ITU-R BS.1770 — The Measurement Underneath Everything

Every loudness number on every delivery spec sheet — the European −23, the American −24, the streaming −14 — is a measurement made by the same algorithm. ITU-R BS.1770 defines it, and the regional standards do not redefine the measurement; they only choose a target value to aim it at.

That is the whole explanation for the most persistent confusion in the field. ATSC, the North American standard, calls the unit LKFS. The EBU, in Europe, calls it LUFS. They are the same unit, produced by the same computation, and one Loudness Unit equals one decibel. A file measuring −23 LUFS measures −23 LKFS. Nothing converts between them because there is nothing to convert.

So the difference between the American target of −24 and the European −23 is not a difference of standards or of units. It is one decibel — small enough that people assume it cannot matter, and large enough that an automated compliance system will reject the file.

A plain average of a programme's loudness would be dragged down by every silence and every quiet passage, so a film with long atmospheric stretches would measure quieter than a continuous dialogue piece even if the dialogue in both sat at the same level. The measurement would then be about pacing rather than about loudness.

BS.1770 solves this with level gating: passages below a relative threshold are excluded from the integrated figure. EBU R 128 specifies the gated form explicitly and identifies it by equation number, and describes the consequence as measuring foreground loudness — the part an audience actually perceives as the programme, rather than the gaps between it.

This is why a quiet, dynamic mix and a dense, compressed one can both deliver at −23 LUFS while sounding nothing alike. The target constrains where the foreground sits. It does not constrain dynamic range, which is measured separately as Loudness Range.

BS.1770-5 and the current EBU R 128-2023 were both approved in November 2023. A spec sheet written before then may cite BS.1770-4, and older delivery paperwork frequently still does. In practice the targets have not moved, so a file that complies under one revision is not suddenly out of compliance under the next — but if a broadcaster's paperwork and your meter disagree about which revision is in play, that is the first thing to check rather than the last.