HDMI-CEC, short for High-Definition Multimedia Interface Consumer Electronics Control, is a low-speed control bus that runs alongside the picture and sound inside every HDMI cable. It carries short commands between devices: wake, go to standby, switch input, report a name, report a power state. The feature exists so a television, a streaming box and a soundbar can be driven from one remote. In advertising it matters for a narrower reason: the bus carries the one signal capable of telling a streaming device whether the screen in front of it is lit, and that signal sits at the centre of the most expensive measurement dispute in connected television.

How the bus works

CEC occupies pin 13 of the nineteen-pin Type A HDMI connector, with pin 17 as its ground. Electrically it is a single-wire bidirectional serial bus, idle at 3.3 volts through a passive pull-up and driven low to transmit. The protocol descends from AV.link, a CENELEC-standardised scheme carried over SCART connectors long before HDMI.

Throughput is roughly 417 bits per second. A frame opens with a start bit held low for 3.7 milliseconds, then carries a header block and up to fifteen data blocks, each ten bits long: eight bits of payload, an end-of-message flag and an acknowledgement bit. Because the line is electrically separate from the high-speed data lanes, a device can shut down its video circuitry entirely and still answer. The bus stays live through equipment that is off.

Two address spaces coexist. The physical address, four nibbles written in the form 1.0.0.0, comes from the extended display identification data (EDID) block a source reads over the display data channel, and describes where a device sits among ports and switches. The logical address, 0 to 14, describes what kind of device it is: 0 for the television, 5 for an audio system, 4, 8 and 11 for playback devices such as streaming sticks and disc players, with 15 acting as broadcast. Fifteen devices is the ceiling.

The messages an ad system would need

Two opcodes carry the argument. Give Device Power Status, opcode 0x8F, asks another device on the bus what state it is in. Report Power Status, opcode 0x90, answers with one of four values: on, standby, in transition from standby to on, or from on to standby. A television powering down also broadcasts Standby, opcode 0x36, to everything attached.

Routing messages answer the second half. Active Source, opcode 0x82, announces which device owns the screen; Request Active Source, Set Stream Path and Inactive Source complete the set. A streaming device therefore has two queries available before the first frame of an advertisement renders: is the display awake, and is this input the one being shown.

Give OSD Name and Give Device Vendor ID let a television ask an attached box what it is called and who made it, which is how a set labels the input a viewer sees.

Origin and evolution

Work on the specification began on April 16, 2002 among founders including Hitachi, Philips, Silicon Image, Sony and Toshiba, with control functions in scope from the start. Version 1.0 was published in December 2002. Early implementations diverged badly, and the correction came on December 14, 2005 with HDMI 1.2a, which fully specified CEC features, command sets and compliance tests. HDMI 1.3a, released on November 10, 2006, reinstated timer control commands and added audio control commands.

One clause explains most of what followed. CEC wiring is mandatory on every compliant device; implementing CEC in software is optional. Manufacturers were obliged to lay the wire and free to ignore it. They were also free to rename it, and did: Anynet+ at Samsung, Bravia Sync at Sony, SimpLink at LG, VIERA Link at Panasonic, EasyLink at Philips, 1-Touch Play on Roku televisions. Vizio and Hitachi use the generic name.

The HDMI Forum took over specification development in 2011, and HDMI 2.0, published in September 2013, brought the extensions now called CEC 2.0. Platform-level standardisation arrived through Android. Before Android 5.0, manufacturers wrote their own CEC stacks or licensed third-party ones; the Android Open Source Project then introduced HdmiControlService, wired into the TV Input Framework, the audio service and the power service, and guarded at the SignatureOrSystem protection level so ordinary applications cannot reach the bus. Android 12 aligned HDMI display power with internal display power: a playback device that wakes attempts to wake the television and claim active source, and switches it off again on sleep.

Why the marketing community argues about it

Television screens switch off while streaming applications keep running, and the resulting impressions are counted and billed. GroupM and iSpot.tv put numbers on it in June 2022, after analysing hundreds of millions of impressions across 20 million Vizio sets between January 1 and June 30, 2021. Between 8 and 10 per cent of streaming impressions were counted with the set off, ranging from 2.5 to 15 per cent by device and publisher. The split inside that figure matters most: 17 per cent for impressions delivered through ancillary devices, and effectively none for applications running on the smart television itself. An application on the set shares an operating system with the panel; a stick on an HDMI port does not, and CEC is the only wire between them.

Stephens analyst Nicholas Zanger valued the overcount at roughly one billion dollars; the Wall Street Journal's estimate ran higher, between 1.5 and 1.9 billion. DoubleVerify, which named the condition TV Off, reported in February 2022 that one in four leading environments kept recording impressions after the set was switched off. Its Fully On-Screen certification, launched in 2020, earned Media Rating Council accreditation for CTV viewability in April 2024. By December 4, 2025 the company was calling publicly for transparency standards, stating that more than one third of connected television impressions deliver into TV-off environments at an annual cost near one billion dollars.

Evidence that the mechanism works arrived in the 2025 North America quality report, which attributed a 16 per cent viewability improvement partly to publishers finally pausing delivery when screens powered down. IAB Europe repeated the one-third figure in March 2026, and a LinkedIn post by ad-fraud investigator Michael M. Maurantonio put the protocol itself into the argument that month, describing CEC as rarely implemented and almost never used in measurement logic.

