FPD-Link - Transferring The RGB Video-interface

Transferring The RGB Video-interface

FPD-Link became successful at transmitting 18-bit RGB raw video because it reduced the cable size and improved the Electromagnetic compatibility by using LVDS. It uses LVDS to transmit the video data on three twisted pair, and another pair to transmit the LVDS clock signal. These four differential pairs are carrying the same information previously transferred over 22 wires, which is almost a 65% wire reduction. In addition, the close coupling of the twisted pair wires improves the EMC because the equal and opposite LVDS currents in the wires create equal and opposite electromagnetic fields that tend to cancel each other's effects. This reduces the radiated emissions. The reduced vulnerability to electrical noise interference comes from the fact the noise affects both signals commonly. Since the LVDS receiver senses the difference between the two commonly-affected signals, it senses no impact from the common mode noise.

The FPD-Link data transmission scheme serializes seven single-ended data bits per clock cycle into each of the LVDS channels. Therefore, the LVDS bit rate is 7 times the frequency of the clock signal. For example, in the 18-bit RGB application, there are 6 bits each for R, G, and B and an additional 3 bits for horizontal and vertical synch and an enable signal. This means there are 21 total data signals in each clock cycle, which means the 7 to 1 serialization reduces this down to 3 data channels. Then if the clock signal is 50 MHz the LVDS streaming video data rate will be 350 Mbit/s per channel, and the total data transfer rate will be 1,050 Mbit/s over the 3 channels.

This same scheme scales to 24-bit and 30-bit color in the following manner. An FPD-Link interface with 4 data channels and 1 clock (4D+C) then reduces the 28-bit input down to 4 pairs plus the clock, which is perfect for 8-bits per RGB plus 4 video control bits. A 5D+C FPD-Link interface serializes 35-bits per clock cycle, which is 10-bits per RGB plus 5 video control bits.