VU Meters: “Virtually Useless” or Very Useful?

After 85 years of active service, the humble VU meter remains as useful as ever in today’s digital studios — despite BBC engineers nicknaming it ‘virtually useless’!

As I was listening to David Mellor’s SOS Podcasts about gain‑staging (www.soundonsound.com/author/david-mellor) recently, my ears pricked up at his passing comment that “BBC engineers refer to the VU meter as virtually useless.” It wasn’t a surprise, exactly, as I was told exactly that during my initial BBC training in the early 1980s and I believed it for years. But I came to understand that the claim was, in fact, based on a fundamental misunderstanding of how the VU meter was designed to be used!

My view today — and it’s shared by many professional mastering and recording engineers all around the world — is that the VU meter remains very useful, even in today’s digital studios. So, in this article, I’ll take you through the virtues and practical benefits of the octogenarian VU meter, and explain how the BBC got it so wrong. To do that, we need to start with a little history...

Origins: PPMs & SVIs

The SVI meter — the first to be scaled in Volume Units — was described in a paper published in 1940, and its core specification remains in use today.The VU meter was conceived in 1939, through a collaboration between research company Bell Labs and the American broadcasters CBS and NBC, and a paper they published in 1940 described what they called the Standard Volume Indicator (SVI). As the SVI meter’s scale was calibrated in ‘volume units’ (and marked ‘VU’), it became known popularly as the VU meter — in much the same way that the modern BS.1770 Integrated Loudness meter is often called simply an ‘LUFS meter’. It’s testament to the genius of those 1940s engineers that the SVI’s core specification lives on today, virtually unchanged, as IEC 60268‑17:1990 Sound System Equipment. Part 17: Standard Volume Indicators.

Of course, various audio meters were already available in the 1930s, so why exactly did CBS, NBC and Bell Labs feel the need to design their own? Well, in the late 1930s, radio broadcasting was a rapidly expanding business globally, and there was a requirement to monitor and control the broadcast programme levels consistently and reliably. Over in Europe, many national broadcasters had developed their own metering systems and nearly all were Peak Programme Meters (PPMs or, more accurately, QPPMs — see the ‘Quasi‑PPM’ box for more on this). To protect the transmitters from overload, European engineers believed it was necessary to monitor the peak programme levels, and their work resulted in the DIN (German), Nordic (Scandinavian) and BBC (British) PPM meters. Although these each had different scales they all employed similarly complicated, valve‑based active electronics to detect and display pseudo‑peak levels.

Such complex metering systems, though, were relatively expensive to manufacture, so although the American broadcasters were aware of the European designs they thought them impractical for use across the vast US radio industry. A simpler, more affordable, passive solution was desired that, preferably, would be capable of indicating the average signal level in a way that reflected how listeners would perceive the volume.

Hence their SVI, and this meter comprised just three passive elements: an adjustable attenuator, a copper‑oxide rectifier and a bespoke moving‑coil meter, built to detailed specifications in terms of its sensitivity and needle ballistics. The rectifier was required to convert an AC audio signal into a DC voltage that could move the meter, and the passive copper‑oxide type they specified avoided the expensive valves employed in European PPMs, as well as the associated power supply. The bespoke moving‑coil meter’s relatively slow (300ms) needle rise and fall times ensured the meter would register low‑frequency and sustained sounds better than brief transient peaks, so it correlated reasonably closely to perceived audio volume, hence the decision to use the Volume Unit scale. By far the most complex and expensive element was the constant‑impedance variable attenuator, which had to maintain a consistent 600Ω load across the line being metered.

A glimpse inside SIFAM’s old MkIV VU meter. Note that, as with all VU meters, there are two scales on the front: one in Volume Units, and the other showing modulation from 0 to 100%.Photo: SIFAMPhoto: SIFAMBack in the 1930s (and well into the ’70s), professional audio interfacing used a matched‑impedance format rather than the matched‑voltage one of today, but the variable attenuator was an essential element to this meter. It adjusted the moving‑coil meter’s native sensitivity to accurately assess the signal level present on the line being monitored. Today, we expect metering to indicate the signal level directly: if you look at a conventional sample‑peak bar‑graph in your DAW, you can see whether the signal is peaking at 0dBFS or ‑10dBFS, or whatever. But the SVI was designed to be used differently, and this explains its relatively limited and cramped scale range. In practice, the SVI was wired across the signal line to be monitored, and the attenuator was adjusted until the meter hovered just below the 0VU mark (nearly 100% modulation). It was actually the resulting attenuator setting rather than the meter display itself that informed the user of the nominal signal level. The SVI’s attenuator covered levels from 0 to +24 dBm, and that made it suitable both for standard studio lines (which typically operated at +8dBm) as well as telecoms lines (which operated at +16dBm).

Although often overlooked, both the SVI meter and the VU meters that evolved from it have two scales. The primary calibration, with which we’re all no doubt familiar, is marked in decibels relative to 0VU. But there’s a secondary scale beneath that shows the programme modulation level between 0 and 100%. ‘Modulation’ is a term rooted in AM radio broadcasting, and it refers to the strength of the audio signal being broadcast: 0% modulation means that the carrier is present but conveying no audio signal, while 100% modulation means it’s carrying as much audio amplitude as is possible without overload. On an SVI/VU meter, the 100% modulation level aligns with 0VU (this is coincidental; it relates to the meter’s slow ballistics) and 0% is a little below the ‑20VU mark. In use, a steady 1kHz or 440Hz tone at the desired Operating Level (whatever that may be) should read 0VU, while a varying audio programme should stay below the 100% mark most of the time.

Studio Operating...