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A volume unit (VU) meter or standard volume indicator (SVI) is a device displaying a representation of the signal level in audio equipment. The Acoustical Society of America 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'Al 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 (), or 1.228 volts RMS across a 600 ohm load, or about 2.5 milliWatts. 0 VU is often referred to as "0 dB".〔(VU Meter )〕 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. In the broadcast industry, loudness monitoring was standardized only recently. == Lead == The original designers of the VU meter were tasked with finding a way to measure complex audio signals with a simple technology. Since a VU meter is a mechanical device it can never reflect the instantaneous signal peaks of complex audio signals. The designers of the VU meter therefore took a different approach. They created a meter that did NOT measure peaks but simply inferred them. A real VU meter has a very specific "ballistic characteristic". This means it responds to changing audio signals at a very precise speed, rising from no signal to 99% of "0VU" when a 1 kHz sine wave tone is applied for 300 milli-seconds. When using a VU meter the audio system is calibrated with a sine wave tone at a "reference level" for the system. At the reference level the VU meter will show "0" for a sine wave tone, but the engineer must know that with music or speech the Peak levels will always be "inferred" to be between 6dB to 10dB higher than the reference level. The genius of the VU meter is that for most types of audio sources the system engineer can count on these peaks being within this range and can design the audio system with confidence. Good engineering practice is to always build in a little extra "headroom" as it is called, to cover the strange conditions where an audio signal might exceed normal peak levels or the equipment operator fails to adjust the levels correctly. Typically the levels to be considered when designing systems using a VU meter are: * Reference Level (typically +4dBu, valid with tones only) * Standard Output Level (10dB above Reference, typical peak levels) * Clip Level (6dB above Standard Output Level, "headroom" to allow for unusual conditions)〔 Schmid, Hans, "Audio, The Stepchild of Television Broadcasting", J. SMPTE, Volume 85, page 6-9 (1976 Jan.). 〕〔 Schmid, Hans, "Audio Program Level, The VU Meter, and The Peak Program Meter", IEEE Transactions on Broadcasting, Volume BC-23, No. 1, page 22-26 (1977 Mar.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「VU meter」の詳細全文を読む スポンサード リンク
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