Alignment level
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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 in broadcast chains and what is commonly known as "0vu" in professional audio (+4dbm which is 1.227 volts RMS across a load of 600 ohms) in places where the signal exists as analogue voltage. Under normal situations the +4dbm or "0vu" reference allowed for a headroom of 18 or more db 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 in order to avoid severe digital distortion if a signal goes beyond 'full scale' or maximum digital levels. 24-bit original or master recordings commonly have alignment level at −24 dB FS in order 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.
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[edit] 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 as late as possible.
The problem of loudness wars, 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 impact, in order to maximise the listener scores on which their advertising revenue was based, they made more use of level compressors, and in particular the multi-band compressors such as optimod that actually change the frequency balance of music. 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 in order 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, 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 useability and ending the loudness war.
[edit] 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 receivers do incorporate a menu setting for automatic loudness compensation which adds extra gain on BBC Radio 3 and BBC Radio 4, to allow for the fact that these programmes adopt lower levels than, for example, the pop station Radio 1. Some television receivers also have a menu setting for loudness equalisation, aimed at helping to reduce excessive loudness on advertisements. However, there is as yet no common agreement to reduce compression and limiting on the basis that these will be applied where desired at the receiver.
[edit] See also
- Loudness war
- Programme levels
- Headroom
- Nominal level
- Standard operating level
- Permitted maximum level
- Measurement level
- Transmission level point
- Maximum coding level
- Full scale
