A VU meter is often included in audio equipment to display a signal level in Volume Units; the device is sometimes also called volume indicator (VI).
It is intentionally a "slow" measurement, averaging out peaks and troughs of short duration to reflect the perceived loudness of the material. It was originally developed in 1939 by the combined effort of Bell Labs and broadcasters CBS and NBC for measuring and standardizing the levels of telephone lines.
Volume Unit (VU) defined: The reading of the volume indicator shall be 0 VU when it is connected to a 600-ohm resistance in which is flowing one milliwatt of sine-wave power at 1000 cycles per second.[1][2]
The typical VU scale is from −20 to +3. The rise and fall times of the meter are both 300 milliseconds, meaning that if a constant sine wave of amplitude 0 VU is applied suddenly, the meter will take 300 milliseconds to reach the 0 on the scale. It behaves as a full-wave averaging instrument, and is not optimal for measuring peak levels.
The behaviour of VU meters is defined in ANSI C16.5-1942, British Standard BS 6840, and IEC 60268-17.
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How to use a VU meter
Tape decks originally used analogue meters with "compass needles". The needles would be "pegged" when they hit the physical pegs which stopped the maximum motion of the needle. One of the first high fidelity tape recorders, Henry Kloss's Advent 201, used only one meter, which could be switched to average both channels, or either channel[1]. When LEDs were developed, they were often used to indicate peak levels, and later arrays of them replaced mechanical meters. Unlike mechanical meters, LEDs are not hampered in their reaction time by inertia. LCD and fluorescent displays have also replaced the mechanical VU meter. Computer recording software often emulates an array of LEDs. The Nagra V, the latest digital version of a professional reel-to-reel recorder dating from the 1960s still uses analog meters.
Peak levels may be displayed in addition to the current level. As a rule, recording levels should be set so that they do not exceed the red area beyond 0 VU or only rarely. If set too high, the sound quality and frequency response is typically poorer at high recording levels, and clipping effects can be especially severe for a digital recording system. If the level is set too low, noise levels will be high relative the the main sound being recorded. Systems tailored for voice often incorporate automatic level control, usually not defeatable in inexpensive recorders.
VCRs only included VU meters when, rarely, they provided a manual level control, which is typically required for recording live music rather than compressed television or radio broadcasts.
See also
- Audio equipment
- Crest factor
- Decibel
- Loudness monitoring
- Mixing console
- Nominal level
- Peak meter
- Peak programme meter
- Programme level
References
External links
- Rane Pro Audio Reference definition for VU meter
- Q&A on the Standard Volume Indicator by John McNight
- A New Standard Volume Indicator and Reference Level by Chinn, Gannett & Morris
- The Bewildering Wilderness - "Navigating the complicated and frustrating world of audio standards"
- Audio Level Meters
- VU meter video
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