Transition-minimized differential signaling

Transition-minimized differential signaling (TMDS), a technology for transmitting high-speed serial data, is used by the DVI and HDMI video interfaces, as well as by other digital communication interfaces.

The transmitter incorporates an advanced coding algorithm which reduces electromagnetic interference over copper cables and enables robust clock recovery at the receiver to achieve high skew tolerance for driving longer cables as well as shorter low-cost cables.

Coding

The method is a form of 8b/10b encoding but using a code-set that differs from the original IBM form. A two-stage process converts an input of 8 bits into a 10 bit code with particular desirable properties. In the first stage, the first bit is untransformed and each subsequent bit is either XOR or XNOR transformed against the previous bit. The encoder chooses between XOR and XNOR by determining which will result in the fewest transitions; the ninth bit encodes which operation was used. In the second stage, the first eight bits are optionally inverted to even out the balance of ones and zeros and therefore the sustained average DC level; the tenth bit encodes whether this inversion took place.