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 Nominal level is the operating level at which an electronic signal processing device is designed to operate. The electronic circuits that make up such equipment are limited in the maximum signal they can handle and the low-level internally genera ...
. 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 Clipping may refer to: Words * Clipping (morphology), the formation of a new word by shortening it, e.g. "ad" from "advertisement" * Clipping (phonetics), shortening the articulation of a speech sound, usually a vowel * Clipping (publications) ...
. 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 (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 The alignment level in an audio signal chain or on an audio recording is a defined anchor point that represents a reasonable or typical level. It does not represent a particular sound level or signal level or digital representation, but it can b ...
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 
volt The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827). Defin ...
s 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, root-power, and field quantities, power or root-power quantity on a logarithmic scale. Two signals whose ...
or 3.47 volts
peak to peak The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of am ...
). In the digital realm, alignment level is −18 dBFS. * AL = analog level * SPL = sound pressure level


See also

* A-weighting *
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 *
Fletcher–Munson curves 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 i ...
* ITU-R 468 noise weighting * 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