Alignment level
Encyclopedia
The alignment level in an audio
Sound recording and reproduction
Sound recording and reproduction is an electrical or mechanical inscription and re-creation of sound waves, such as spoken voice, singing, instrumental music, or sound effects. The two main classes of sound recording technology are analog recording and digital recording...

 signal chain
Signal chain
Signal chain, or signal-processing chain is a term used in signal processing and mixed-signal system design to describe a series of signal-conditioning electronic components that receive input in tandem, with the output of one portion of the chain supplying input to the next...

 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 be defined as corresponding to particular levels in each of these domains.

For example, alignment level is commonly 0 dBu
DBU
DBU may refer to:* Dansk Boldspil-Union, in English known as the Danish Football Association* dBu, a decibel measurement of voltage* 1,8-Diazabicyclo[5.4.0]undec-7-ene...

 (Equal to 0.775 Volts RMS) in broadcast
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...

 chains and in professional audio what is commonly known as "0VU", which is +4dBu (Equal to 1.227 Volts RMS) in places where the signal exists as analogue
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...

 voltage. Under normal situations the "0VU" reference allowed for a headroom of 18dB or more above the reference level without significant distortion. This is largely due to the use of slow responding VU meters in almost all analog professional audio equipment which, by their design, and by specification responded to an average level, not peak levels. It most commonly is at −18 dB FS (18 dB below full scale
Full scale
In electronics and signal processing, full scale or full code represents the maximum amplitude a system can present.-Electronics and signal processing:...

 digital) on digital recordings for programme exchange, in accordance with EBU
European Broadcasting Union
The European Broadcasting Union is a confederation of 74 broadcasting organisations from 56 countries, and 49 associate broadcasters from a further 25...

 recommendations. Digital equipment must use peak reading metering systems to avoid severe digital distortion caused by the signal going beyond 'full scale' or maximum digital levels. 24-bit original or master recordings commonly have alignment level at −24 dB FS to allow extra headroom, which can then be reduced to match the available headroom of the final medium by audio level compression
Audio level compression
Dynamic range compression, also called DRC or simply compression reduces the volume of loud sounds or amplifies quiet sounds by narrowing or "compressing" an audio signal's dynamic range...

. FM broadcasts usually have only 9 dB of headroom as recommended by the EBU, but digital broadcasts, which could operate with 18dB of headroom, given their low noise floor even in difficult reception areas, currently operate in a state of confusion, with some transmitting at maximum level while others operate at much lower level even though they carry material that has been compressed for compatibility with the lower dynamic range of FM transmissions.

Alignment level as used in the EBU

The EBU uses the term "alignment level" not for levelling any real audio signals. In the EBU documents "alignment level" just defines -18 dBFS as the level of the Alignment Signal (1 kHz sinus tone generator resp. 997 Hz in the digital domain).

The reason for alignment level

Using alignment level rather than maximum permitted level as the reference point allows more sensible headroom management throughout the audio chain, so that quality is only sacrificed through compression
Audio level compression
Dynamic range compression, also called DRC or simply compression reduces the volume of loud sounds or amplifies quiet sounds by narrowing or "compressing" an audio signal's dynamic range...

 as late as possible.

The problem of loudness war
Loudness war
The loudness war or loudness race is a pejorative term for the apparent competition to digitally master and release recordings with increasing loudness.The phenomenon was first reported with respect to mastering practices for 7" singles...

s, which have caused a general fall in audio quality, initially on radio stations, but more recently on CDs too, is well known. As radio stations competed for attention and raise the listener scores their ad revenue is based on, they used audio compression to give their sound more impact. They used level compressors, and in particular multi-band compressors such as optimod that compress different parts of the frequency independently. Such compressors usually incorporate fast acting limiters to eliminate brief peaks, since brief peaks, though they may not contribute much to perceived loudness, limit the modulation level that can be applied to FM transmissions in particular, if serious clipping and distortion are to be avoided. Digital broadcasting has changed all this, since stations are no longer found by tuning across the band, so the loudest ones no longer stand out. Low noise level is also guaranteed regardless of signal level, so that it is no longer necessary to fully modulate to ensure acceptable clarity in poor reception areas. Many professionals feel that the more widespread adoption and understanding of alignment level throughout the audio industry could help bring modulation levels down, leaving headroom to cope with brief peaks, and using a different form of level compression that reduces dynamic range on programmes where this is considered desirable, but does not remove the brief peaks which add 'sparkle' and contribute to clearer sound. CDs in particular have suffered a loss of quality since they were introduced through the widespread use of fast limiting, which, given their very low noise level is quite unnecessary.

