HDMI Redrv
Sits in series with each TMDS lane (D0/D1/D2/Clock), boosts the high-frequency content with a programmable continuous-time linear equaliser (CTLE), and re-launches the signal with a controlled amplitude and pre-emphasis profile.
Each lane has a CTLE whose zero-frequency boost is set by registers (typically 0–13 dB at 3 GHz), feeding an output driver with programmable de-emphasis and swing. There's no clock-data recovery — it's a memoryless analogue stage, so jitter passes through unaltered. An I²C control bus exposes per-channel configuration; some redrivers also auto-detect TMDS rate and pick a CTLE profile.
In plain terms
Like a megaphone for a tired voice on a long hallway — the speaker's words are still the same but louder and crisper, just enough that the listener at the far end can still parse the syllables.
Why designers use it
- Restore margin on long HDMI cables — a 5 m passive Cat-2 cable hits 6 dB of loss at 3 GHz and pushes a 6 Gb/s lane outside the receiver eye mask without help.
- Cross board-to-board connectors and ribbon cables — every additional connector eats a dB or two; redrivers buy back the budget.
- Cheaper than a retimer — no PLL, no CDR, no protocol awareness, so power and silicon area are a fraction of a retimer's.
- Transparent to HDCP and audio — because the redriver doesn't reclock, the signal still has the source's encryption and audio tunnels intact.
Best for
- Long cables
- Source devices
- HDMI splitters
Key specifications
- Operating limits: Check the exact manufacturer's datasheet (A family name does not establish voltage, current, temperature or timing limits.)
- Pin assignment: Match the complete part and package code (A similar name or function does not guarantee the same wiring.)
- Mechanical fit: Use the exact package drawing (Check pad layout, dimensions and viewing direction before building.)
When not to use it
- When jitter dominates over loss — a redriver passes jitter through; if the source is the noisy one, only a retimer (with CDR) helps.
- At HDMI 2.1's FRL 12 Gb/s rates over very long links — losses exceed CTLE's reach; switch to a retimer or active optical cable.
Common mistakes
- Cranking the CTLE boost to maximum — over-equalisation creates pre-cursor ISI that closes the eye more than the under-equalised case.
- Putting the redriver right next to the source's pre-emphasis — both stages add boost and the receiver sees ringing instead of recovery.
Where you will find it
- An LG OLED C2 65-inch TV uses a Texas Instruments TMDS181 redriver behind each HDMI 2.1 input: the chip's 12 dB CTLE boost reopens the eye after a 3 m AmazonBasics 'High Speed' cable, letting the panel handle 4K-120 Hz from a PlayStation 5 without the random 'No Signal' drops cheap cables otherwise cause.
- An ATEN VS0801H 8-port HDMI switch sits four Parade PS133 redrivers across its lane multiplexer: each redriver compensates for the trace and analog mux loss between the eight inputs and the single output, meaning the source still meets the HDMI 2.0 6 Gb/s eye even after the longest internal route.
- A Belkin AVC012 HDMI 2.1 active optical cable embeds a small ICN1117 redriver at each end: the source-side redriver pre-equalises before the optical modulator, and the sink-side redriver post-equalises out of the photodiode TIA, letting the cable deliver 12 Gb/s FRL across 30 m without the consumer paying for a full retimer.
A short history
An HDMI redriver is a signal-conditioning device placed in the HDMI path to counteract deterministic jitter, such as inter-symbol interference caused by predictable cable or trace attenuation. It works through equalization, compensating for input channel loss without the clock data recovery found in retimers. Redrivers are often used in source-side devices like PCs or players, where signal loss is largely predictable, helping support compliance and interoperability.