K projector, native or pseudo, in a dedicated dark room, will always provide the most immersive video experience, even if it doesn’t quite match the technical performance of a flat panel.
Click on the following links for more product information and to see some of the latest designs.
- JVC Projectors
- BENQ projectors
- Epson Projectors
- LG displays
- Sony displays and projectors
- Samsung displays
Click here for part one of this post on Flat Panel Display technology.
