Why is the display rotated forty five degrees?

So that the vertical axis carries the sum of the two channels and the horizontal carries their difference. That orientation is what makes mono appear as a vertical line and inverted content as a horizontal one, which is the convention every goniometer uses.

What does the shape of a good stereo image look like?

A fountain: dense along the vertical where the two channels agree, spreading outward where they differ, and staying symmetric about the centre. A lopsided fountain suggests an unbalanced image, and a flat horizontal line means the sides are inverted.

Why only 4000 points?

Plotting every sample pair of a long recording would fill the square with ink and hide the distribution. Sampling four thousand pairs across the whole file shows the shape and the density while keeping the drawing readable.

Does the density of the points mean anything?

Yes. Points are drawn with additive transparency, so regions where the audio spends more time accumulate brightness. A bright compact core with faint outliers means the recording sits mostly in the centre with occasional excursions, which is typical of a centred mix with effects.

Can it tell me whether a specific instrument is out of phase?

Not reliably. The figure summarises the whole file, so one instrument's phase relationship is mixed with everything else and with any frequency-dependent behaviour. To isolate a part, render it on its own or cut that section out first and then measure it.

Goniometer / Vectorscope

You Might Also Need

Correlation Meter

Measure stereo correlation

Phase Meter

Check stereo phase correlation

Mid/Side Processor

Process mid and side separately

Channel Isolator

Isolate left or right channel

Goniometer / Vectorscope plots the stereo field of a file as a rotated Lissajous figure, so the shape of the image is visible rather than reduced to a number. Mono, inverted and genuinely wide material each produce a recognisable figure.

What is Goniometer / Vectorscope?

Left and right channels drive the two axes of a scatter plot rotated forty five degrees, which is what makes the usual reading possible: identical channels collapse to a vertical line, inverted channels to a horizontal one, and real stereo spreads as a fountain around the vertical.

What the display draws:

A square canvas with a dark background, a white reference cross and a surrounding circle to anchor the shape.
Up to 4000 stereo sample pairs plotted as small points with additive alpha, so overlap builds density and the outline of the distribution becomes readable.
A vertical axis carrying the sum of the two channels and a horizontal axis carrying their difference, which is why mono lands on the vertical and anti-phase content on the horizontal.
A caption explaining how to read the three characteristic shapes.

The rotation is not decoration: it is the reason the picture can be judged by eye at all. With sum on the vertical and difference on the horizontal, how tall the figure is shows how much the two channels share, while how wide it is shows how much they differ. A tall narrow figure means a nearly mono recording, a broad fan means a wide image with real side information, and a flat horizontal line means the sides are inverted. Only four thousand sample pairs are plotted, sampled across the file rather than taken from the beginning, because a scatter plot of every sample in a long recording would simply fill the square and lose its shape. Points are drawn with additive transparency so that regions where the signal spends most of its time appear brightest, which turns the display into a density map as well as a shape. The figure is a Lissajous pattern, the same construction a hardware vectorscope draws, rotated so that the vertical axis carries the mono sum and the horizontal axis carries the side difference. Reading it gets quicker with practice: a narrow ribbon means the stereo field is close to mono, a broad even fan means the two channels genuinely differ across the range, and a tilted shape means the image leans to one side. Because 4000 points are plotted across the whole file rather than only from its opening, the figure describes the recording as a whole instead of just its first seconds.

What the figure cannot show:

It is a whole-file summary, so a section that goes out of phase is diluted by the rest.
Four thousand points is a sample of the material, not every pair, so fine structure is approximate.
Frequency information is absent: nothing here distinguishes a wide bass from a wide cymbal.
No numbers are reported, so the display complements measurement rather than replacing it.
Silence produces a single dense dot at the centre with no useful shape.
Nothing is exported or recorded over time.

Use it when you want to see the stereo image rather than measure it: judging whether a reverb is genuinely wide, whether a doubled vocal has pushed the image off centre, or whether a remaster has narrowed a classic recording. Pair it with the correlation meters when a decision has to be defended with a number as well as a picture.

How to use Goniometer / Vectorscope

Drop a stereo audio file onto the panel; mono files are reported as unsupported.
Read the overall shape first: vertical for mono, horizontal for inverted, a fountain for real stereo width.
Judge brightness as dwell time, since the densest regions are where the signal spends most of its energy.
Use the reference cross and circle to compare how far the distribution reaches from the centre.

When to use Goniometer / Vectorscope vs related tools

Goniometer / Vectorscope is the visual counterpart to the correlation meters, best when the stereo image needs to be seen rather than scored. Correlation Meter and Phase Meter give the numbers, with the latter adding a channel balance figure. When the shape shows the image sitting off centre or leaning to one side, Mid/Side Processor and Channel Isolator are the tools that let you work on the sides directly.

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