Today let’s talk about a technology we all interact with almost daily: HDMI. It stands for High-Definition Multimedia Interface and is currently the defacto digital interface for transmitting video and audio signals in a single cable. HDMI was first introduced in 2002 as an alternative to a popular standard at the time called DVI(Digital Visual Interface). It was developed by a group of companies including Sony, Philips, Toshiba, and Hitachi. The HDMI’s first version in 2002 supported 1080p video and 8-channel audio with 4.95Gbps bandwidth. The latest version(v2.1b) supports 8k@60fps and 4k@120fps with a bandwidth of 48Gbps.

Let’s discuss the electrical side.
An HDMI connector consists of 19 pins.
TMDS (Transition Minimized Differential Signaling) Channels (Pins 1-3, 7-9, 13-15): These are the three(D+, D- & Shield) pairs of main channels that carry video and audio data using differential signalling.
Clock(Pins 10-12): TMDS differential clock for synchronization between systems
CEC (Consumer Electronics Control) (Pin 13): This channel allows devices to send control commands to each other, like turning on your TV with your set-top box.
DDC (Display Data Channel) (Pins 15-16): Used for communication between the source and display. It’s I2C based. The display uses it to tell the source what modes/formats it supports.
Hot Plug Detect(Pin 19): Signals the source device when a display is connected/disconnected.
On the physical connector design side, there are 3 main versions. Type A Standard is the most common and biggest one, Type C Mini and Type D Micro are other smaller space-constrained versions. Pin count-wise all are the same except for size reduction. From the PCB routing POV, all the high-speed 4 differential pairs of TMDS signals are to be routed with a single-ended impedance of 50Ω and differential impedance of 100Ω.
HDMI has undoubtedly transformed how we connect our devices over the years. Personally, though I think, its future might be limited, especially with USB Type-C now supporting HDMI signals through Alternate Mode. Probably a decade away from USB Type-C becoming the universal connector for everything.
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