In the display industry, the consistent trend is to move toward higher-resolution displays. However, increasing the resolution results in increasing the overall data rate both from the host to the panel, and within the panel itself. By the late 1990s, resolutions for LCD (liquid crystal display) panels were moving from VGA (video graphics array) resolutions with a cumulative bandwidth requirement of slightly more than 300 Mbits/s to XGA (extended graphics array) resolutions that required 850 Mbits/s. In addition, UXGA (ultra extended graphics array) resolution and its 2-Gbit/s requirement loomed on the horizon. The increasing frequency was creating problems with the TTL interface between the host and the LCD panel. Power consumption was ballooning, electromagnetic interference (EMI) was on the rise, and larger connectors and cables were required to meet the expanding number of data lines.
In 1999, National Semiconductor released the Open Low-Voltage Differential-Signaling Display Interface, or OpenLDI specification, which serialized 22 TTL signals down to four differential pairs. Because the new interface was low swing (±400 mV versus several volts for TTL) and differential, the total power and EMI were significantly reduced. Also, as the total number of wires was reduced from 22 down to eight, the connectors and cabling shrank, saving system cost and improving the mechanical connection between the host and the panel. OpenLDI is now the de facto standard interface between the graphics controller and the LCD panel in notebook computers.
Once the issues of the host-panel interface were solved, similar issues existed within the panel. For XGA resolutions, system designers were using a dual TTL bus with 36 data signals per bus. National Semiconductor used the success of LVDS and OpenLDI as a baseline for creating another open standard for the Reduced-Swing Differential-Signaling (RSDS) interface to solve intra-panel interface issues. The RSDS interface reduced the total number of wires from 72 (two 36-wide buses) to 20 (10 differential pairs). The voltage swings were ±200 mV differential, reducing both the power and the EMI of the panel. Following the success of OpenLDI, RSDS technology has become the de facto standard for intra-panel interfaces for all notebook and monitor resolutions from XGA through UXGA.
LVDS and RSDS technologies accomplished the same end goal at two different points in the data flow. LVDS technology lowered the power, improved EMI, and reduced the wire count between the graphics controller and the LCD panel. RSDS technology provided the same benefits between the timing controller and the column drivers within the panel. More information on LVDS and RSDS technologies is available at www.national.com.
