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In digital and analog audio, headroom refers to the amount by which the signal-handling capabilities of an audio system can exceed a designated nominal level. Headroom can be thought of as a safety zone allowing transient audio peaks to exceed the nominal level without damaging the system or the audio signal, e.g., via clipping. Standards bodies differ in their recommendations for nominal level and headroom.


Digital audio

In digital audio, headroom is defined as the amount by which digital full scale (FS) exceeds the nominal level in
decibel The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a Power, root-power, and field quantities, power or root-power quantity on a logarithmic scale. Two signals whos ...
s (dB). The
European Broadcasting Union The European Broadcasting Union (EBU; , UER) is an alliance of Public broadcasting, public service media organisations in countries within the European Broadcasting Area (EBA) or who are member states of the Council of Europe, members of the ...
(EBU) specifies several nominal levels and resulting headroom for different applications.


Analog audio

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

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 Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...
levels at different points in the analog chain. Typically, nominal (not alignment) level is 0 dB, corresponding to an analog
sine wave A sine wave, sinusoidal wave, or sinusoid (symbol: ∿) is a periodic function, periodic wave whose waveform (shape) is the trigonometric function, trigonometric sine, sine function. In mechanics, as a linear motion over time, this is ''simple ...
of voltage of 1.23 
volt The volt (symbol: V) is the unit of electric potential, Voltage#Galvani potential vs. electrochemical potential, electric potential difference (voltage), and electromotive force in the International System of Units, International System of Uni ...
s 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


See also

* A-weighting *
Audio system measurements Audio system measurements are used to quantify audio system performance. These measurements are made for several purposes. Designers take measurements to specify the performance of a piece of equipment. Maintenance engineers make them to ensur ...
*
Equal-loudness contour An equal-loudness contour is a measure of sound pressure level, over the frequency spectrum, for which a listener perceives a constant loudness when presented with pure steady tones. The unit of measurement for loudness levels is the phon an ...
* ITU-R 468 noise weighting * Loudness war *
Noise measurement In acoustics, noise measurement can be for the purpose of measuring environmental noise or measuring noise in the workplace. Applications include monitoring of construction sites, aircraft noise, road traffic noise, entertainment venues and neighb ...
* Rumble measurement *
Weighting filter A weighting filter is used to emphasize or suppress some aspects of a phenomenon compared to others, for measurement or other purposes. Audio applications In each field of audio measurement, special units are used to indicate a weighted measure ...


References


Further reading

*BS.1726 "Signal level of digital audio accompanying television in international programme exchange" (2005) *BS.1864 "Operational practices for loudness in the international exchange of digital television programmes" (2010) *BS.1770-3 "Algorithms to measure audio programme loudness and true-peak audio level" (2012)


External links


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
{{Webarchive, url=https://web.archive.org/web/20090909130542/http://www.ebu.ch/CMSimages/en/tec_doc_t3282_tcm6-10542.pdf , date=2009-09-09
AES17-1998 (r2004): AES standard method for digital audio engineering -- Measurement of digital audio equipment
Audio engineering Broadcast engineering Sound production technology Sound recording Sound