Transition-Minimized Differential Signaling

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Transition-Minimized Differential Signaling (TMDS) is an interface standard developed by Silicon Image for the serial transmission of digital video signals. The standard is used for interfaces such as Digital Visual Interface (DVI) and High-Definition Multimedia Interface (HDMI). Up to 5.94 Gbit / s can be transmitted per TMDS transmission line, which is also referred to as “lane”.

With video interfaces such as DVI (specification from 1999) in the simple basic variant, three lanes are used to transmit the digital RGB signal , one for each color channel. A clock signal is also transmitted. Due to the cable attenuation, up to 165 megapixels per second can be transmitted between the computer and the monitor with the maximum cable lengths of 15 meters , which enables a maximum resolution of 1600 × 1200 pixels ( UXGA ) or 1920 × 1200 (WUXGA) at 60 Hz refresh rate if the graphics card and monitor support " reduced blanking ". In an extended version with a dual link (24 + x contacts) DVI connector, six lanes are used to connect max. 330 megapixels per second to transmit what a resolution of z. B. 2048 × 1536 pixels ( QXGA ) at 60 Hz makes possible.

General

TMDS consists of two main functional blocks:

  1. A special channel coding to minimize the signal transitions in order to minimize high-frequency signal components and to keep the electromagnetic interference during the transmission low.
  2. A physical transmission layer with symmetrical signal transmission in the Current Mode Logic (CML). A lane consists of an impedance-controlled pair of wires in the form of a twin wire .
Schematic and simplified structure of a TMDS video interface

Channel coding

TMDS uses a specific 8b10b code to expand bytes to 10-bit long sequences that contain the same number of ones and zeros on a long-term average and at the same time contain as few changes as possible. Of the 1024 possible combinations with 10 bits, only a certain number of combinations is permitted:

  • 460 combinations are used to map the 256 possible combinations of the 8-bit wide net data word. Many of the 8 bit wide net data words have two possible and equivalent combinations in the 10 bit data word. Among other things, this serves to ensure that the signal formed is free of DC components.
  • 4 combinations serve as special control words which are used to represent the frame of the video signal. Their function is roughly comparable with the horizontal and vertical image synchronization pulses required for analog video transmission.
  • The remaining 560 combinations are not used or represent prohibited data words.

In the receiving device - this is usually a monitor with digital video input - decoding takes place in the reverse order to the coding in order to obtain the original video data for the display.

Individual evidence

  1. ↑ Digital Visual Interface & TMDS Extensions, White Paper. Silicon Image, 2004, accessed February 19, 2015 .
  2. ↑ Digital Visual Interface DVI, V1.0. April 1999, accessed February 17, 2015 .
  3. ↑ a b DVI, HDMI Retimer with Input Equalization and Output De-Emphasis: DS34RT5110. Texas Instrument, 2013, accessed February 19, 2015 .