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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 be defined as corresponding to particular levels in each of these domains.
For example, alignment level is commonly 0 dBu (Equal to 0.775 Volts RMS) in broadcast 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 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 digital) on digital recordings for programme exchange, in accordance with EBU 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. 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).
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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