Digital audio players such as the iPod
IPod
iPod is a line of portable media players created and marketed by Apple Inc. The product line-up currently consists of the hard drive-based iPod Classic, the touchscreen iPod Touch, the compact iPod Nano, and the ultra-compact iPod Shuffle...

, demonstrate the need for a common alignment level convincingly. While tracks taken from recent CDs sound loud enough, many older recordings (such as Pink Floyd albums which notably allowed lots of headroom for stunning dynamic range and rarely reach peak digital level) are far too quiet, even at full volume setting. Older audio systems incorporated typically 12dB of 'overvolume', meaning that it was possible to turn up the loudness on a quiet recording to make maximum use of amplifier output even if peak level was never reached on the recording. Modern devices, however, tend to produce maximum output at full volume only on recordings that reach full-scale digital level. If extra gain is added, then playing a modern CD after listening to a well recorded older one is likely to deafen, requiring the volume control to be turned down by a huge amount. Again, the adoption of a common alignment level (early CDs allowed around 18dB of headroom by common consent) would make sense, improving quality and usability and ending the loudness war.

Making compression a listening option

The incorporation of (switchable) level compression in domestic music systems and car in-car systems would allow higher quality on systems capable of wide dynamic range and in situations that allowed realistic reproduction. Such compression systems have been suggested and tried from time to time, but are not in widespread use — a 'chicken and egg' problem since producers feel they must make programmes and recordings that sound good in car with high ambient noise or on cheap low-power music systems. In the UK, some DAB
Digital audio broadcasting
Digital Audio Broadcasting is a digital radio technology for broadcasting radio stations, used in several countries, particularly in Europe. As of 2006, approximately 1,000 stations worldwide broadcast in the DAB format....

 receivers do incorporate a menu setting for automatic loudness compensation which adds extra gain on BBC Radio 3
BBC Radio 3
BBC Radio 3 is a national radio station operated by the BBC within the United Kingdom. Its output centres on classical music and opera, but jazz, world music, drama, culture and the arts also feature. The station is the world’s most significant commissioner of new music, and its New Generation...

 and BBC Radio 4
BBC Radio 4
BBC Radio 4 is a British domestic radio station, operated and owned by the BBC, that broadcasts a wide variety of spoken-word programmes, including news, drama, comedy, science and history. It replaced the BBC Home Service in 1967. The station controller is currently Gwyneth Williams, and the...

, to allow for the fact that these programmes adopt lower levels than, for example, the pop station Radio 1
BBC Radio 1
BBC Radio 1 is a British national radio station operated by the British Broadcasting Corporation which also broadcasts internationally, specialising in current popular music and chart hits throughout the day. Radio 1 provides alternative genres after 7:00pm including electronic dance, hip hop, rock...

. Some television receivers also have a menu setting for loudness equalisation, aimed at helping to reduce excessive loudness on advertisements. However, there is no common agreement to reduce compression and limiting and leave these tasks to the receiver.

See also

  • Audio normalization
    Audio normalization
    Audio normalization is the application of a constant amount of gain to an audio recording in order to bring the average or peak amplitude to a target level ....

  • Full scale
    Full scale
    In electronics and signal processing, full scale or full code represents the maximum amplitude a system can present.-Electronics and signal processing:...

  • Headroom
  • Loudness war
    Loudness war
    The loudness war or loudness race is a pejorative term for the apparent competition to digitally master and release recordings with increasing loudness.The phenomenon was first reported with respect to mastering practices for 7" singles...

  • Maximum coding level
  • Measurement level
  • Nominal level
    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 output and the low-level internally-generated electronic noise they add to the signal...

  • Permitted maximum level
  • Programme levels
  • ReplayGain
  • Standard operating level
  • Transmission level point
    Transmission level point
    In a telecommunications system, a transmission level point is a test point or interface, i.e. a physical point in an electronic circuit where a test signal may be inserted or measured, and for which the nominal power of the test signal is specified....


External links

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