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The response of a VU meter (black line) compared to instantaneous input level (grey area) of a drum beat. Level is in dB and time is in seconds. The rise time, defined as the time it takes for the needle to reach 99% of the distance to 0 VU when the VU-meter is submitted to a signal that steps from 0 to a level that reads 0 VU, is 300 ms.
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.
Broadcast engineers in North America usually line up their audio gear to nominal reference level of 0 dB on a VU meter aligned to +4 dBu or -20 dBFS, in Europe equating to roughly +4 dBm or -18 dBFS. Peak signal levels must not exceed the nominal level by more than +10 dB. [10]
"The VU (or Volume Unit) system is a hangover from early radio usage when 0 VU meant 100% of the legal modulation for the particular radio station." [6] "When the VU meter indicates "0" (typically a +4 dBm level), " [7] — Omegatron 01:13, 13 September 2006 (UTC) [ reply ]
To aid alignment on both VU meters and PPMS, ABC in New York used a special test signal known as ATS. A 440 Hz tone alternated between steady tone at +8 dBu (indicated at 0 VU and −8 PPM) and tone bursts at +16 dBu (indicated at 0 VU and 0 PPM). [19]
UK broadcasters, alignment level is taken as 0 dBu (PPM 4 or −4 VU) The American SMPTE standard defines −20 dBFS as the alignment level. European and UK calibration for Post & Film [clarification needed] is −18 dBFS = 0 VU. US installations use +24 dBu for 0 dBFS. American and Australian Post: −20 dBFS = 0 VU = +4 dBu.