Clearcast — UK Television Advertising Clearance

Clearcast clears advertising for the UK commercial broadcasters. An advert does not reach UK television without passing through it, so its technical specification functions as a delivery requirement regardless of what any loudness recommendation says.

On loudness it defers to the European standard: integrated loudness is stated as EBU R128, −23 LUFS. What makes the document worth reading is the requirement that has no equivalent anywhere else.

Loudness measurement is indifferent to phase. A stereo mix with a polarity-inverted element, an over-wide stereo plug-in on a bus, or a mono element duplicated and delayed can integrate to exactly −23 LUFS, sit under every true-peak ceiling, and still fail this check.

The consequence is not academic. A mix with poor phase coherence partially cancels when summed to mono — which is what happens on a phone speaker, a television with a single driver, and a large part of broadcast distribution. The gate exists because the audience frequently hears the mono sum, and a number caught it more reliably than an engineer listening in stereo would.

The practical answer is to check the mono sum before delivery rather than after rejection, and to treat aggressive stereo widening on a full mix bus as something to justify rather than default to.