HDMI 2.1 HDMI Specification Version 2.1

Specification basics

The HDMI Forum released Version 2.1 of the HDMI Specification on November 28, 2017, after a January 4, 2017 announcement earlier in the year. The headline number is bandwidth: HDMI 2.1 raises the link ceiling to 48 Gbps, distributed across four data channels each running at 12 Gbps.

Reaching that ceiling required a new signaling scheme. HDMI 2.1 introduces Fixed Rate Link (FRL), a packet-based format with an embedded clock that frees what used to be the dedicated TMDS clock channel and turns it into a fourth data channel. FRL also moves to 16b/18b encoding rather than the 8b/10b encoding TMDS used. TMDS itself is retained for backwards compatibility — if FRL link training fails, the connection falls back to TMDS.

Features introduced

On the video side, HDMI 2.1 supports 4K at 120 Hz, 8K at 60 Hz, and resolutions up to 10K, with Display Stream Compression brought in for the highest formats.

Variable Refresh Rate (VRR) lets the display's refresh rate track the GPU's frame output, reducing or eliminating lag, stutter, and frame tearing during gaming. Auto Low Latency Mode (ALLM) lets a source — typically a console — tell the display to switch into its lowest-latency picture mode automatically, then switch back when the source changes. Quick Frame Transport (QFT) reduces latency a different way: each frame is bursted across the link faster than the display's refresh rate would require, finishing the transfer earlier in the frame interval.

On the audio side, HDMI 2.1 introduced Enhanced Audio Return Channel (eARC). eARC carries object-based and high-bitrate formats — Dolby Atmos, DTS:X, Dolby TrueHD, DTS-HD Master Audio — at up to 192 kHz / 24-bit and up to 32 channels uncompressed, well beyond what legacy ARC could handle.

Differences from HDMI 2.0

HDMI 2.0 ran TMDS (Transition-Minimized Differential Signaling) over three data lanes plus one clock lane, with a maximum link rate of 18 Gbps — about 14.4 Gbps usable after the 8b/10b overhead. HDMI 2.1's 48 Gbps ceiling is roughly 2.7× higher, which is why a new cable was needed alongside the new signaling.

Reaching the full 48 Gbps requires the Ultra High Speed HDMI Cable, often called the "48G cable." FRL operates at roughly 6 GHz fundamental frequency — about double HDMI 2.0's TMDS ceiling — so cables need tighter shielding to suppress EMI ingress and emission. The Ultra High Speed cable was introduced as part of the HDMI 2.1 spec and reached the market in 2020.

Real-world implementation

One piece of HDMI 2.1 catches buyers out, and it is in the spec by design rather than a manufacturer mistake. Every HDMI 2.1 capability — FRL, the higher bandwidths, VRR, ALLM, QFT, and the rest — is optional, not mandatory. A device can carry the HDMI 2.1 label while implementing none of them.

This is compounded by the HDMI Forum's position that HDMI 2.0 no longer exists as a referenceable spec. Its features are now considered a subset of HDMI 2.1, and devices are not supposed to claim compliance to 2.0 anymore. The practical consequence is that some "HDMI 2.1" ports run only TMDS at 18 Gbps and top out at roughly 1080p/240 Hz or 4K/60 Hz, with no FRL, no 4K/120, and no 8K. Xiaomi disclosed exactly this on at least one monitor: "The HDMI 2.1 interface of this product supports the TMDS protocol, the maximum supported resolution is 1920×1080, and the maximum refresh rate is 240Hz." The HDMI label, on its own, does not tell you what a port can actually do — feature lists and signaling details do.

The Ultra High Speed cable side of the spec is policed more tightly. Ultra High Speed HDMI Cables must be certified through HDMI Forum Authorized Testing Centers, and certified cables carry an authentication label with a scannable QR code. The program also runs ongoing market sampling.

HDMI 2.1 is no longer the newest spec on the books. HDMI 2.2 was announced on January 6, 2025 and released on June 25, 2025, raising maximum bandwidth to 96 Gbps via a next-generation FRL, introducing a new Ultra96 HDMI Cable certification, and supporting formats such as 8K at 60 Hz 4:4:4 and 4K at 240 Hz 4:4:4 at 10-bit and 12-bit color. HDMI 2.1 and the 48 Gbps Ultra High Speed cable remain valid; HDMI 2.2 is the successor spec, not a renaming of 2.1 device labeling.