How HDMI Cables Work: Technology & Background

Plugged in, switched on - and the picture and sound are already there in the best HD quality. That's how we know HDMI cables and that's how we love them. But what looks so simple is actually a relatively complex process.

Today, we'll explain how an HDMI cable works, what the underlying technology is, and why each cable has a "hot connector."

TMDS - The basic technology
The key word to HDMI technology is Transition-Minimized Differential Signaling (TMDS). TMDS is a digital transmission standard from Silicon Image designed to transmit uncompressed multimedia data.

Compared to analog signal transmission, TMDS has the advantage that electromagnetic interference is normally eliminated. Transition Minimized Differential Signaling is used for transmission via DVI and HDMI, especially when high-resolution displays are to be controlled.

Via TMDS, a maximum of 165 megapixels per second can be transmitted with a cable length of 15 meters, which corresponds to a maximum screen resolution of 1,920 x 1,200 pixels at a refresh rate of 60 hertz, provided that the graphics card and monitor support reduced blanking. If this is not the case, TMDS supports a maximum screen resolution of 1,600 x 1,200 pixels.

Three TMDS lines are available as standard, each differential TMSD line can transmit a maximum of 1.65 Gbit/s, the clock frequency is one tenth of the data rate, i.e. a maximum of 165 MHz.

In practical applications, TMDS offers the advantage that very high data volumes can be transmitted digitally and without loss when using a relatively long cable.

By the way: Transition-Minimized Differential Signaling (TMDS) is used in all HMDI cables - regardless of whether the cable costs five or 200 euros.

Structure of an HDMI cable in detail


Each HMDI connector has a total of 19 contacts (pins) that perform different functions. Ten contacts are located at the top of the interface, and nine more at the bottom. The numbering starts from the first pin in the top row (pin 1), pin 2 is the first contact in the bottom row of contacts, pin 3 is the second contact in the top row, and so on.

Contacts 1 to 9 are reserved for the three TMDS channels, these enable the transmission of digital multimedia signals and contain audio and video information.

Contacts 10 to 12 are provided for the synchronization of the signals, these are called the TMDS clock channel.

Contact 13 transmits the CEC channel (Consumer Electronics Control), this sends commands between the connected devices and controls them.

Currently contact 14 has no function, it is reserved for future use scenarios and ensures that the HDMI standard remains expandable.

Contacts 15 and 16 are called the display data channel, also known as DDC (Display Data Channel). EDID (Extended Display Identification Channel) is used to exchange information between connected devices.

Contact 17 is a data shield for the CEC and DDC channels, while contact 18 is responsible for the power supply (+ 5 volts).

Contact 19 is called Hot Plug Detect, this detects if a device is switched on or off or if the HDMI cable is plugged in or unplugged. Just a "hot plug" ;-)

Other special features of a HMDI cable
HDMI cables are always backward compatible. In practice, this means that, for example, a TV that works with HDMI 1.2 or HDMI 1.3 can also be operated with an HDMI 1.4 cable without any problems.

So thank goodness no one has to bother with silly cable versions these days!

HDMI expansion made easy: Multiple devices on one HDMI port without reconnection
HDMI Wireless: Establish Wireless HDMI Connection - How to do it
HDMI 2.0 or: the step to Ultra HD
Convert VGA to HDMI - this is how it works
Flat HDMI cables - a new old trend?
Connect iPhone to TV: AirPlay beats the HDMI adapter
Mini HDMI vs. HDMI vs. Micro HDMI - 3 connections in comparison
slanted! Innovative HDMI cable for very flat TVs
Lightning Adapters: HDMI, VGA and more - an overview