Transition Minimized Differential Signaling
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Transition Minimized Differential Signaling (TMDS) is a technology for transmitting high-speed serial data and is used by the DVI and HDMI video interfaces.
The transmitter incorporates an advanced coding algorithm which has reduced electromagnetic interference over copper cables and enables robust clock recovery at the receiver to achieve high skew tolerance for driving longer cable lengths as well as shorter low cost cables.
The encoding is similar to 8B/10B encoding. It is a two-stage process resulting in ten bits being used to represent eight bits. In the first stage each bit is either XOR or XNOR transformed against the previous bit, whilst the first bit is not transformed at all. The encoder chooses between XOR and XNOR by determining which will result in the fewest transitions; the ninth bit is added to show which was used. In the second stage, the first eight bits are optionally inverted to even out the balance of ones and zeroes and therefore the sustained average DC level. The tenth bit is added to indicate whether this inversion took place.
The 10-bit TMDS symbol can represent either an 8-bit data value during normal data transmition, or 2 bits of control signals during screen blanking. Of the 1024 possible combinations of the 10 transmitted bits:
- 460 combinations are used for representing an 8-bit data value, as each of the 256 possible values has two encoded variants (except some values which have one),
- 4 combinations are used for representing 2 bits of control signals (such as HSync and VSync); these combinations have such properties, that they can be reliably recognised even if sync is lost and are therefore used also for synchronizing the decoder,
- 560 remaining combinations are reserved and forbidden.
TMDS was developed by Silicon Image Inc. as a member of the Digital Display Working Group.
