The What, Why & How Of Stereo Audio Metering

Typical stereo phase‑coherence meter displays. +1 denotes a mono signal (fully phase‑coherent), and most stereo material should sit in the region between here and 0 in the centre. ‑1 denotes that the channels’ signals are out of phase — in other words a Sides only signal.

Today, we have access to all sorts of audio metering options. But with a little understanding and experience, you’ll find you can glean a surprising amount of information with the quickest of glances at the humble vectorscope...

Recording in stereo dates back 94 years, to 1932, when Harvey Fletcher in the Bell Labs Research department recorded the Philadelphia Orchestra under Leopold Stokowski. Two years later, in Britain, Alan Blumlein’s research into stereophonic sound led to EMI’s first stereo recording of the London Philharmonic Orchestra, conducted by Sir Thomas Beecham. These, though, were both academic research projects, and commercial stereo recording didn’t really take off until the end of the 1950s. And that’s when the challenge of how to meter two audio channels simultaneously needed to be addressed.

The most obvious and simplest solution was to use two standard mono meters (most commonly two VU meters), one for each channel, mounted alongside each other. Although pragmatic and cost‑effective, this solution wasn’t without problems. Most notable was the difficulty of keeping an eye on two physically separated meter needles flicking independently. Particularly problematic is that this metering arrangement makes it quite difficult to spot small level offsets between the two channels.

The widespread adoption of LED bar‑graph meters in the mid‑1970s solved that problem reasonably well; closely adjacent bar‑graph meters make the viewing and comparison of channel levels much easier, particularly around the 0VU region where the scale is relatively expanded. It’s hardly surprising that this has become the default meter format for most consumer and professional stereo and multi‑channel audio equipment, whether hardware or software. But bar‑graph meters weren’t the only answer, and UK company Sifam came up with a unique, elegant alternative: the BBC Twin‑Twin PPM (more about that in the separate box) (the online version of this review describes that in more detail).

Phase Correlation

Critically important though dual meters are, when you’re working with stereo material they don’t tell the whole story, because they only indicate the absolute and/or relative signal levels of the two channels. That limitation matters, because the positional information about a source within a stereo sound stage is conveyed by both inter‑channel level differences (which these audio meters can show), and inter‑channel phase differences (which they can’t). Consequently, another form of metering is required to inform the user about phase relationships between the two channels.

Look at any serious studio, mastering or broadcast console from the 1970s and ’80s (and even today), and you’ll typically see a trio of meters in the centre section: one each to display the left and right channel levels, plus a ‘phase meter’ or ‘stereo correlation meter’. All three are needed to provide full information about the stereo image being created.

Phase meters have appeared in a variety of formats over the years (mechanical, LED and virtual), but most have employed the same simple scale marked with +1 at the top or right, 0 in the middle, and ‑1 at the bottom or left. A reading of +1 on the phase meter scale indicates identical signals with perfect phase correlation between the two channels — dual mono, in other words. The ‑1 end of the scale indicates identical signals with a phase difference of 180 degrees (ie. opposite polarity).

Often, the region between 0 and +1 is coloured green, and between 0 and ‑1 red — green for safety, red as a warning. Normal stereo material has randomly changing levels and phase relationships between the two channels and, as a result, the needle normally does hover in the positive, green half of the display. A narrow stereo image will pull the needle closer to +1, while a wider stereo signal stretching across the full sound stage pulls the needle closer to 0, in the centre. Brief and small excursions below 0, into the red side of the scale, are rarely problematic in practice — they’re common when monitoring the output of spaced mic arrays, for example — but do indicate reduced mono‑compatibility.

The trio of left, right and phase meters, then, can tell us a great deal. But it’s still less than ideal, because one must keep an eye on three different meters all at once! Wouldn’t it be helpful if there were some form of integrated metering system, showing all of this relative level and phase information in one easily viewable and understandable place? Thankfully, such a thing exists: the audio vectorscope.

Enter The Goniometer

An audio vectorscope, or ‘goniometer’, is essentially a real‑time, two‑dimensional graphical plot of the left and right channel signal amplitudes. Plotting the two audio signals in this way reveals both their relative amplitudes and phase relationships as a momentary dot on the graph. As the audio signals change, moment by moment, the dot moves, leaving a trace that resembles a continuously changing, tangled ball of string.

They often take the form of a Lissajous display. This is a geometric pattern commonly created using an oscilloscope by plotting two different periodic signals simultaneously on the X and Y axes (rather than plotting a signal against time). Its invention is often credited to Holger Lauridsen, the Chief Engineer at Danmarks Radio in Denmark in the early 1950s, who is also cited as popularising the use of Mid‑Sides microphone arrays around the same time.

In most applications, plots are oriented such that the Y axis is vertical. But tilting the plot by 45 degrees makes the vectorscope much more useful...

To better understand the audio vectorscope display, imagine a conventional graph with the left channel amplitude plotted on the vertical axis, and the right channel on the horizontal axis. These two axes cross at zero amplitude, with positive‑ and negative‑going half‑cycles drawn either side. To make the resulting display more representative and meaningful in the stereo audio sense, the entire plot is rotated 45 degrees anticlockwise, so that the positive half of the left channel is indicated at the top left, and the positive half of the right channel at top right. Thus a dual‑mono (or centre‑panned) signal appears as a straight vertical line in the middle. So it’s a very intuitive and informative display of stereo positioning.

An important point to note is that, to keep the display readable and meaningful over a wide range of audio levels, most vectorscope displays employ an automatic gain function (AGC) — this way, they maintain a relatively consistent image size. Very quiet signals collapse into, or grow from, the central crossing point of the two axes, of course, but all signals across the normal working level range produce a similar‑sized tangled ball of string. Excessively loud signals that overload the display circuitry will result in a hard‑edged diamond shape instead of a soft‑edged ellipse (hard edges for lower signal levels indicate limiting or clipping somewhere in the signal path)....