View source for VU meter

{{Short description|Audio signal level measurement device}}
[[Image:VU Meter.jpg|thumb|An analog VU meter with peak LED]]
A '''volume unit''' ('''VU''') '''meter''' or '''standard volume indicator''' ('''SVI''') is a device displaying a representation of the [[signal level]] in [[sound|audio]] equipment.
The original design was proposed in the 1940 [[Institute of Radio Engineers|IRE]] paper, ''A New Standard Volume Indicator and Reference Level'', written by experts from [[CBS]], [[NBC]], and [[Bell Labs|Bell Telephone Laboratories]].<ref>{{Cite web|url=http://www.aes.org/aeshc/pdf/chinn_a-new-svi.pdf|title=A New Standard Volume Indicator and Reference Level|last=Chinn|first=H. A.}}</ref> The [[Acoustical Society of America]] then standardized it in 1942 (ANSI C16.5-1942)<ref>{{cite web |title=Some Questions and Answers on the Standard Volume Indicator ("vu meter") |first=John. G. (Jay) |last=McKnight |url=http://www.aes.org/aeshc/pdf/mcknight_qa-on-the-svi-6.pdf |date=2006-07-27 |website=Audio Engineering Society}}</ref><ref>{{cite web |title=VU Meters Information |url=http://www.globalspec.com/learnmore/test_measurement_equipment/multimeters_electrical_test_meters/vu_meters |website=Engineering 360 |access-date=2017-03-23}}</ref> for use in telephone installation and radio broadcast stations.
Consumer audio equipment often features VU meters, both for utility purposes (e.g. in recording equipment) and for aesthetics (in playback devices).
[[File:Vumeters-audio-5.1.png|thumb|[[Surround audio]] VU meter graphic]]
The original VU meter is a passive electromechanical device, namely a 200 μA DC [[Jacques-Arsène d'Arsonval|d'Arsonval]] movement [[ammeter]] fed from a [[Metal rectifier|full-wave copper-oxide rectifier]] mounted within the meter case. The mass of the needle causes a relatively slow response, which in effect [[Integral|integrates]] or smooths the signal, with a [[rise time]] of 300&nbsp;ms. This has the effect of averaging out peaks and troughs of short duration, and reflects the perceived [[loudness]] of the material more closely than the more modern and initially more expensive [[Peak programme meter|PPM]] meters. For this reason many audio practitioners prefer the VU meter 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.
0&nbsp;VU is equal to +4&nbsp;[[dBu]], or 1.228&nbsp;[[volt]]s [[Root mean square|RMS]], a power of about 2.5&nbsp;milliwatts when applied across a 600-[[ohm]] [[Electrical load|load]]. 0&nbsp;VU is often referred to as "0&nbsp;dB".<ref>[http://www.mediacollege.com/audio/monitoring/vu.html VU Meter].</ref> The meter was designed not to measure the signal, but to let users aim the signal level to a target level of 0&nbsp;VU (sometimes labelled 100%), so it is not important that the device is non-linear and imprecise for low levels{{says who?|date=July 2020}}. In effect, the scale ranges from −20&nbsp;VU to +3&nbsp;VU, with −3&nbsp;VU right in the middle (half the power of 0&nbsp;VU). Purely electronic devices may emulate the response of the needle; they are VU-meters in as much as they respect the standard.
In the broadcast industry, [[loudness monitoring]] was standardized, in 2009 in the United States by the ATSC A/85, in 2010 in Europe by the [[EBU R 128]], in 2011 in Japan by the TR-B32, and in 2010 in Australia by the OP-59.
== 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 that it responds to changing audio signals at a very precise speed, rising from no signal to 99% of "0&nbsp;VU" when a 1&nbsp;kHz sine wave tone is applied for 300&nbsp;milliseconds.
