Matrix-addressed displays, which include LCDs, LEDs, and almost every kind of display except the CRT, are currently refreshed one row or one column at a time. This is required for all digitally controlled displays and causes bandwidth limitations due to the high resolutions present in today’s displays. This bandwidth limitation is usually compensated for by using a faster processor, usually at increased expense and power consumption in the display.
Signal Display Systems has patented a method of refreshing all pixels in the display at the same time using the concept of OFDM (Orthagonal Frequency Division Multiplexing), which is commonly used in wireless communications for bandwidth efficiency. With this method and the electronics developed by Signal Display Systems, a 1920 x 1080 display can be refreshed simultaneously with carrier frequencies no higher than 400 KHz.
Standard refresh of an active matrix display requires N writes to the display in order to update the entire display, where N is either the number of rows or the number of columns. Signal Display Systems’ method of using OFDM for displays allows simultaneous refresh for all pixels in the display, so that only one write to the display (N = 1) is required to update the display. This increases the efficiency of the display by a factor of N, where N is typically 1920 or 1080 in high-definition displays such as HDTV.
The OFDM method uses carriers of equally spaced frequencies to transport pixel data in the same row or column simultaneously. If the rows are used for the OFDM carriers, then the columns deliver one carrier frequency each so that the pixel intersection of row and column allows for the demodulation of each carrier on the OFDM row signal. The OFDM modulation of the pixel data improves the signal-to-noise ratio of the data throughout the matrix of the display over the common line-by-line refresh. This improvement in signal-tonoise ratio is similar to the one that occurs for frequency-modulation data over amplitude-modulation data in the commercial radio bands.
The reduced bandwidth required to drive a high-definition display with OFDM also reduces the power consumption of the core processor in the display, resulting in longer battery life for portable display devices. The technology has been demonstrated with LED prototypes using the same elements that are in each cell of a liquid crystal display. This includes single-transistor demodulators and a simple capacitor filter. A version of the technology has been tested that uses OFDM row signals but eliminates the need for column signals from the core processor. This reduces wiring and processing expense. Another version of the technology has been tested for modulation of OFDM carriers over a single power signal to drive multiple LEDs on the same power line. Many more variations are possible.
Development of this technology is at the commercialization stage for LED displays, with LED modules that use OFDM over power lines being licensed and sold in the LED display and lighting market. Versions of the LED displays have been tested that demonstrate the pixel layout of LCD displays, providing a proof-ofconcept for the LCD market. Several LED prototypes have been tested including a version that uses a single-transistor demodulator for comparison in activematrix LCDs.
This technology is available for licensing. For more information, visit www.greendesignbriefs.com/LT0712B.