HDMI supports video, audio, and data transmission down the cable. Looking at the pin-out, there are 19 pins in an HDMI cable (any version), with most pins allocated to TMDS lines. TMDS, or transition-minimized differential signaling, aids in reducing static noise (EMI, impedance, crosstalk) in the data. This is done by splitting the signal down a positive and negative line, then merging them at the end of transmission to effectively perform a parity check. The process helps strip static introduced during transmission by cross-checking the lines against one another.
As an interface, HDMI 2.0 hosts three 6Gbps TMDS channels that offer a maximum theoretical throughput of 18Gbps. The pixel clock ticks at 600MHz on HDMI 2.0 – 600 million oscillations per second – which is a marked improvement over HDMI 1.4's 340MHz pixel clock.
Let's do some math to look at the throughput potential.
At 4K / 60Hz, a display commands almost the entire interface for data throughput. 4096 * 2160 equals approximately 8.8 million pixels per frame; there are 60 refreshes per second, giving us a pixel-rate of 530 million pixels transferred per second (8.8m px * 60Hz). For clarity, 530 million pixels per second could also be written as 530MHz – now the picture comes together. On HDMI 2.0, we're falling within that 600MHz pixel clock, supporting 4K60 only just. That was the big marketing push with HDMI 2.0.
1080p at 120Hz, then, would fit within even HDMI 1.4b's 340MHz pixel clock (1920 * 1080 = 2.07 million * 120 = 248 million pixels per second). There's more to it than that, though, and HDMI 1.4b simultaneously supports and does not support 1920x1080 at 120Hz. It supports the 120Hz 1080p throughput only for 3D processing, which it does by cloning the data packet for concurrent output to the display device. For 2D viewing – which is what almost all gamers mean when they want “120Hz” – HDMI 1.4b is stuck at just 60Hz for 1080p. This is commonly misunderstood in the HDMI 1.4b spec language.
For HDMI 2.0, 1080p120 falls well within the 600MHz pixel clock and 18Gbps throughput cap (even accounting for 8b/10b encoding). It is possible for 1080p to be supported at 120Hz on HDMI 2.0; unfortunately, very few solutions on-market are capable of actually utilizing this potential. Most monitors selling with HDMI 2.0, like the $500 ASUS PB287Q, are 4K displays that cap-out at 60Hz. This is because HDMI is ultimately a consumer interface that's targeted at TVs, but finds its way into desktop displays. For gaming use cases at higher refresh rates, it is still safest and easiest to just opt for DisplayPort or Dual-Link DVI (something we've previously explained).
For HDMI 2.0 to properly work for higher display refresh rates, the monitor and the video card must both host the HDMI 2.0 interface and, obviously, the monitor has to operate at a higher refresh rate.
One quick note, just to address some other confusion: There's no such thing as “HDMI 2.0 cables.” You can use “old” HDMI cables for HDMI 2.0 devices; just make sure the display device and video card both support the HDMI 2.0 interface.
- Steve “Lelldorianx” Burke.