The standards response runs through the Open Measurement SDK. IAB Tech Lab announced a connected television extension on June 8, 2022, shipping that quarter with two new signals: last activity, to distinguish a watched binge from an abandoned one, and display connection status, which flags that a display is off while an application keeps playing. Coverage widened to Samsung and LG sets in May 2024. The gap is that display connection status is reported by the platform rather than queried from the bus, so its accuracy inherits whatever the manufacturer implemented.

The other thing the bus reveals

CEC also answers questions about the household rather than the screen. On September 12, 2026, Gamers Nexus published packet captures showing an LG television transmitting device identifiers, coordinates, city, state, postcode and the nearest major road to Alphonso, the company's advertising subsidiary, twenty times a minute while the set was in use purely as a computer monitor. The audio fingerprinting payloads were linked in telemetry to HDMI device identifiers retrieved over the source product description InfoFrame and over CEC. The same identity messages that populate an input label can therefore populate a targeting attribute. LG maintains the claims are untrue, and nothing has been tested in court.

Limitations and disputes

The protocol's weaknesses are structural rather than technical. Software support is optional, so no device can be assumed to answer. Hot plug detect, a separate pin, often stays asserted while a television sits in standby, leaving a source rendering video to a dark panel. Implementations disagree about what power state means, and the Android documentation states that two products claiming CEC support cannot be assumed compatible. Nothing on the bus indicates a person is present, so a correct On answer proves only that a backlight is running.

Commercial incentives compound the technical ones. Near-total completion rates support premium pricing, a point made in why 98 per cent completion is automatic rather than earned and in the case against treating connected television like display. Measurement practice often assumes the opposite of what CEC could confirm: Google Ad Manager's inferred viewability treats qualifying connected television inventory as 100 per cent on screen and viewable after two seconds of playback, a presumption rather than an observation.

Not the same as

ARC and eARC share the cable and the marketing copy but move audio rather than commands, sending sound from a television back to a receiver. HDCP is copy protection negotiated over the display data channel, unrelated to control. EDID is the capability block a display advertises to a source over that same channel; CEC borrows a physical address from it and nothing else. ACR, automatic content recognition, fingerprints what appears on screen to identify content, a different question from whether the screen is on, though the LG case shows the two streams travelling together.

Recent developments

Screen-state signalling is moving from product differentiator toward standard. DoubleVerify committed to supporting the Open Measurement SDK for connected television in December 2025, and IAB Tech Lab opened public comment on standardised CTV ad portfolio signals in June 2026. Pre-bid attention products from OpenX and TVision and from Index Exchange route around the problem, buying away from weak environments rather than reading the bus. The Texas action against Hisense has meanwhile made the HDMI port a regulatory subject as well as a measurement one.

Timeline

  • April 16, 2002: HDMI founders begin specification work, with consumer electronics control in scope
  • December 2002: HDMI 1.0 published, defining CEC
  • December 14, 2005: HDMI 1.2a fully specifies CEC features, command sets and compliance tests
  • November 10, 2006: HDMI 1.3a reinstates timer commands, adds audio commands and revises CEC capacitance limits
  • 2011: HDMI Forum established to take over specification development
  • September 2013: HDMI 2.0 introduces CEC extensions, known as CEC 2.0
  • November 2014: Android 5.0 ships HdmiControlService as a standard platform CEC implementation
  • 2020: DoubleVerify launches Fully On-Screen certification for connected television
  • February 2022: DoubleVerify reports one in four leading environments playing on after the set is switched off
  • June 8, 2022: IAB Tech Lab announces Open Measurement SDK for connected television, adding display connection status
  • June 13, 2022: GroupM and iSpot.tv publish findings of 8 to 10 per cent of impressions counted with the television off
  • October 2021: Android 12 aligns HDMI display power control with internal display power
  • April 2024: DoubleVerify receives Media Rating Council accreditation for CTV viewability
  • May 2024: Open Measurement SDK for connected television extends to Samsung and LG platforms
  • December 4, 2025: DoubleVerify calls for streaming transparency standards, citing more than one third of impressions in TV-off environments
  • March 2026: Screen-state detection surfaces as a public industry argument, with HDMI-CEC named as the unused signal
  • June 2026: IAB Tech Lab opens public comment on standardised CTV ad portfolio signals
  • September 12, 2026: Packet captures published showing HDMI device identifiers retrieved over CEC and bundled with location telemetry

Summary

Who: The HDMI Forum and HDMI Licensing Administrator maintain the specification; television and streaming device manufacturers decide whether to implement it; the Android Open Source Project and LG's webOS provide platform-level services for it; DoubleVerify, IAB Tech Lab, GroupM and iSpot.tv have driven the advertising argument about its absence.

What: A single-wire, roughly 417 bit per second control bus on pin 13 of the HDMI connector, addressing up to fifteen devices, carrying power, routing and identity commands including Give Device Power Status and Report Power Status.

When: Defined in HDMI 1.0 in December 2002, fully specified with compliance tests in HDMI 1.2a on December 14, 2005, extended in HDMI 1.3a in 2006 and HDMI 2.0 in 2013, and standardised at platform level in Android from version 5.0 onward.

Where: Inside every compliant HDMI cable and connector worldwide, with the advertising dispute concentrated in the United States connected television market, where the largest volume of streaming inventory trades programmatically.

Why: The protocol can establish whether a television is powered on and which input is displayed, which is precisely what impression counting in connected television does not verify. Estimates of the resulting waste have ranged from one billion to 1.9 billion dollars a year, and the same identity messages that could protect that spend are now documented feeding household data into advertising telemetry.