Panel display interfaces and bandwidth: From TTL, LVDS, TDMS to DisplayPort

The panel industry is rapidly accelerating to higher resolution and a greater number of bits per color. Breakthroughs in resolution and color are possible only when increasing the overall data rate from the host to the panel. Advances beyond this limitation therefore are placed squarely on the shoulders of panel display interfaces.

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Figure 1: Display Interface Roadmap: Seeking Higher Bandwidth. This image shows the increase in bandwidth of displays since 1994.

Panel display interface standards
To best see the future, it's appropriate to recap the evolution of panel display interface standards. From Transistor-Transistor Logic (TTL) all the way to today's DisplayPort digital display interface, here is an overview of panel display interfaces from the early 1960s to 2007 and a bird's-eye view of what's next.

Transistor-Transistor Logic
The classic digital interface, TTL, was the choice of interface standards when the display panel was first introduced. At that time, the panel size was less than 10 inches with the resolution at VGA in 6-bit color and bandwidth requirements at 300 megabits per second (Mbps). TTL integrated circuits (ICs) represent small-scale to large-scale integration, with each chip housing up to a few hundred transistors compared with today's microprocessors at tens of millions of transistors.

TTL's popularity was based on Texas Instruments' (TI) introduction of the 7400 series of ICs. While TI's family rapidly became the standard, Motorola, Signetics, SGS-Thomson, National Semiconductor and others joined TI with their own devices. TTL represented a low-cost IC that enabled economically feasible digital techniques vs. analog solutions.

As soon as panel size grew to the 15-inch range in the latter half of the 1990s, resolution became XGA and bandwidth requirements jumped to 850Mbps. Challenges included power consumption and electromagnetic interference (EMI), making the slow TTL interface a bottleneck on the display panel.

Next: LVDS Display Interface (LDI)

LVDS Display Interface
Low Voltage Differential Signaling (LVDS) Display Interface (LDI) is a differential signaling system transmitting two different voltages on twisted-pair copper cables. Compared with TTL, the small amplitude of the signal and tight coupling between the twisted pair reduced power consumption and EMI challenges inherent in TTL.

LVDS represents an electrical signaling method that runs high speed over inexpensive copper cable. Running two different voltages at the receiver, LVDS capitalizes on the difference in voltage (typically 350 mV) to encode information. The polarity of the voltage is sensed by the receiver, which determines the logic level. EMI is reduced since there is a very small signal amplitude and tight electric/magnetic field coupling between the wires. The average voltage on the wires is 1.25 volts. Adoption of this interface took place during the last half of the 1990s.

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Figure 2: LVDS Video Links, an example of display interface

LVDS is predominantly a serial data transmission choice rather than parallel transmission. LVDS combines high-speed and in-channel synchronization, allowing a greater amount of data to transmit over fewer wires.

Based on LVDS, National Semiconductor released its Open LDI specification in 1999, which reduced the total number of wires from TTL's 22 down to eight. As a result, connectors and cabling also shrank. Most important, LDI broke the TTL bottleneck and increased the bandwidth to approximately 2.8 gigabits per second (Gbps). As an open standard, LDI does not require a royalty, which prompted it to quickly become the de facto panel display interface standard.

Again, bottlenecks began to develop. As panel makers consistently increased the size of their panels and improved the color richness, 40- to 50-inch 1080p panels became mainstream. This panel in 8-bit color requires approximately 3Gbps bandwidth, which exceeds the capability of the 4-pair LVDS interface. Even larger panels with Digital Cinema resolution at 4096×2160 and the ability to display deeper color at 10 bits and 12 bits are currently on the drawing table. To meet the bandwidth requirements of Digital Cinema resolution, up to 8-links and bus widths of 40 pairs (808 wires) are used.

Challenges again arise at this level with complex connectors and cabling as well as crosstalk noise, data misalignment and others. LDI is now the bottleneck on the panel display interface, and another interface revolution is waiting in the wings.

Next: Transition Minimized Differential Signaling and DisplayPort

Transition Minimized Differential Signaling
In the late 1990s, Silicon Image began to push its proprietary standard — Transition Minimized Differential Signaling (TMDS) — to the display industry in the form of PanelLink, Digital Visual Interface (DVI) and High-Definition Multimedia Interface (HDMI). In this case, the transmitter incorporates an advanced coding algorithm featuring reduced EMI over copper cable, enabling robust clock recovery at the receiver.

This 8-bit/10-bit encoding is a two-stage process that converts an input of 8-bit into 10-bit code. Similar to LVDS, it uses differential signaling that reduces EMI and speeds up accurate signal transfers. Also similar to LVDS, it is a serial transmission scheme.

The technology has been successful for DVI in the PC arena and HDMI in the consumer electronics market space. However, TMDS has been unable to become a widely used panel interface standard. The royalty-free LVDS has been commonly used instead. In addition, the current version of DVI cannot be updated and has physical, functional and cost limitations.

New on the scene, DisplayPort is a digital display interface standard put forth by the Video Electronics Standards Association (VESA) that was initially approved in May 2006 with version 1.1 approved on April 2, 2007. The standard is recommended between a computer and a monitor or a computer and a home theater system.

Figure 3: DisplayPort Data Transport Channels (source: VESA)

DisplayPort consists of a unidirectional Main Link for transporting audio/video data streams and a half-duplex bi-directional auxiliary channel (AUX CH) used for plug-and-play. Main Link and AUX CH are made up of AC-coupled differential pairs. The Main Link has 1, 2 or 4 pairs or lanes, while AUX CH has one pair and no link is required for the clock. This results in maximum usage of differential pairs.

For example, a 1680×1050 panel may be supported via a single Main Link lane. Currently, the DisplayPort specification supports bandwidths up to 10.8 Gbps and offers WQXGA (2560×1600) resolution over a 15-meter cable.

Next: License-free, royalty-free; comparison chart

License-free, royalty-free
DisplayPort is license-free, royalty-free, digital audio/video interconnect that specifically addresses the link between a computer and display monitor. It has support from AMD/ATI, Dell, IDT, Genesis, HP, Intel, Lenovo, Quantum Data, Molex, NVIDIA, and the list of companies supporting this standard continues to expand. DisplayPort is also accepted by panel makers as a panel display interface standard. Product adoption is already underway.

The advantages of DisplayPort include the ability to have a more-affordable liquid crystal monitor, performance scalability to meet a wide range of requirements and applications, from entry-level to high-performance displays. It also supports future innovation through its micropacket architecture. DisplayPort is currently being marketed to computer monitors.

Table 1 shows the differences between DisplayPort and the other previous standards. Looking at the standards side by side, it is clear that DisplayPort offers many advantages.

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Figure 4: Comparing DisplayPort with LVDS, DVI and HDMI

Although panel display technology keeps advancing, current display interface standards reach their bandwidth limit and become bottlenecks. The LVDS interface, which served as the display interface for the past decade, has run out of steam. This technology limitation is bringing drastic architecture changes in PC and notebook displays as well as in LCD TV. With higher performance capability and industry backing, DisplayPort-based products are positioned to be the next technology that will drive the display interface market.

About the author
Henry Zeng is the Director of Application Engineering and Technical Marketing for the Digital Display Operation Group at Integrated Device Technology. Mr. Zeng has more than 20 years of experience dealing with display, graphics control and video processors. He holds a Bachelor of Computer Science degree from Fudan University, China and a Master of Computer Science degree from Beijing University, China. He can be reached at .

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