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 shows "0" for a sine-wave tone, but the engineer must know that, with music or speech, to always infer that peak levels are always between 6&nbsp;dB and 10&nbsp;dB higher than the reference level. The usefulness of the VU meter comes from the fact 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 considered when designing systems using a VU meter are:
* Reference level (typically +4&nbsp;dBu, valid with tones only);
* Standard output level (10&nbsp;dB above reference, typical peak levels);
* Clip level (6&nbsp;dB above standard output level, "headroom" to allow for unusual conditions)<ref>{{cite journal |last=Schmid |first=Hans |title=Audio, The Stepchild of Television Broadcasting |journal= SMPTE Journal|volume=85 |pages=6&ndash;9 |date=January 1976 |doi=10.5594/J07600}}</ref><ref>{{cite journal |last=Schmid |first=Hans |s2cid=40700960 |title=Audio Program Level, The VU Meter, and The Peak Program Meter |journal=IEEE Transactions on Broadcasting |volume=BC-23 |issue=1 |pages=22&ndash;26 |date=March 1977 |doi=10.1109/TBC.1977.266233}}</ref>
== Standard characteristics ==
The behaviour of VU meters is defined in [[ANSI]] C16.5-1942, [[British Standard]] BS 6840, and [[International Electrotechnical Commission|IEC]] 60268-17.
=== Reference level ===
{{quotation|'''VU defined:'''
“The reading of the volume indicator shall be 0 VU when it is connected to an AC voltage equal to 1.228 Volts RMS across a 600 ohm resistance (equal to +4 [dBu]) at 1000 [[cycles per second]].”<ref>[http://www.sizes.com/units/volume_unit.htm volume unit or VU]</ref><ref>[http://yarchive.net/phone/decibels.html] Re: -dBm?</ref>
Note: The reference above is generally true now and was always true in the recording industry. However in some North American broadcast installations up until late in the 20th century, the Reference level ("0VU") was +8dBm at large studio installations and some used 150 ohm impedance throughout the studio.(CFRB Toronto and CFPL London Canada) This was yet another "standard" established in the early years of audio and the VU meter was altered by changing the series resistors to adjust its sensitivity. This had no effect on the ballistics.
<ref> Source: the author worked on these systems and with the designers in the 1970s </ref>
}}
=== Rise time ===
[[Image:VU-meter-reponse-graph.svg|thumb|The response of a VU meter (black line) compared to instantaneous input level (grey area) of a [[drum beat]].
Level is in [[Decibel|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.
The [[Overshoot (signal)|overshoot]] must be within 1 to 1.5%.
The fall time is the same as the rise time, 300 ms.
=== Frequency response ===
The level specification is meant at 1000 [[Hertz|Hz]]. The reading should not depart from the reading at 1000&nbsp;Hz by more than 0.2 dB from 35&nbsp;Hz to 10&nbsp;kHz or more than 0.5 dB between 25&nbsp;Hz and 16&nbsp;kHz.
'''Note''' that the specification mentions only sinusoid waveforms. Given the electromechanical principle of the meter, the deviation of the needle is actually approximately proportional to the ''average'' of the part of the signal with more than approximately 0.4 V instantaneously because of the two copper-oxide rectifiers always in series, which transfer function curve has a knee around 0.2 V. Signals generally do not have a sinusoidal waveform by far, even if they all fall within the VU-meter bandpass. The reading is the average of the voltage,<ref>[http://www.electronics-tutorials.ws/accircuits/average-voltage.html Average Voltage Tutorial]</ref> and is not an indication of the power of the signal, which is proportional to the average of the square of the voltage, or the root-mean-square (RMS) value. As a conventional ''VU reading'', however, it served its purpose as an indication a) of the overall level and dynamics of the signal and b) of the proximity to the maximum admitted level, to the operators of recording and broadcasting equipment. Maintenance staff could also use it as a measurement apparatus, to check for losses in transmissions and level alignment, provided that they used exclusively sine waves as test signals.
=== Impedance ===
The VU meter and its attenuator should present a 7,500-ohm impedance to the circuit it is applied to, measured with a sinusoid signal that sets the indicator to 0 dB.
