Headroom (audio signal processing)
In digital and analog audio, headroom refers to the amount by which the signal-handling capabilities of an audio system exceed a designated level known as Permitted Maximum Level (PML). Headroom can be thought of as a safety zone allowing transient audio peaks to exceed the PML without damaging the system or the audio signal, e.g., via clipping. Standards bodies differ in their recommendations for PML.
In digital audio, headroom is defined as the amount by which digital full scale (FS) exceeds the PML in decibel (|dB). The European Broadcasting Union (EBU) specifies a PML of 9 dB below 0 dBFS (-9 dBFS), thus giving 9 dB of headroom. An alternative EBU recommendation specifies 24 dB of headroom, which might be used for 24-bit master recordings where it is useful to allow more room for unexpected peaks during live recording.
In analog audio, headroom can mean low-level signal capabilities as well as the amount of extra power reserve available within the amplifiers that drive the loudspeakers.
Alignment level is an anchor point 9 dB below the nominal level, a reference level that exists throughout the system or broadcast chain, though it may imply different voltage levels at different points in the analog chain. Typically, nominal (not alignment) level is 0 dB, corresponding to an analog sine wave of voltage of 1.23 volts RMS (+4 dBu or 3.47 volts peak to peak). In the digital realm, alignment level is −18 dBFS.
- AL = analog level
- SPL = sound pressure level
- Audio quality measurement
- Noise measurement
- Programme levels
- Rumble measurement
- ITU-R 468 noise weighting
- Weighting filter
- Equal-loudness contour
- Fletcher-Munson curves
- Loudness war
- EBU Recommendation R68-2000
- AES Preprint 4828 - Levels in Digital Audio Broadcasting by Neil Gilchrist (not free)
- EBU Recommendation R117-2006 (against loudness war)
- AES Convention Paper 5538 On Levelling and Loudness Problems at Broadcast Studios
- EBU Tech 3282-E on EBU RDAT Tape Levels
- AES17-1998 (r2004): AES standard method for digital audio engineering -- Measurement of digital audio equipment