Electronic Design

Use DisplayPort In Your Next TV Application

DisplayPort, a new interface standard from the Video Electronic Standards Association (VESA), simplifies display design and its associated connections. It also supports higher resolutions with robust electrical characteristics. Although the immediate application of the DisplayPort interface is in notebooks and display monitors, it is designed to be robust for many embedded and internal applications, such as digital TVs.

This high-bandwidth, bidirectional display interconnect enables a common next-generation interface approach for internal and external display connectivity. With scalable serializer/deserializer (SERDES) technology and by changing the number of lanes and the speed, display systems can use DisplayPort to support a wide range of resolutions, making it a perfect fit for use in digital TVs. The table shows some examples of resolutions supported by DisplayPort with different speed and lane combinations.

Myriad interfaces are present in today’s digital TVs, including HDMI, VGA, RS-232, and RGB. Multiple interfaces increase the panel weight and thickness, as well as the number of cables that go to the TV, making it difficult to hang the device on the wall. To overcome these issues, designers are including all of the interfaces in a set-top box and running only one cable between the box and the panel (Fig. 1). This keeps the panel light and thin, making it easy to hang on the wall.

The system cable is an external display and audio interface, and it brings new requirements to the TV display interface. For example, it has to support a long reach and a 120-Hz refresh rate display data speed. The data on this interface should be protected with a protocol, such as high-definition content protection (HDCP).

For the internal interface, most digital TVs still use low-voltage differential signaling (LVDS) between chips in the system or between boards. Used for many years for its simplicity, it has been scaled in speed and distance far beyond initial expectations. However, LVDS is running out of steam as its high voltage and swing prohibit the technology from supporting higher resolutions and higher refresh rates as well as long-reach applications.

Dual-LVDS links with frequencies going up to 135 MHz can support 120-Hz 1080p resolutions, but the bit error rate goes up beyond this speed, and higher resolutions can’t be supported. If quad-LVDS links with half the speed are used, the number of connections gets so large that board layout becomes an issue.

Each LVDS link runs at seven times the specified frequency—that is, a 135-MHz frequency corresponds to a 945-MHz link speed. Also, the increasing number of links, the high common voltage (3.3 V) and the swing, and the unscrambled data on LVDS links all contribute to the overall electromagnetic-interference (EMI) problem.

The next-generation TV display interface has the challenge of not only replacing the internal interface, but also satisfying the needs of the external interface. DisplayPort is a viable and robust replacement for LVDS, not only because of higher-resolution support but also for lower EMI features. The embedded clock and the use of 8B10B code enables much lower EMI levels than LVDS.

DisplayPort also is an ideal external interface, as it can reach up to 15 to 30 m and comes with HDCP content protection. It additionally can enable a DirectDrive-type of TV panel where the scaler or video processor is integrated into a timing controller (TCON) or pushed into the set-top box (Fig. 2).

Furthermore, DisplayPort is a smart display interface with a dedicated bi-directional command channel (AUX). The AUX benefits are endless, as it enables a paradigm shift in TV design. It is designed to replace existing slow command interfaces, such as I2C, and enable a better coherence between the software of the system and the display signaling, which will benefit heavy image processing in these devices.

The AUX channel can be used for easy debugging of a TV system during the design phase. The processor on the set-top box can be programmed to run pre-set debug or test software that enables run-time checks or recovery from an error state.

During operation, the AUX channel can be used to communicate control information between the panel and the set-top box. The control information can be used in administering newer algorithms, such as LED backlight control, gamma table, or any other image-based display adjustment.

DisplayPort is also a better choice as a next-generation interface than HDMI because it is an open interface standard supported by a large number of companies ranging from OEMs to cable manufacturers. It is also designed for internal and external interfaces and has better EMI characteristics as the link speed is abstracted from the pixel clock and is scrambled to lower EMI.

This technically robust interface can address the internal and external interface needs of next-generation TV platforms. Its flexibility to use its features for content protection, adaptable SERDES speed, or increasing number of links enables it to go into TV applications with varying screen sizes, resolutions, and refresh rates.

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