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VU Meters on Compressor Plug-ins - How Useful Are They?

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VU Meters on Compressor Plug-ins - How Useful Are They?

Postby ITHertz » Wed Mar 03, 2021 7:45 am

Hi Everyone,

I've been doing a bit of compressor homework recently (as you do!) and realised something that I'd never noticed before - that some plug-in emulations of older compressors (particularly Vari-Mu types) often have VU meters rather than meters that show dB (e.g. MJUC, VCL-25A, various 670s, etc.). What makes this more confusing is that IK's Opto Compressor has a "VU" meter that shows gain reduction in dB!

Aside from the misnaming of meters (a VU Meter shows VUs, right?), how useful are VU meters, especially in showing compressor gain reduction, which is often transitory? Are they all that was available at the time?

And what of something like SPL's Iron (plug-in) - would it be better with a dB meter rather than VU?

BTW, if I'm completely off-track with any of this I'm happy to be enlightened!

And I know, "just use my ears", however I do like to understand what I'm doing when I'm mangling sounds :D

Cheers,

Chris
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Re: VU Meters on Compressor Plug-ins - How Useful Are They?

Postby Hugh Robjohns » Wed Mar 03, 2021 12:40 pm

ITHertz wrote:...some plug-in emulations of older compressors (particularly Vari-Mu types) often have VU meters rather than meters that show dB...

Erm... not sure what I'm missing... VU meters are definitely calibrated in decibels..

Aside from the misnaming of meters (a VU Meter shows VUs, right?)

Kind of... Short history lesson coming up:

What you know as the VU meter was actually born as the SVI -- Standards Volume Indicator -- and came out of a paper published in 1940 from a collaborative project between Bell Labs, CBS and NBC in America. The full paper is available here: https://www.aes.org/aeshc/pdf/chinn_a-new-svi.pdf

Originally the SVI comprised a variable, constant-impedance attenuator, a copper-oxide rectifier, and a moving-coil ammeter -- all passive to keep the costs down compared to the active PPMs being developed in Europe at the same time. With the technology of the day, this meter system was essentially an RMS voltmeter measuring across a 600 Ohm line termination -- so the scale is, strictly speaking, calibrated in dBm -- and the meter's movement has roughly 300ms attack and release times and effectively serves to integrate the energy in the signal, giving an approximation to perceived loudness or volume.

The upper 'VU' scale shows decibels relative to the '0VU' reference mark, and it was/is intended for use in level-setting with static line-up tones. So 0VU was intended to represent the alignment level of the system being metered. The lower percentage scale was/is intended to show broadcast modulation levels with normal programme material.

The fact that the 100% modulation and 0VU line-up marks are coincident on the scale often causes confusion, but the apparent gain offset between them is entirely due to the inherently slow response of the meter to fast moving audio signals -- the meter under-reads considerably on programme material; a fact that was known and recognised in its scale markings.

The meter itself was designed such that the 0VU indication was achieved for a signal level at the meter of +4dBm -- that was simply the way the design worked out with the voltmeter and rectifier. The variable constant-impedance attenuator was provided to adjust the incoming signal level such that the 0VU reading could be achieved for whatever the reference alignment level was on the line being monitored (typically +8dBm for American broadcast networks, or +16dBm for telco lines). Impressively, the variable attenuator in the original SVI could cope with signal reference levels up to +24dBm!

Later, with the rise of consumer and low-cost pro-audio equipment needing audio metering, the expensive constant-impedance variable attenuator was omitted for cost-saving purposes, and that resulted in a simple VU meter with a fixed calibration of 0VU = +4dBm -- and that's fundamentally why the standard music studio line up level became established as +4dBu.

However, the SVI or VU meter was originally intended to be calibrated to any desired reference level, so, for example, European broadcasters could/should calibrate the meter such that 0VU = 0dBu... and in fact Mackie mixers are setup at the factory for this same alignment because that company's consoles' nominal operating level is 0dBu by design.

how useful are VU meters, especially in showing compressor gain reduction, which is often transitory? Are they all that was available at the time?

VU meters were and are provided on compressors as a low-cost way of enabling the user to monitor the output (and sometimes input) level so that the correct gain structure alignment could be established through the unit by using line-up tones -- a role for which the VU meter is perfectly suited.

With a meter on the box, it's obviously sensible and practical to make it switchable to indicate the applied gain-reduction too. Yes, its inherently slow attack and release times mean it can only provide an indication of GR -- it will miss any very fast gain-reduction peaks, rather than provide a precisely accurate real-time measurement -- but absolute precision is not important in this GR-meter role since the ears are the final arbiter of whether the compressor is working as desired.

And what of something like SPL's Iron (plug-in) - would it be better with a dB meter rather than VU?

