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Issue 3, 2013
Understanding HDMI 2.0: Enabling the Ultra-High Definition Experience
Chip developers targeting multimedia applications have a new standard to embrace: HDMI 2.0 for ultra-high definition video. Manmeet Walia, senior product manager for mixed-signal PHY IP at Synopsys, explains the new features that define the standard and summarizes Synopsys’ DesignWare® HDMI offering.
In specifying HDMI 2.0, the main objective has been to support an “ultra-HD experience”, which means upping the number of pixels to more than four times that used in today’s HD devices – from full HD’s 1920 x 1080 to ultra HD’s 4096x2160 pixels. Supporting this “4K” resolution will enable viewers to watch video on very large screens with high-definition clarity.
Figure 1 summarizes the most important features of the HDMI 2.0 standard.
Figure 1: Key features of the HDMI 2.0 standard
Why 4K Mode?
Do we really need ultra-high definition screens? Figure 2 shows how viewing distance affects how much screen resolution we actually need, depending on the screen size. Generally speaking, the closer the viewing distance, the more pixels we need when viewing larger screens. For example, a 40-inch, full HD (1080p) display is fine if the viewing distance is more than about 18 feet away – any closer and 4K would be a noticeable improvement.
Figure 2: The relationship between display size, screen definition and viewing distances
A Look at the Numbers
Today’s full HD 1080p uses 1920 x 1080 active video pixels, or 2.1 megapixels. 4K Ultra HD comprises 4096 x 2160 active video pixels, or 8.84 megapixels –more than four times as many pixels overall for carrying video data. In fact, real implementations require more pixels than the numbers quoted above in order to carry additional control and audio information in addition to the video signals.
Supporting more than four times the pixel count of today’s screens requires design teams to implement more bandwidth in their HDMI 2.0-compatible systems. The HDMI 2.0 “4K mode” requires the use of 2,250 horizontal lines and 4,400 vertical pixels. HDMI 2.0 supports a 60 frame per second refresh rate. Each pixel comprises 8 bits of color information for three separate colors, such as red, green, and blue (RGB) or yellow, blue, and red (YCbCr).
Furthermore, HDMI protocol requires transition-minimized differential signaling (TMDS) encoding to transfer audio-video content. The TMDS encoding process converts 8-bit data into 10-bit codes (called “8b10b” coding) that have desirable properties, specifically to equalize the number of 1s and 0s in the bit sequence. The two additional control bits are transmitted along with the 8 bits of data so that the TMDS signal can be transcoded.
HDMI 2.0 takes advantage of TMDS encoding, which means that the number of bits required for each pixel is actually 3 x 10, or 30 bits.
The total bandwidth required for all of this data is 2440 x 4400 x 60 x 30 = 17.82 gigabits per second (Gbps).
Figure 3: Bandwidth of a 4K x 2K Video Frame
Goodbye Screen Flicker
HDMI version 1.4 had a bandwidth limit of 10.2 Gbps, which constrained the frame rate to 30 Hz, rather than the 60 Hz supported by HDMI 2.0.
Unfortunately, our eyes can detect the fading between each individual frame at 30 frames per second, a phenomenon known as “flicker”. Doubling the bandwidth enables the frame rate to increase, which allows flicker-free viewing. This provides a significant boost in image quality for viewers.
While 8b10b encoding produces equal numbers of 1s and 0s, at very high frequencies there is a trade-off – switching between repetitive patterns results in energy peaks and high Electro Magnetic Interference (EMI). With HDMI 2.0, the data rate on each data lane (red, green, blue) increases from 3.4 Gbps to 6 Gbps.
To alleviate the energy peaks, HDMI 2.0 introduces scrambling to the TMDS encoding. Scrambling produces longer runs of continuous 1s and 0s, which substantially reduces the energy peaks and hence the EMI (Figure 3).
Figure 4: TMDS scrambling reduces energy peaks and EMI
To maintain backwards compatibility, HDMI 2.0 only requires the use of scrambling with data rates of above 3.4 Gbps per lane.
Color Coding for a Visual Lossless Experience
YCbCr 4:2:0 is a new colorimetry format introduced in HDMI 2.0 and enables the 4K mode to be implemented with fewer pixels than the 8-bits per pixel color (resulting in 24-bits per color) identified above. Color coding effectively halves the overall bandwidth required to support the 4K mode at 60 Hz.
In the previous colorimetry scheme, each color is represented by 8 bits: 8 bits each for yellow (Y), blue (Cb) and red (Cr), which is represented by YCbCr 4:4:4.
Color coding looks at groups of four adjacent pixels at a time instead of each pixel individually (Figure 4). Each individual pixel within the group of four retains its 8 bits for yellow, but the coding averages the blue and red colors across the group of four.
The number of bits required to represent the color information for a group of four pixels now halves from 96 (4 x 24) to 48 (4 x 8 for yellow, 8 for red and 8 for blue). This bit reduction requires half the bandwidth to implement and in terms of quality the difference is imperceptible to most viewers – hence the “visual lossless” experience. Dramatically reducing the bandwidth required for HDMI 2.0 helps to ease the demands on wireless networks and cable manufacturers.
