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...