A VU meter is a 'dB meter'.....

So in short... the VU meter is perfectly serviceable as an indicator of gain reduction... :wave:
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Re: VU Meters on Compressor Plug-ins - How Useful Are They?

Postby desmond » Wed Mar 03, 2021 12:44 pm

I was going to reply to this earlier, but I hung back, as Hugh was bound to get to it, and would do it far better than me... :thumbup:
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Re: VU Meters on Compressor Plug-ins - How Useful Are They?

Postby CS70 » Wed Mar 03, 2021 12:59 pm

Ahah same here, and Hugh has overperformed as usual. :D

In short: yes they are useful.
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Re: VU Meters on Compressor Plug-ins - How Useful Are They?

Postby Zukan » Wed Mar 03, 2021 1:13 pm

Some plugs, like Brainworx HG2, come with both VU and PPM.
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Re: VU Meters on Compressor Plug-ins - How Useful Are They?

Postby The Elf » Wed Mar 03, 2021 1:38 pm

I prefer a peak meter on the compressor's output, since I want to preserve peak, but I'm more forgiving of whatever denotes GR.
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Re: VU Meters on Compressor Plug-ins - How Useful Are They?

Postby ITHertz » Wed Mar 03, 2021 1:50 pm

Thanks Hugh, that clears it up.

For some reason I was thinking of VUs an an entity unto themselves. I think it's because they're called volume "units" that I'd never really made the connection that they were another dB unit.

Cheers,

Chris
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Re: VU Meters on Compressor Plug-ins - How Useful Are They?

Postby Hugh Robjohns » Wed Mar 03, 2021 2:00 pm

The Elf wrote:I prefer a peak meter on the compressor's output, since I want to preserve peak...

Some form of peak metering should really be on your console (real or virtual) which is where you would normally be assessing the programme (mix) levels.

The meter on the compressor is really only there for the initial installation calibration/line up and to indicate that it's doing something.
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Re: VU Meters on Compressor Plug-ins - How Useful Are They?

Postby Hugh Robjohns » Wed Mar 03, 2021 2:14 pm

ITHertz wrote:For some reason I was thinking of VUs an an entity unto themselves. I think it's because they're called volume "units" that I'd never really made the connection that they were another dB unit.

I've always been bemused as to why the Americans decided to suggest the meter was calibrated in 'Volume Units' since they didn't actually exist as as defined 'thing' in the audio world at the time (or since), and when they were actually describing conventional decibels graduations!

We do now have 'Loudness Units', of course, and proper metering system to measure them accurately!

I always thought the 'volume unit' was the cubic metre... or a litre... :lol:

However, the fact that the meter inherently integrates the signal energy over time when displaying programme audio, the indication is loosely related to perceived volume... so it sort of works...

However, the lower scale, intended for use with programme audio actually provides an indication of approximate programme modulation, so the meter was really intended as a 'modulometer' (when it wasn't being used to align signal levels with sine tones) -- a term which Nagra later claimed for their portable tape recorders!
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Re: VU Meters on Compressor Plug-ins - How Useful Are They?

Postby Hugh Robjohns » Wed Mar 03, 2021 2:27 pm

By the way... although the spec for the SVI is very precise and has been maintained and updated over the years, not all VU meters are built to meet those standards!

So while most will give a stable indication of 0VU for a +4dBu sine wave input tone, their indications with programme material are often wildly adrift. A proper VU meter which is genuinely compliant with the standard and is useful as a reference audio metering tool is expensive!

A cheap meter with a VU scale stuck on is only really of any use for line-up calibrations.
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Re: VU Meters on Compressor Plug-ins - How Useful Are They?

Postby Zukan » Thu Mar 04, 2021 9:31 am

And let us not even begin to discuss the detailed customisable configurations available for various VU meters nowadays...K-standards, built-in dynamic eqs, BBC/DIN/Nordic, HP and LP filters and so on and on and on.....
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Re: VU Meters on Compressor Plug-ins - How Useful Are They?

Postby Martin Walker » Thu Mar 04, 2021 2:46 pm

Am I the only one who bemoans how small some of these VU meters are on compressor plug-ins?

Despite them being 'calibrated' with microscopic dB numbers, all you can really tell at their miniscule size is your current gain reduction is a twitch or a lurch :D


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Re: VU Meters on Compressor Plug-ins - How Useful Are They?

Postby desmond » Thu Mar 04, 2021 2:53 pm

That problem is most easily fixed by just compressing more.

;)
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Re: VU Meters on Compressor Plug-ins - How Useful Are They?

Postby Hugh Robjohns » Thu Mar 04, 2021 3:19 pm

desmond wrote:That problem is most easily fixed by just compressing more.

;)

:bouncy: :bouncy: :bouncy: I love a simple solution!
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