Figure 5: Visual representations of YCbCr 4:4:4 (on the left) and YCbCr 4:2:0 (on the right)
Toward Horizontal Aspect Ratios
Most current TVs support an aspect ratio of 16:9. The new HDMI standard adds support for 21:9 displays because most motion in life takes place on a horizontal plane, rather than a vertical one. Interestingly, for many years it has been standard practice to shoot movies in the 21:9 format, so unlike 3D, a lack of content will not inhibit the uptake of 21:9 by the viewing public.
Consumer Electronic Control
Consumers find it very frustrating that electronics manufacturers haven’t worked together to standardize the remote control. The result is that most homes have multiple remotes to operate their DVDs, TVs and set top boxes.
Today, each consumer electronics manufacturer has its own proprietary Bluetooth link for its remote control system that includes various optional features and will not talk to equipment designed by other manufacturers. The universal remote controls that are available on the market today tend to be difficult to use, requiring awkward setup routines and sometimes only having the ability to control a subset of equipment features.
As an alternative to proprietary remote technology, HDMI 2.0 introduces technology called consumer electronic control – or CEC 2.0, which defines a set of operational codes to enable true universal remote controls. Once instantiated, CEC 2.0 lets consumers control up to 15 CEC-enabled devices, each connected through HDMI, by using only one of their remote controls (for example, controlling a television set, set-top box, and DVD player using only the remote control of the TV).
Low-Level Error Detection
Transmitting very high bandwidth signals without errors requires the use of high-quality cables (the shorter the better) and connectors. If there is a problem with the cable, the result is often poor picture quality.
Because previous HDMI standards only support unidirectional communication, for example, from the set-top box to the TV or source to sink, any transmission errors go undetected by the system.
HDMI 2.0 uses a new error detection capability to support two-way communication, which will enable the TV to report back any errors to the source. The source will then be able to count the number of errors, and, if necessary, sacrifice some bandwidth in order to deliver a viewable picture, albeit not at full resolution.
For viewers who find that the sound experience offered by 8-channel audio is not quite enough, HDMI 2.0 will raise the number of audio channels it supports to an impressive 32 channels.
Multi-View Audio and Video
HDMI 2.0 supports a multi-view mode, which consists of two separate video and audio channels that will let viewers in slightly different positions enjoy different images.
One application could be in-car consoles, where a front-seat passenger may want to watch a movie while the driver views the map displayed by the car’s GPS unit. Video gaming will provide plenty of opportunities for the application of this feature, where two players can see the game situation from different contexts on a single screen.
Multi-view mode works a little like 3D TV – using overlapped polarised images that appear distinct when viewed from different angles.
Selecting HDMI 2.0 IP
The key features enabled by HDMI 2.0 center on the big jump in pixel count to support ultra HD.
Commercial IP solutions should support the plethora of new features within HDMI 2.0 designed for premium home theater systems. True IP solutions are more than just the hardware netlist. Design teams need the PHY, controllers, verification IP, software, operating system drivers and emulation kits. By having a comprehensive IP solution, design teams will be able to integrate their HDMI 2.0 designs faster and with less risk.
Synopsys HDMI 2.0 DesignWare IP
Synopsys is the first IP vendor to announce the availability of HDMI 2.0 IP with several design wins and test chips available in 28-nm technology nodes. The DesignWare solution includes a complete set of software drivers for the Linux platform, which reduces designers’ software development time from weeks to hours and accelerates the time-to-market for products incorporating HDMI 2.0 technology.
DesignWare HDMI 2.0 IP gives design teams an extremely competitive implementation in terms of area, power and performance. The layout and small area of the DesignWare HDMI 2.0 TX “4K” PHY in 28-nm technology is shown in Figure 5.
Figure 6: DesignWare HDMI 2.0 TX “4K” PHY layout in 28-nm technology
Synopsys plays a key role in the HDMI ecosystem development of new standards. This includes participation in the HDMI Forum to help architect new specifications and nurture the ecosystem by working closely with the test equipment developers, consumer electronics giants, foundries and other interested companies.
“We selected Synopsys' HDMI IP solution due to the company's strong mixed-signal and system-level expertise in the multimedia domain. Synopsys’ early readiness with HDMI 2.0 has helped us incorporate the specification's advanced features into our 4K HDTV SoC. As a trusted IP vendor, Synopsys provided us with a high-quality product and expert technical support so we could focus resources on other aspects of our chip."
Michael Mo, senior director of business development at Amlogic
About the Author
Manmeet Walia is a senior product manager for Mixed-Signal PHY IP at Synopsys. He has over 15 years of experience holding product marketing, product management and system engineering positions covering ASSP, ASIC, and IP products for broad range of applications. Manmeet holds a Master of Science degree in Electrical Engineering from University of Toledo, and an MBA from San Diego State University.