Headroom (audio signal processing)
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In digital and analog
audio Audio most commonly refers to sound, as it is transmitted in signal form. It may also refer to: Sound *Audio signal, an electrical representation of sound *Audio frequency, a frequency in the audio spectrum * Digital audio, representation of sou ...
, 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 decibels (dB). The
European Broadcasting Union The European Broadcasting Union (EBU; french: Union européenne de radio-télévision, links=no, UER) is an alliance of public service media organisations whose countries are within the European Broadcasting Area or who are members of the C ...
(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 In audio and recording, what is known colloquially as an anchor point is a center position in a stereo mix reserved for only three or four important tracks.Hodgson, Jay (2010). ''Understanding Records'', p. 165. . Most modern pop productions are ...
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 electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to ...
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 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 or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a po ...
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 A-weighting is the most commonly used of a family of curves defined in the International standard IEC 61672:2003 and various national standards relating to the measurement of sound pressure level. A-weighting is applied to instrument-measured ...
*
Audio quality measurement Audio system measurements are a means of quantifying system performance. These measurements are made for several purposes. Designers take measurements so that they can specify the performance of a piece of equipment. Maintenance engineers make ...
*
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 and ...
* Fletcher–Munson curves *
ITU-R 468 noise weighting ITU-R 468 (originally defined in CCIR recommendation 468-4, therefore formerly also known as CCIR weighting; sometimes referred to as CCIR-1k) is a standard relating to noise measurement, widely used when measuring noise in audio systems. The ...
* Loudness war * Noise measurement * Programme levels * 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