What is TMDS?
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Learn how TMDS encoding reduces EMI and improves high-speed data transmission for digital video signals.
TMDS stands for Transition Minimized Differential Signaling. It’s a method for transmitting high speed digital data. It incorporates a very unique and very clever algorithm that reduces electromagnetic interference (EMI) and enables the clock recovery at prodigious distances, up to 100ft at 1920×1200. It also enables high skew tolerance on cables that are really complex, and based on their original design, should not be able to produce video images from one end to the other. It does all of this with a very high level of confidence.
TMDS is a way of encoding the signal to protect it from degrading as it travels down the length of the cable. TMDS is a lot like RGBHV, and much like the analog world we live in today, in that it uses four channels: Red, Green, Blue and Clock.
Here’s what happens:
- The sending device, such as an HD-DVD player, encodes the signal to reduce the number of transitions between one (on) and zero (off). Think of each transition as a sharp drop-off — as the signal travels, this drop-off can begin to wear away, degrading the signal. The encoding step helps protect signal quality by reducing the number of chances for the signal to degrade.
- One of the cables in the twisted pair carries the signal itself. The other carries an inverse copy of the signal.
- The receiving device, such as an HDTV, decodes the signal. It measures the differential, or the difference between the signal and its inverse. It uses this information to compensate for any loss of signal along the way.
HDMI also has the ability to protect data from piracy. It uses high-bandwidth digital copy protection(HDCP) to accomplish this. HDCP is an authentication protocol. Basically, each home-theater device has identification data and encryption data stored on its extended display identification data(EDID) chip. The source device, such as a Blu-Ray player, checks the authentication key of the receiving device, such as an HDTV. If both keys check out, the sending device moves on to the next step. It generates a new key and shares it with the receiving device. In other words, it creates a shared secret. Ideally, this whole process, known as a handshake, takes place almost instantaneously.
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