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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.

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.

The 10-bit TMDS symbol can represent either an 8-bit data value during normal data transmission, or 2 bits of control signals during screen blanking. Of the 1,024 possible combinations of the 10 transmitted bits:

Control data is encoded using the values in the table below. Control data characters are designed to have a large number (7) of transitions to help the receiver synchronize its clock with the transmitter clock.

On Channel 0 the C0 and C1 bits encode the HSync and VSync signals. On the other channels they encode the CTL0 through CTL3 signals which are unused by DVI but in the case of HDMI are used as a preamble indicating the type of data about to be transferred (Video Data or Data Island), the HDCP status and so on.

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Wikipedia