Open Dynamic Range

ODR specification, version 1.0 — 2026-08-28. This document is normative. Where an implementation and this document disagree, this document is right and the implementation has a bug. The reference implementation is Open Audio Analyzer, and its conformance suite asserts every case in § 7.

Two readings, one definition: a true peak minus the loudness of the same audio, in loudness units.

The arithmetic is that of the peak-to-loudness ratios of AES TD1004 (its PSR and PLR). What this document adds is every choice that note leaves open — the peak, the window, the gate, the statistic, the display — so that two implementations of it cannot disagree by more than the tolerance in § 7.

Licence. The text of this specification is © the Open Audio Analyzer contributors and is published under CC BY 4.0, so that it can be reproduced in another product’s documentation. Implementing it requires no licence at all. The reference implementation is GPL-3.0-or-later, which binds its code and not the measurement.

1 Scope

This document defines two measures of the dynamics of a programme, for use in metering, in delivery specifications and in offline analysis. It applies to any audio for which ITU-R BS.1770-4 loudness and true peak are defined: any channel count that standard assigns weights to, at any sample rate.

It does not define loudness, loudness range or true peak. Those are taken from the documents in § 2 unchanged, and an implementation of this specification is first an implementation of them.

2 Normative references

Informative: AES TD1004.1.15-10, Recommendation for Loudness of Audio Streaming and Network File Playback, which names PLR and PSR and is where the arithmetic comes from.

3 Terms

• LU — loudness unit; one LU is one dB of the difference between two loudness values or between a loudness and a true peak, per BS.1770-4.
• LUFS-S — short-term loudness: the K-weighted, channel-weighted mean square over the last 3 s, ungated, per EBU Tech 3341.
• LUFS-I — integrated loudness: the gated measure of BS.1770-4 § 3, over the programme.
• TP — true peak, dBTP: the largest inter-sample magnitude found by oversampling per BS.1770-4 Annex 2, over all channels.
• Absolute gate — −70 LUFS, the level below which a 400 ms block is not programme (BS.1770-4 § 3; EBU R 128).
• Programme — everything measured since the last reset. A reset is the event that restarts the integrated measure; it restarts every quantity in this document at once.
• Undefined — a reading that does not exist yet or cannot exist. It shall be carried as not a number and displayed as a mark that is not a numeral (§ 6.2). It shall never be carried or displayed as zero.

4 ODR-S, the short-term reading

4.1 Definition

ODR-S = TP(3 s) − LUFS-S

where LUFS-S is the short-term loudness of the last 3 s, and TP(3 s) is the highest true peak within the same 3 s — the largest value reached, in any channel, by the oversampled signal over the window LUFS-S averages.

4.2 The peak

The peak shall be true peak per BS.1770-4 Annex 2, oversampled at least 4× at every sample rate. It shall not be sample peak: a limited master’s inter-sample overs are what a dynamics reading is for, and a ratio built on sample peak understates them by up to 3 dB (§ 7, case 3).

The peak shall be taken over every channel that carries programme — the loudest channel’s peak — and not averaged across channels. It is the number a peak ceiling is checked against, and it is the same number here.

4.3 The window

Both operands shall cover the same audio. The peak shall be the maximum over the 3 s window LUFS-S is computed over, aligned with it to within one loudness sub-block (10 ms; EBU Tech 3341 permits the window to advance in steps of up to 100 ms). It shall not be a held or decaying meter reading, and it shall not be the peak of a single block.

4.4 The gate

ODR-S is undefined while LUFS-S is at or below the absolute gate, −70 LUFS. Below that line there is no programme to describe, and an ungated subtraction of two floored quantities reads a number — 0 LU for digital silence on an implementation that floors dB at −144, about 8 LU for a noise floor at −90 dBFS — that describes nothing a listener hears (§ 7, case 4).

ODR-S is also undefined until 3 s of signal have been measured, because LUFS-S is.

4.5 Minimum

An implementation that reports on a programme after the fact shall report the minimum ODR-S reached over the programme, taken over every window in which ODR-S was defined. The minimum is the most limited three seconds of the programme, and it is the statistic a floor on dynamics is checked against (§ 6.3); ODR-I cannot serve, for the reason in § 5.3.

