Stereo & phase

Global polarity

If one channel pushes while the other pulls, they can cancel when mixed to mono. This analysis examines their relationship across the whole track.

Reading the result

Near +1: the channels move together. Near −1: they oppose each other. Listen in mono before making a correction: intentional stereo effects can also create opposition.

Illustrative equal-gain sine waves. Correlation runs from −1 (opposition) to +1 (matching polarity). Real mixes can differ by time or frequency.

Go deeperMethod, limits, verification and sources

The polarity indication compares the two decoded stereo channels over the whole file. It flags a likely L/R polarity inversion when they are strongly opposed. It cannot determine absolute polarity, and it cannot decide whether an intentional artistic phase relationship is a defect.

Calculation#

For exactly two channels, FlacCompagnon accumulates left energy E_L, right energy E_R and cross energy C = Σ(L×R). The reported correlation is:

correlation = C / sqrt(E_L × E_R)

It is clamped to the interval −1 through +1. A value at or below −0.95 produces the polarity indication. The threshold accepts relationships such as R = −kL, where different channel gains prevent complete cancellation in a mono fold-down.

No correlation is reported when either channel is silent, either energy is invalid, or the file is not exactly stereo.

Interpretation#

A value near +1 means the channels move together, a value near 0 means little whole-track linear correlation, and a value near −1 means they move in opposite directions. The indication is deliberately conservative: values above −0.95 are still useful information for mono compatibility, but are not labelled as an inversion by the app.

The result is whole-track average behaviour. A brief inverted section can be diluted by the rest of a track, while intentional spatial effects, stereo ambience and phase processing can make correlation a poor quality judgement. The separate local and frequency-band phase analysis measures short windows and four frequency ranges to reveal opposition hidden by this average. Listen in mono and inspect the source before changing audio.

Tests and manual fixture#

cargo test -p flaccompagnon-core analysis::stereo

ffmpeg -n -f lavfi -i 'aevalsrc=0.1*sin(2*PI*1000*t)|-0.1*sin(2*PI*1000*t):s=48000:d=5' -c:a pcm_s24le polarity.wav

Import polarity.wav: the correlation should be approximately −1 and the polarity indication should appear. Open it in Audacity, invert one channel, then export a comparison file to verify that the indication disappears.

Integrating the engine in your software?

Browse the Rust API in Rustdoc ↗