Fixed Rate Link HDMI Fixed Rate Link (FRL)

What Fixed Rate Link is

HDMI.org says the 48 Gbps bandwidth of HDMI 2.1 required a new signaling technology, Fixed Rate Link, with a fundamental frequency up to 6 GHz. That is double the HDMI 2.0 limit and twelve times that of a Category 6A cable. HDMI.org also states the consequence: the only way 4K at 120 Hz can be achieved is FRL, which is needed for higher frame rates and resolutions, while FRL is not needed for some other features such as eARC. Allion Labs puts it in terms of wires. FRL lanes 0 to 3 correspond to the TMDS clock and data channels 0 to 2, and because FRL carries no separate clock, all four pairs move data. Transmission is AC-coupled, with a coupling capacitor defined at 100 nF to 250 nF, where TMDS was DC-coupled.

How FRL works: lanes, rates and coding

FRL runs either three or four lanes. Allion lists the tiers: 3 lanes at 3 Gbps (9 Gbps total), 3 at 6 Gbps (18 Gbps), 4 at 6 Gbps (24 Gbps), 4 at 8 Gbps (32 Gbps), 4 at 10 Gbps (40 Gbps) and 4 at 12 Gbps (48 Gbps). AVPro Global names them FRL1 through FRL6. Coding moves from 8b/10b to 16b/18b, so 16 of every 18 bits are payload, 88.9% against TMDS's 80%. AVPro gives HDMI 2.1 a maximum data rate of 42.6 Gbit/s inside the 48.0 Gbit/s transmission rate, which matches 48 x 16/18 = 42.67 Gbps.

FRL is packetized and protected. Allion describes a character block of up to 502 FRL characters plus 8 characters of Reed-Solomon FEC parity, making a 510-character FRL block, and four blocks form a super block. Before video flows, source and sink run link training through six states: the source reads the sink's EDID to verify FRL support, prepares, and starts training; the sink can request a new rate; state P means training passed and FRL transmission begins; state L means training failed and TMDS is initiated instead. Allion also says FRL adopts VESA DSC and supports up to 10K at 120 Hz.

Electrical testing differs from TMDS as well. Allion says the HDMI 2.1 specification calls for a TP2_EQ eye compliance test rather than TP1, and that FRL jitter characteristics include random and deterministic jitter instead of clock jitter. Its test list keeps FRL electrical, protocol, pixel decoding, video timing and EDID tests separate from the TMDS ones, so a port can pass TMDS compliance and still never have been tested for FRL.

FRL vs TMDS: the bandwidth math

HDMI 2.0 TMDS is 3 lanes at 6 Gbps, or 18 Gbps, and 8b/10b coding leaves 14.4 Gbps usable. FRL's top tier leaves 42.67 Gbps. The figures below count active pixels only and ignore blanking, so real requirements are higher.

4K60 at 8-bit RGB needs 3840 x 2160 x 60 x 24 = 11.94 Gbps, which fits TMDS. 4K60 at 10-bit needs 14.93 Gbps, already past 14.4 Gbps. 4K120 at 10-bit needs 29.86 Gbps, fine for FRL but impossible on TMDS; TFTCentral notes TMDS 4K120 only works at 8-bit with 4:2:0 chroma subsampling. 8K60 at 8-bit needs 47.78 Gbps and at 10-bit 59.72 Gbps, both beyond 42.67 Gbps, so 8K60 over HDMI 2.1 needs compression or reduced chroma. For the tiers, see HDMI 2.1 bandwidth tiers, and for the compression, display stream compression.

The naming trap: HDMI 2.1 without FRL

HDMI.org says support for each HDMI 2.1 feature is optional, and a product can claim HDMI 2.1 compliance while supporting only TMDS and 18 Gbps, as long as it lists the features it supports. HDMI.org adds that products can no longer be certified for HDMI 2.0, only as HDMI 1.4b or HDMI 2.1. TFTCentral found the result in the Xiaomi Fast LCD Monitor 24.5-inch 240Hz version: it advertised 2 HDMI 2.1 ports, but its small print limits it to 1920 x 1080 at 240 Hz over TMDS. Check the feature list, not the version number. A genuine FRL port should state 4K120 or higher. See HDMI 2.1 and 4K120.

Common confusions

FRL is a signaling mode, not a cable or a version number. The 48 Gbps figure is the transmission rate and 42.6 Gbps is the data rate, so treating 48 Gbps as payload overstates capacity by 12.5%. Allion notes that FRL link training is compatible with TMDS, and a failed training run initiates TMDS, so a source and display that cannot agree on an FRL rate still show a picture at a lower mode instead of a blank screen.

FRL and cables

The Ultra High Speed HDMI Cable program tests cable signal quality to the full 48 Gbps bandwidth, low EMI and FRL signaling, and every length of every cable must be tested individually. The practical limit is distance: HDMI.org notes that passive cables have become shorter or thicker as speeds rose, and many makers now use active optical cables; see HDMI cable length limits and Ultra High Speed HDMI cable.

What comes next: Next-Gen FRL

HDMI 2.2 carries a Next-Gen Fixed Rate Link at 96 Gbps. HDMI.org lists uncompressed 8K60 4:4:4 and 4K240 4:4:4 at 10-bit and 12-bit, and compressed modes up to 4K at 480 Hz, 8K at 240 Hz and 12K at 120 Hz, with a matching Ultra96 cable. See HDMI 2.2 and Ultra96 HDMI cable. For gaming displays and consoles that need the extra headroom, PS5 Pro 120Hz TV picks and the HDMI cable guide cover what to buy today.