KNOW THE LINGO — WHAT IS TMDS?

By: Joseph D. Cornwall, CTS-D
Technology Evangelist — C2G

Technical lingo is a kind of shorthand that's used to express concepts common to that specific topic or area of study. Technical lingo is important because it provides a very precise or unique "shorthand" description of a device, effect or concept. Unfortunately, if you aren't comfortable and familiar with the lingo of a topic it can be a tall hurdle to communicate efficiently with folks who consider the jargon of their field to be "self-explanatory." In this series of articles we'll lift the veils of misunderstanding from the lingo of the A/V industry.

​​​​WHAT IS TMDS?

Transition Minimized Differential Signaling (TMDS) is the algorithm that underlies both DVI and HDMI digital video interfaces, and is an output option in DisplayPort ++ (dual mode DisplayPort, aka DisplayPort 1.2) enabled devices. TMDS is a foundational element of contemporary A/V integration and needs to be a familiar concept to anyone involved in the design, installation or deployment of presentation systems.

Uncompressed digital video data is organized into groups of 8-bit words (assuming standard bit depth—this number can increase to 16-bits in xvYCC content—a topic for another day). Each digital word is used to describe the amplitude of the red, green and blue components (one word per color) that together comprise an individual pixel in a high definition digital video image. The electronic waveform that represents the transition from a digital "0" to a "1"—essentially a square wave—is very rich in high frequency content. Square waves are difficult to transport and condition without distortion. A method to minimize these transitions was needed in order to help limit the bandwidth of the digital data stream to a practical and controllable level. Limiting and controlling signal bandwidth makes the data stream easier to send over the physical layer of cables and components.

To minimize symbol transitions, a line code called 8b/10b is used. It maps the 8-bit uncompressed digital data to 10-bit symbol words. The two added bits allow an encoder to balance the number of "0" and "1" states in the data stream to an even amount, resulting in a sustained average DC level. In other words, the total amount of energy in the wire doesn't change much over time, allowing for stable electromagnetic fields. And because each transition between a low state (0) and high state (1) includes a large amount of harmonics (take a look at that square wave again), then minimizing the number of those transitions ensures the resulting electromagnetic waveform carried by copper conductors is bandwidth constrained.

​​​​​​​In TMDS, the transition minimized content is transmitted as two complementary signals in a differential mode over a shielded twisted pair (STP) of wires. Differential signals are mirror images of each other, where one side of the pair is exactly opposite, in both polarity and amplitude, from the other. The information (signal) is found in the electronic difference between the two wires. This is called a "balanced" state. Balanced connections, by their very nature, minimize induced noise (noise from outside the system) and improve resistance to electromagnetic interference (hence their ubiquitous use in microphone connections, for example).In a TMDS system, one twisted pair is used to transmit each of the red, green and blue (RGB) video components, for a total of three links. A fourth shielded twisted pair of wires is used to send clock information. In short, TMDS is a digital version of sRGB.