Up & Down
Around the same time, radio broadcasters in Germany and the UK independently developed versions of a meter that became known as the Peak Program Meter (PPM). PPMs are quasi-peak meters and, as their name suggests, they’re designed more to measure peaks rather than the average values of the VU. This distinction is important because in radio broadcasting, and particularly those early days of AM broadcasting, it was vital to ensure the peak levels were as high as possible without exceeding the maximum.
The British PPM was developed by the BBC and features white markings on a black background, fast integration and slow return times, and a scale that goes from 1 to 7. The German PPM has faster integration and return times, wider dynamic range, and a semi-logarithmic scale (calibrated in dB relative to permitted maximum levels).
The German version is more commonly known as the DIN PPM, and both it and the British versions were later defined in IEC 60268-10 as the Type I and Type II PPM respectively. Further regional variants include the Nordic, used in Scandinavia, which uses the same integration and return times as the DIN PPM but employs a more logarithmic scale with a smaller dynamic range, and the EBU, named after the European Broadcast Union, which is identical to the British PPM except for the scale.
A commonly held misconception is that the PPM was designed as a superior alternative to the VU, but in truth, the PPM came about a bit before the VU, so it might be more accurate to say that the VU was developed as a more economical alternative to the PPM. Both ruled the roost for many years, but their relatively large physical size soon became a problem as channel and track numbers increased, particularly in recording studio and live applications in the 1970s.
Expanded Focus
At that point, a new meter began appearing on mixing consoles and other audio equipment: a vertical arrangement of colored light-emitting diodes known as an LED bargraph. The smaller size enabled meter bridges to show the level for every channel aligned directly above the channel, and in some cases, the meters sat right next to the fader.
The color scheme typically starts out as green and changes to yellow roughly halfway up and then to red at the very top, which enables the operator to aim for the “sweet spot,” much like the VU meter. Also, because there is no needle mass or ballistics involved, LED bargraphs can be true peak meters, although many are designed to mimic the averaging of VU meters (and some are even switchable).
And that just about brings us up to date.
Bargraphs are ubiquitous while VU and PPM meters are still widely used, and all exist in both hardware and software forms. Most of the latest developments in metering are based around continuing efforts to measure perceived loudness, which quickly plunges us into a whole new realm of standards and acronyms.
In 2001, the International Telecommunications Union (ITU) published the global standard ITU 85.1770, which used a K weighed filter curve (offering a bridge between subjective impression and objective measurement) to define the LKFS scale. This stands for loudness, K-weighted relative to full scale, and was designed to enable normalization of audio levels for broadcast television.
The European Broadcast Union (EBU) then pointed out that LKFS doesn’t comply with scientific naming conventions so they suggested changing it to loudness units relative to full scale (LUFS) in the EBU R128 standard. Subsequent revisions have brought both standards into line so LKFS and LUFS are now the same thing — both units are equal to 1 decibel and give negative values (as they’re relative to full scale where the maximum value is 0 dB).
Loudness Unit (LU) can also be used relative to other scales where 0 LU corresponds to the target level defined by various international loudness normalization standards such as EBU R128 (Europe), OP-59 (Australia), ARIB (Japan), AGCOM (Italy), ATSC A/85 and CALM (USA).
While this may seem like it’s only of interest to those in broadcasting, online streaming services and makers of portable playback devices are all exploring the implementation of some form of standardized loudness normalization to ensure consistent program levels. As a result, these measurements should be of great interest to anyone involved in the production of recorded music. (For further information check out the Music Loudness Alliance)
We’ve clearly come a long way since we measured signal levels in terms of miles of cable, and in the modern age of digital audio, it’s as important as it ever was to ensure audio levels are both adequate and consistent. Therefore, understanding the different types of meters and the information they convey is vital to ensure the success of any project.