![Schematic TMDS link](https://upload.wikimedia.org/wikipedia/commons/d/d1/Schematic_TMDS_link.svg)
Transition-minimized differential signaling (TMDS), a
technology for transmitting high-speed
serial data
In the pursuit of knowledge, data (; ) is a collection of discrete values that convey information, describing quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted ...
, is used by the
DVI
Digital Visual Interface (DVI) is a video display interface developed by the Digital Display Working Group (DDWG). The digital interface is used to connect a video source, such as a video display controller, to a display device, such as a comp ...
and
HDMI
High-Definition Multimedia Interface (HDMI) is a proprietary audio/video interface for transmitting uncompressed video data and compressed or uncompressed digital audio data from an HDMI-compliant source device, such as a display controller, ...
video interfaces, as well as by other digital communication interfaces.
The transmitter incorporates an advanced coding algorithm which reduces
electromagnetic interference
Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electros ...
over copper cables and enables robust
clock recovery
In serial communication of digital data, clock recovery is the process of extracting timing information from a serial data stream itself, allowing the timing of the data in the stream to be accurately determined without separate clock information. ...
at the receiver to achieve high
skew
Skew may refer to:
In mathematics
* Skew lines, neither parallel nor intersecting.
* Skew normal distribution, a probability distribution
* Skew field or division ring
* Skew-Hermitian matrix
* Skew lattice
* Skew polygon, whose vertices do not ...
tolerance for driving longer cables as well as shorter low-cost cables.
Coding
The method is a form of
8b/10b encoding
In telecommunications, 8b/10b is a line code that maps 8-bit words to 10-bit symbols to achieve DC balance and bounded disparity, and at the same time provide enough state changes to allow reasonable clock recovery. This means that the diff ...
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
The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represente ...
is either
XOR
Exclusive or or exclusive disjunction is a logical operation that is true if and only if its arguments differ (one is true, the other is false).
It is symbolized by the prefix operator J and by the infix operators XOR ( or ), EOR, EXOR, , ...
or
XNOR
The XNOR gate (sometimes XORN'T, ENOR, EXNOR or NXOR and pronounced as Exclusive NOR. Alternatively XAND, pronounced Exclusive AND) is a digital logic gate whose function is the logical complement of the Exclusive OR ( XOR) gate. It is equival ...
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:
* 460 combinations are used to represent an 8-bit data value, as most of the 256 possible values have two encoded variants (some values have only one),
* 4 combinations are used to represent 2 bits of control signals (C0 and C1 in the table below); unlike the data symbols these have such properties that they can be reliably recognized even if sync is lost and are therefore also used for synchronizing the decoder,
* 2 combinations are used as a guard band before HDMI data,
* 558 remaining combinations are reserved and forbidden.
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
Horizontal synchronization (HSync) and
Vertical synchronization
Analog television is the original television technology that uses analog signals to transmit video and audio. In an analog television broadcast, the brightness, colors and sound are represented by amplitude, phase and frequency of an analog s ...
(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
High-bandwidth Digital Content Protection (HDCP) is a form of digital copy protection developed by Intel Corporation to prevent copying of digital audio and video content as it travels across connections. Types of connections include DisplayPort ...
status and so on.
TMDS was developed by
Silicon Image Inc.
Silicon Image is a provider of semiconductors for the mobile, consumer electronics and personal computers (PCs). It also manufactures wireless and wired connectivity products used for high-definition content. The company’s semiconductor and IP p ...
as a member of the
Digital Display Working Group.
TMDS is similar to
low-voltage differential signaling
Low-voltage differential signaling (LVDS), also known as TIA/EIA-644, is a technical standard that specifies electrical characteristics of a differential, serial signaling standard. LVDS operates at low power and can run at very high speeds ...
(LVDS) in that it uses
differential signaling
Differential signalling is a method for electrically transmitting information using two complementary signals. The technique sends the same electrical signal as a differential pair of signals, each in its own conductor. The pair of conducto ...
to reduce
electromagnetic interference
Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electros ...
(EMI) which allows faster signal transfers with increased accuracy. TMDS also uses a
twisted pair
Twisted pair cabling is a type of wiring used for communications in which two conductors of a single circuit are twisted together for the purposes of improving electromagnetic compatibility. Compared to a single conductor or an untwisted ba ...
for noise reduction, rather than
coaxial cable
Coaxial cable, or coax (pronounced ) is a type of electrical cable consisting of an inner conductor surrounded by a concentric conducting shield, with the two separated by a dielectric ( insulating material); many coaxial cables also have a p ...
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 different
RGB
The RGB color model is an additive color model in which the red, green and blue primary colors of light are added together in various ways to reproduce a broad array of colors. The name of the model comes from the initials of the three addi ...
component.
The physical layer for TMDS is
current mode logic
Current mode logic (CML), or source-coupled logic (SCL), is a digital design style used both for logic gates and for board-level digital signaling of digital data.
The basic principle of CML is that current from a constant current generator is ...
(CML),
DS34RT5110 DVI, HDMI Retimer with Input Equalization and Output De-Emphasis
/ref> DC coupled
In electronics, direct coupling or DC coupling (also called conductive coupling and galvanic coupling) is the transfer of electrical energy by means of physical contact via a conductive medium, in contrast to inductive coupling and capacitive coup ...
and terminated 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.
See also
*S/PDIF
S/PDIF (Sony/Philips Digital Interface) is a type of digital audio interface used in consumer audio equipment to output audio over relatively short distances. The signal is transmitted over either a coaxial cable (using RCA or BNC connectors) ...
*LVDS
Low-voltage differential signaling (LVDS), also known as TIA/EIA-644, is a technical standard that specifies electrical characteristics of a differential, serial signaling standard. LVDS operates at low power and can run at very high speeds ...
*display controller
A video display controller or VDC (also called a display engine or display interface) is an integrated circuit which is the main component in a video-signal generator, a device responsible for the production of a TV video signal in a computing ...
References
{{reflist
Digital display connectors
Line codes