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Transition-minimized differential signaling (TMDS) is a technology for transmitting high-speed serialdata used by theDVI[1] andHDMI video interfaces, as well as by other digital communication interfaces.
The transmitter incorporates a coding algorithm which reduceselectromagnetic interference over copper cables and enables robustclock recovery at the receiver to achieve highskew tolerance for driving longer cables as well as shorter low-cost cables.
The method is a form of8b/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 subsequentbit is eitherXOR orXNOR 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 averageDC 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.
| Input control bit | Output codeword | |
|---|---|---|
| C0 | C1 | 0 ... 9 |
| 0 | 0 | 0010101011 |
| 0 | 1 | 0010101010 |
| 1 | 0 | 1101010100 |
| 1 | 1 | 1101010101 |
On Channel 0 the C0 and C1 bits encode theHorizontal synchronization (HSync) andVertical synchronization (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), theHDCP status and so on.
TMDS was developed bySilicon Image Inc. as a member of theDigital Display Working Group.
TMDS is similar tolow-voltage differential signaling (LVDS) in that it usesdifferential signaling to reduceelectromagnetic interference (EMI) which allows faster signal transfers with increased accuracy. TMDS also uses atwisted pair for noise reduction, rather thancoaxial cable that is conventional for carrying video signals. Like LVDS, the data is transmitted serially over the data link. When transmitting video data and used in HDMI, three TMDS twisted pairs are used to transfer video data. Each of the three links corresponds to a differentRGB component.
The physical layer for TMDS iscurrent mode logic (CML),[2]DC coupled andterminated to 3.3 Volts. While the data is DC balanced (by the encoding algorithm), DC coupling is part of the specification. TMDS 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.