Transition Minimized Differential Signaling
Encyclopedia
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.
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.
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.
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 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 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.
Digital Visual Interface
The Digital Visual Interface is a video interface standard covering the transmission of video between a source device and a display device. The DVI standard has achieved widespread acceptance in the PC industry, both in desktop PCs and monitors...
and HDMI
High-Definition Multimedia Interface
HDMI is a compact audio/video interface for transmitting uncompressed digital data. It is a digital alternative to consumer analog standards, such as radio frequency coaxial cable, composite video, S-Video, SCART, component video, D-Terminal, or VGA...
video interfaces, as well as other digital communication interfaces.
The transmitter incorporates an advanced coding algorithm which reduces electromagnetic interference
Electromagnetic interference
Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
over copper cables and enables robust clock recovery
Clock recovery
Some digital data streams, especially high-speed serial data streams are sent without an accompanying clock signal. The receiver generates a clock from an approximate frequency reference, and then phase-aligns to the transitions in the data stream with a phase-locked loop...
at the receiver to achieve high skew
Skew
Skew may refer to:In mathematics:* Skew lines, lines that are neither parallel nor intersecting* Skew-symmetric matrix, a square matrix whose transpose is also its negative...
tolerance for driving longer cables as well as shorter low cost cables.
Coding
The method is a form of 8b/10b encoding8B/10B encoding
In telecommunications, 8b/10b is a line code that maps 8-bit symbols to 10-bit symbols to achieve DC-balance and bounded disparity, and yet provide enough state changes to allow reasonable clock recovery. This means that the difference between the count of 1s and 0s in a string of at least 20 bits...
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
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
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
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
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 for representing 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 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.
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 | 0 | 0010101011 |
| 0 | 1 | 0010101010 |
| 1 | 0 | 1101010100 |
| 1 | 1 | 1101010101 |
TMDS was developed by Silicon Image Inc.
Silicon Image Inc.
Silicon Image is a provider of wireless and wired connectivity solutions that enable the reliable distribution and presentation of high-definition content for consumer electronics, mobile, and PC markets...
as a member of the Digital Display Working Group
Digital Display Working Group
The Digital Display Working Group was organized by Intel Corporation, Silicon Image, Inc., Compaq Computer Corp., Fujitsu Limited, Hewlett-Packard Company, International Business Machines Corp., and NEC Corporation....
.
TMDS is similar to low-voltage differential signaling (LVDS) in that it uses differential signaling
Differential signaling
Differential signaling is a method of transmitting information electrically by means of two complementary signals sent on two separate wires. The technique can be used for both analog signaling, as in some audio systems, and digital signaling, as in RS-422, RS-485, Ethernet , PCI Express and USB...
to reduce electromagnetic interference (EMI) which allows faster signal transfers with increased accuracy. TMDS also uses a twisted pair
Twisted pair
Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...
for noise reduction, rather than coaxial cable
Coaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...
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
Current mode logic
Current mode logic , or source-coupled logic , is a differential digital logic family intended to transmit data at speeds between 312.5 Mbit/s and 3.125 Gbit/s over a standard printed circuit board....
(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 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.