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.

Analogue
In analogue systems, alignment level is commonly 0 dBu (0.775 volts RMS) in broadcast chains and in professional audio is commonly 0 VU, which is +4 dBu (1.228 volts RMS). Under normal situations, the 0 VU reference allows for a headroom of 18 dB or more above the reference level without significant distortion. This is largely due to the use of slow-responding VU meters in almost all analogue professional audio equipment, which, by their design and by specification, respond to an average level, not peak levels.

Digital
In digital systems alignment level commonly is at −18 dBFS (18 dB below digital full scale), in accordance with EBU recommendations. Digital equipment must use peak-reading metering systems to avoid severe digital distortion caused by the signal going beyond digital full scale. 24-bit original or master recordings commonly have an alignment level at −24 dBFS 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 18 dB 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 a much lower level, even though they carry material that has been compressed for compatibility with the lower dynamic range of FM transmissions.

EBU
In EBU documents alignment level defines −18 dBFS as the level of the alignment signal, a 1 kHz sine tone for analog applications and 997 Hz in digital applications.

Motivation
Using alignment level rather than maximum permitted level as the reference point allows more sensible headroom management throughout the audio signal chain; compression happens only where intended.

Loudness wars have resulted in increasing playback loudness. Loudness normalisation to a fixed alignment level can improve the experience when listening to mixed material.