The minimum shall be sampled at least once per loudness sub-block (10 ms) where the implementation measures offline. A live display that keeps a running minimum may sample it at its own display rate, and shall say so if the two are compared.

5 ODR-I, the integrated reading

5.1 Definition

ODR-I = TP Max − LUFS-I

where LUFS-I is the gated integrated loudness of the programme, and TP Max is the highest true peak reached, in any channel, since the last reset.

5.2 The gate

ODR-I is undefined for exactly as long as LUFS-I is — until at least one 400 ms block has cleared the absolute gate. Nothing else gates it.

5.3 The peak is not gated

TP Max counts every true peak since reset, including one that fell in a block the loudness gate excluded. A transient in a passage the relative gate throws out is still a peak a converter will see, and belongs in the ratio (§ 7, case 5).

The consequence is that ODR-I alone cannot say how hard the loud passages of a programme were limited: one transient in a quiet introduction rescues a flattened chorus. That is not a defect in ODR-I, which describes the whole programme’s headroom; it is why § 4.5 exists.

6 Values, display and use

6.1 Unit and resolution

Both readings are in LU. They shall be displayed to a resolution of 0.1 LU and computed at a resolution at least ten times finer. Rounding shall be to nearest.

6.2 Undefined readings

An undefined reading shall be carried as not a number (IEEE 754 NaN, or the equivalent) and displayed as an em dash or another mark that cannot be read as a numeral. It shall never be carried as zero, clamped to a floor, or held at the last defined value. A serialised report shall write null or omit the field.

6.3 Floors

A delivery specification may state a floor on ODR-I, on the minimum ODR-S, or on both, in LU. A programme meets a floor when the reading is greater than or equal to it. A floor is the only limit in a delivery specification that runs upward, and a reading below it is a failure in the same sense as a true peak over its ceiling. An undefined reading meets no floor and fails none; it is reported as not measured.

No streaming platform publishes a floor. This document recommends none.

6.4 Naming

The readings shall be labelled ODR-S and ODR-I, and the minimum of § 4.5 as the minimum of ODR-S. An implementation that also offers the same subtractions under another name (PSR, PLR, DR-S, DR-I) offers one measurement twice, and shall not present the two as distinct.

6.5 Properties an implementation may rely on

• Gain invariance. Scaling the programme by a constant moves its peak and its loudness together; neither reading changes. A platform that normalises a master does not change its ODR.
• Consistency. Two implementations that agree with BS.1770-4 on LUFS-S, LUFS-I and TP to within the tolerances of EBU Tech 3341 agree with each other on ODR-S and ODR-I to within the sum of those tolerances.

7 Conformance

An implementation conforms to this specification when it produces the readings below, within the stated tolerances, for the signals described. Every signal is a sine or a sequence of sines and can be generated exactly; no recording is needed. The expected values follow from BS.1770-4: a sine of peak amplitude A has an RMS of A/√2, 3.0103 dB below its peak; a stereo pair sums to +3.0103 dB; the K filter’s gain at 1 kHz is +0.691 dB, which cancels the −0.691 dB offset in the loudness equation.

Notes to the cases:

• Case 1 is the identity at the heart of the definition: the crest of a sine, the second channel’s contribution and the K filter’s gain at 1 kHz cancel to nothing, so a reading of anything but zero is one of those three terms applied twice or not at all.
• Case 3 is the one that distinguishes true peak from sample peak: on sample peak the reading is 3 dB low. The oversampled peak lands within a few hundredths of 0 dBTP; the tolerance covers the interpolation filter.
• Case 4c is not exactly zero for ODR-I because the 400 ms blocks that straddle the step from −80 to −60 dBFS land between the two levels, above the absolute gate and inside the relative gate, and pull the integrated reading a tenth or two under the tone. That is BS.1770-4 gating as specified.
• Case 5 places a peak in a block the relative gate excludes: the click’s block reads about −43 LUFS, above the absolute gate and ten LU under the relative gate the tone sets. The integrated reading is the tone’s; the peak is the click’s; ODR-I is the difference. The tolerance covers the oversampling filter’s response to a single sample.
• Cases 6 and 7 are inherited from the loudness conformance suite and are what this document means by “independent of the buffer” and “at any sample rate”.

8 Relation to other measures (informative)

9 Revision history