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Updated on 21/10/2022
HDMI (High-Definition Multimedia Interface) and DisplayPort are modern serial interfaces designed to direct audio and video to a compatible display. These standards replace the aging VGA (Video Graphics Array), DVI (Digital Video Interface), and FPD-Link standards for increased data bandwidth, resolutions, and refresh rates to drive today’s high-pixel-density televisions and monitors. . HDMI 2.1 and DisplayPort 2.0 are the latest revisions and offer new features such as 10K (@120Hz) panel support, Dynamic HDR, AR/VR support, up to 77.37Gbps throughput data and more.
Background
Video monitors for personal computers are analog products. They get their input from video controllers that use digital-to-analog converters (DACs) to convert digital video display content to analog signals. There are analog interfaces such as VGA and other variants depending on the display resolution. Today, a digital video interface called DisplayPort is quickly becoming the primary video standard for PCs, laptops, and other computers.
Additionally, all video and audio used to be analog. But with the advent of CDs and DVDs, video has gone digital. In 2009, television broadcasting became fully digital. While analog interfaces such as composite video, S-video, and RGB were used to transport video from one device to another, an all-digital interface was needed. Intel’s Digital Video Interface (DVI) filled part of that need, but today HDMI has become the de facto standard for consumer electronics.
hdmi
HDMI is a standard from the HDMI Forum (Fig.1). This non-profit organization develops and maintains the standard, manages its interoperability testing and certification, and promotes it. More than 80 member companies support the Forum. The standard became available in 2003 and has been continuously updated. The current version is 2.1. It complies with EIA/CEA-861 standards that define video formats and transmission methods for compressed and uncompressed video and audio.
The consumer electronics industry has almost universally adopted HDMI as “the” video transmission standard. It appears in HDTVs, DVD/Blu-ray players, cable and satellite boxes, AV receivers, digital games, digital cameras, video projectors and some video monitors. It’s even available on some tablets and smart phones.
Technically, the standard supports three differential serial data channels that carry uncompressed video and audio. The protocol is called transition minimized differential signaling (TMDS), developed by Silicon Image. An 8B/10B encoding scheme converts bytes to 10-bit codes for error detection and correction. Video is transferred as 24-bit pixels in sync with a separate clock channel. Ten bits are transferred per pixel clock period. The standard supports up to 48 bits of pixel data. Pixel clock frequencies can be any value in the range of 25 MHz to 340 MHz. This supports 720p and 1080p resolution video with a 60Hz refresh rate. The overall maximum possible composite data rate is 10.46 Gbps.
As for audio, HDMI uses Linear Pulse Code Modulation (LPCM). Support is provided for the most popular digital audio formats such as Dolby Digital, DVD and Super CD audio and DTS HD audio. Audio capability supports up to eight channels. HDMI also includes the Audio Return Channel (ARC), which carries audio from your TV to an AV receiver without a separate audio connection, as is common in some systems.
HDMI also incorporates High Bandwidth Digital Copy Protection (HDCP) to encrypt copyrighted video and audio material. It prevents the reading or copying of protected material during transmission over the interface.
A separate Display Data Channel (DDC) link provides two-way communication capability between the transmitting source and receiver. It is based on the popular I2C. DDC interface is used to inform the transmitter of the configuration and functionality of the receiver. Another separate consumer electronics control (CEC) channel allows multiple devices to be controlled from a single source. Up to 15 devices are supported, so a single remote control can manage all HDMI-connected products.
These multi-wire bundles include four shielded twisted pair lines (100 Ω impedance) for data and clock, plus individual data lines for DDC and CEC connections, as well as ground and power lines. A maximum cable length has not been specified and a wide range of lengths are available. High-speed signals degrade quickly over length, so cables should be kept short. Cables as long as 50 feet are available, but considerable degradation occurs. Longer distances can be achieved by using repeaters or active cables that include signal regeneration circuitry. Fiber optic cables can be used over distances of several hundred feet if needed.
There are four connectors defined for different applications. The best known is Type A with 19 pins which is used on most consumer electronics. The Type B connector uses 29 pins and doubles the six channel data connections to provide a higher pixel rate for carrying high definition formats. The Type C connector is a mini version of Type A with the same 19 pins. It is used on portable devices like laptops and tablets. A micro D-type connector is also available for even smaller devices. It also has 19 pins, but the pinout is different from other connectors. An E-type connector targets automotive applications and includes a locking pin to prevent accidental disconnections due to vibration. A variety of adapters are available to convert from one type of connector to another.
An interesting feature of the latest version of HDMI is the HDMI Ethernet Channel (HEC). By using two pins in the standard connector plus a special cable, a single 100 Mbit/s Ethernet channel is implemented. HEC is mainly used for audio control.
A major factor in designing HDMI into your product is the cost of adopting the standard. Your company must join the HDMI Forum with a membership fee of $15,000 per year and pay a royalty in the range of 3 to 15 cents per product sold. Version 2.0, expected in the near future, increases the clock rate to 600 MHz and supports 4K and 3D HD video resolution as well as faster refresh rates and newer audio standards.
Display Port
DisplayPort (DP) is a standard of the Video Electronics Standards Association (VESA), which develops, certifies and promotes it (Fig.2). More than 200 member companies support it. DP defines a standard for connecting PCs, laptops and other computers to video monitors. It is also available on video graphics cards, docking stations, projectors and other computing devices. It is primarily designed to replace VGA and DVI ports. DP can be used for consumer electronics but is not common. As a result, it does not directly compete with HDMI. It is an open standard that does not require a royalty payment. Introduced in 2006, the standard is now at version 2.0.
DP uses one, two or four differential serial data paths to transport video and audio. Data is self-clocking at a bit rate of up to 77.37 Gbps per path using clock frequencies of 162, 270, or 540 MHz. Data is transmitted in packets with an 8B/10B encoding scheme. The maximum possible net data rate is 17.28 Gbit/s with encoding using all four paths. Most major video formats are supported, including 720p and 1080p, 24 bits per pixel, at 60 Hz refresh rate and up to 3840 by 2160, 30 bits per pixel, at 60 Hz refresh rate.
Digital audio capability in LPCM (Linear Pulse Code Modulation) format supports up to eight channels of audio with 16- or 24-bit sample rates, from 32 to 192 kHz. Compressed audio formats are encapsulated. DP can also use multiple monitors. Up to four monitors with a resolution of 1920 by 1200 can be daisy-chained. With two monitors connected, the resolution can go up to 2560 by 1600.
A dual-mode feature called DP++ can carry single-channel HDMI or DVI using a special adapter. This feature is found on some monitors and graphics cards. The DisplayPort Content Protection (DPCP) encryption system protects copyrighted material from copying or transmission. DP may also implement HDCP if licensed separately by Intel.
Conclusion
Both HDMI and DisplayPort technologies have improved dramatically over the past decade, and both will continue to evolve as the latest technologies come to fruition. The standards will continue to drive multi-panel displays to higher resolutions beyond 10K at increased bandwidths and frequencies. That said, HDMI and DisplayPort will only be able to handle so much bandwidth before a new standard is needed, and AI and Domain Specific Processors (DSPs) will usher in wireless connectivity that will make these technologies obsolete. The idea is to place neural processors embedded in SoCs (System on a Chip) to run super-resolution algorithms to scale today’s lower resolutions (2, 4, 8 and 10K ). It will be interesting to see how far developers can push HDMI and DisplayPort beforehand.
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