Transition Minimized Differential Signaling

Transition Minimized Differential Signaling (TMDS) is a technology for transmitting high-speed serial data and is used by the DVI and HDMI video interfaces, as well as other digital communication interfaces. It replaces RAMDAC, used in analog VGA video interface.

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 method is a form of 8b/10b encoding but using a non-standard code-set that differs from the IBM originated form. A two-stage process converts an input of 8 bit into a 10 bit code with particular desirable properties. 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 zeros 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 transmission, 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 recognized 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.

TMDS is similar to Low-Voltage Differential Signaling (LVDS) in that it uses differential signaling to reduce electromagnetic interference (EMI) which allows faster signal transfers with increased accuracy. TMDS signaling also uses a twisted pair for noise reduction, rather than coaxial cable that is conventional for carrying video signals. Like LVDS, the data is transmitted serially over the data link. When transmitting video data, three TMDS twisted pairs are used to transfer video data. each of the three links corresponds to a different RGB component.

The physical layer for TMDS is Current Mode Logic (CML), DC coupled and terminated to 3.3 Volts. While the data is DC balanced (by the encoding algorithm), the DC coupling is part of the specification. TMDS signals can be switched or repeated by any method applicable to CML signals. However, if DC coupling to the transmitter is not preserved, some transmitters' "monitor detection" features may not work properly.

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