HDMI 2.1 Bandwidth Tiers HDMI 2.1 Bandwidth Tiers (40 Gbps vs 48 Gbps)

How HDMI 2.1 bandwidth tiers work

HDMI 2.1 relies on a signaling method called Fixed Rate Link (FRL), used instead of the older TMDS signaling found in previous HDMI generations. FRL is necessary to reach uncompressed resolutions above 4K60 and to achieve the ultra-high-speed bandwidths that scale up to the HDMI Forum's stated maximum of 48 Gbps.

The reason two ports can both carry the "HDMI 2.1" label while supporting different maximum bandwidths comes down to how the specification itself is written: all of the new capabilities and features associated with HDMI 2.1 are optional. That includes FRL, the higher bandwidth tiers, Variable Refresh Rate, and Auto Low Latency Mode. A manufacturer can implement a subset of these and still call the port HDMI 2.1, because the spec does not mandate a single fixed bandwidth floor for the name to apply.

The two real-world tiers

At the full 48 Gbps tier, a device has the complete HDMI 2.1 bandwidth ceiling as defined by the HDMI Forum. This gives the most headroom for demanding combinations of resolution, refresh rate, color depth, and chroma sampling. It is the tier tied to the specification's marquee feature set: resolutions up to 8K60 and 4K120 (and as high as 10K), Dynamic HDR, eARC, VRR, Quick Media Switching, Quick Frame Transport, and ALLM.

At 40 Gbps, many current TVs, gaming displays, and source devices can still handle 4K at 120Hz and 8K in certain formats, but without the same headroom as the full-bandwidth tier. To hit 4K120 within this lower ceiling, a 40 Gbps port typically depends on either chroma subsampling or Display Stream Compression (DSC) rather than passing every combination of resolution, frame rate, bit depth, and chroma fully uncompressed.

Bit depth and chroma sampling directly affect how much bandwidth a given resolution and frame rate actually consume. Full 4:4:4 chroma (useful for desktop use, text clarity, or professional display applications) requires more bandwidth than 4:2:0 or 4:2:2 sampling at the same resolution and frame rate, which is why the same nominal resolution/refresh combination can fit inside different bandwidth tiers depending on how the chroma and bit depth are configured.

It's also worth separating two distinct paths to 4K120: one route uses FRL and the higher-bandwidth tiers described above; a separate, lower-bandwidth route exists using older TMDS-level signaling, where 4K120 is reachable only by dropping to 8-bit color depth with 4:2:0 chroma subsampling. These are two different technical paths to the same nominal resolution and frame rate, not a contradiction. FRL is what's required to reach 4K120 uncompressed or at higher fidelity, while the TMDS path reaches the same numbers only by subsampling more aggressively.

Common confusions

A spec sheet that simply states "HDMI 2.1" is not sufficient to know a port's actual capability, precisely because every headline feature of the specification (the bandwidth tier itself, VRR, ALLM, and the rest) is optional rather than mandatory. Two products can both be truthfully labeled HDMI 2.1 while one ships at 40 Gbps and the other at the full 48 Gbps, or while one supports VRR and the other does not.

DSC, often described as "visually lossless," is not the same thing as uncompressed 4:4:4 signal transmission. It is a compression scheme applied to fit a given resolution/frame rate/chroma combination into less bandwidth than it would require uncompressed. For 8K at 60Hz specifically, some HDMI 2.1 implementations rely on DSC while others depend on lower chroma sampling or specific color settings to fit within their bandwidth tier, so the underlying method varies by device and is worth checking rather than assuming.

What this means when reading a spec sheet

Because the "HDMI 2.1" label covers a range of optional bandwidth tiers and features, confirming the number itself (40 Gbps versus 48 Gbps) along with which specific features (VRR, ALLM, particular resolution/refresh combinations) a given port actually implements is the only way to know what a port can do. The label indicates that a port follows the HDMI 2.1 specification's structure; it does not by itself indicate which of that specification's optional capabilities are present.