A Volume Unit (VU) meter or Standard Volume Indicator (SVI) is a device displaying a representation of the signal level in audio equipment. The ASA standardized it in 1942 ( C16-5 - 1942) for use in telephone installation and radio broadcast stations. Consumer audio equipment often features VU meters, both for utilitarian purposes (e.g. in recording equipment) and for aesthetics (in playback devices).
The original VU meter is a passive electromechanical device, namely a 200 µA DC d'Arsonval movement ammeter fed from a full wave copper-oxide rectifier mounted within the meter case. The mass of the needle causes a relatively slow response, which in effect integrates the signal, with a rise time of 300 ms. 0 VU is equal to +4 [dBm], or 1.228 volts RMS across a 600 ohm load. 0 VU is often referred to as "0 dB". The meter was designed not to measure the signal, but to let users aim the signal level to a target level of 0 VU (sometimes labelled 100%), so it is not important that the device is non-linear and imprecise for low levels. In effect, the scale ranges from −20 VU to +3 VU, with −3 VU right in the middle. Purely electronic devices may emulate the response of the needle; they are VU-meters inasmuch as they respect the standard.
The VU-meter (intentionally) "slows" measurement, averaging out peaks and troughs of short duration, and reflects more the perceived loudness of the material than the more modern and initially more expensive PPM meters. For this reason many audio practitioners prefer it to its alternatives, though the meter indication does not reflect some of the key features of the signal, most notably its peak level, which in many cases, must not pass a defined limit.
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