== History ==
[[File:PPM IEC 268-10 IIb EBU.jpg|49 px|thumb|Neon bar graph VU meter]]
The VU-meter was originally developed in 1939 by the combined effort of [[Bell Labs]] and broadcasters [[CBS]] and [[NBC]].<ref>{{cite web|last=Robjohns|first=Hugh|title=What's the difference between PPM and VU meters?|url=http://www.soundonsound.com/sound-advice/q-whats-difference-between-ppm-and-vu-meters|publisher=Sound on Sound|date=July 2013}}</ref> In the 1970s–80s, [[neon]]-filled, planar dual displays with up to 201 segments per stereo channel<ref>{{cite web |url=http://tubehobby.com/datasheets/plasma.pdf |publisher=[[Vishay Intertechnology|Vishay Dale]], Columbus, Nebraska, USA |title= ''Plasma Panel Displays - Dual Linear Bar Graph'' |date=November 2000 |access-date=8 March 2014}}</ref><!-- Alternatively http://www.dj9kw.de/dj9kw/projekte/audio/plasmabargraph/similar.pdf - application ftp://ftp.studer.ch/Public/Products/Mixing_Analog/Analog_Mixer_Assemblies/Technical_Info/1913111/1913111.pdf --> were popular among broadcasters as fast bar graph VU meters.
== Other level meters ==
The consumer audio industry often uses volume indicators that do not comply with any standard.<ref>[http://www.recordingmag.com/resources/resourceDetail/246.html Meter Madness] {{Webarchive|url=https://web.archive.org/web/20150402153425/http://www.recordingmag.com/resources/resourceDetail/246.html |date=2015-04-02 }} – Mike Rivers.</ref>
BBC sound engineers would refer to the VU meter as the "Virtually Useless" meter,<ref>{{cite web |url=https://www.theguardian.com/notesandqueries/query/0,5753,-18540,00.html |title=On audio equipment, sound level meters are sometimes called VU meters. What does VU stand for? |publisher=Guardian News and Media Ltd |date=2011 |access-date=8 August 2019}}</ref> preferring the [[peak programme meter|PPM]].
Academic research has shown that an SVI or VU meter behaves differently to the average value and RMS meters. The ballistics shown by this instrument, in response to signals with a large crest factor, position its readings halfway between both. For example, an increment of 3&nbsp;dB in the crest factor of an audio signal gives approximately a fall of −3&nbsp;dB in a RMS meter, −6&nbsp;dB in an average meter, and −4&nbsp;dB in a VU meter.<ref>[http://www.aes.org/e-lib/browse.cfm?elib=17844 Method to Evaluate the Ballistics of Audio Meters] – Victor M. Acuña.</ref>
[[Image:2008-01-22 Miniature magnetoelectric VU meter.jpg|thumb|Another miniature magnetoelectric meter]]
== See also ==
*[[Audio equipment]]
*[[Crest factor]]
*[[Decibel#Acoustics|Decibel]]
*[[Loudness monitoring]]
*[[Mixing console]]
*[[Noise pollution]]
*[[Nominal level]]
*[[Peak meter]]
*[[Peak programme meter]]
==References==
{{Reflist}}
== External links ==
{{commons category|VU meters}}
*[http://www.aes.org/par/v/#VU_meter AES Pro Audio Reference definition for VU meter]
*[http://www.aes.org/aeshc/pdf/chinn_a-new-svi.pdf A New Standard Volume Indicator and Reference Level] by Chinn, Gannett & Morris
*[http://www.rane.com/pdf/bewilder.pdf The Bewildering Wilderness - "Navigating the complicated and frustrating world of audio standards"]
*[https://web.archive.org/web/20130627234540/http://www.darkwood.demon.co.uk/PC/meter.html Audio Level Meters]
*[http://www.dailymotion.com/video/x6i4ik_tuto-after-effects-vu-meter_creation VU Meter Video]
*[http://www.soundonsound.com/sos/jul13/articles/qanda-0713-2.htm Q What’s the difference between PPM and VU meters?]
*[http://www.tonnesoftware.com/meter.html Software to Design & Print Custom VU Meter Panels]
*[http://www.sengpielaudio.com/MeterComparisonTable.htm VU Meter dBu & dBv Reference Chart]
{{Electrical and electronic measuring equipment}}
[[Category:Sound production technology]]
[[Category:Measuring instruments]]