Audio Phase Inverter & Mid/Side Decoder

When two microphones record the same sound, or two speakers play it, their waveforms either reinforce or fight each other. If one is wired backwards, the speaker cone moves out while the other moves in, and the bass in the middle of the image disappears. The same thing happens in a recording where one channel was captured upside down. This tool flips and re-routes the two channels of a stereo file, and converts between the left/right form and the mid/side form that many engineers prefer to mix in.

Polarity versus phase

Inverting polarity multiplies every sample by −1, turning peaks into troughs. On its own it is inaudible. It matters in combination: add a signal to its inverted copy and you get silence, which is how noise-cancelling and some mixing tricks work. Phase shifts, in contrast, move different frequencies by different amounts and come from delays, filters and microphone distances. Inverting polarity can cure a wiring mistake, but not a timing offset between two microphones.

Mid and side

Any stereo pair can be written as what the two channels share, the mid, and how they differ, the side. The tool defines mid as (L + R) ÷ 2 and side as (L − R) ÷ 2. To go back, left is mid plus side and right is mid minus side. A mono sound panned dead centre has all its energy in the mid and nothing in the side. Reverb and wide synthesisers carry a lot of side. Mastering engineers adjust the two separately to widen a mix or tighten the bass.

The nine operations

Invert both channels, Invert left only and Invert right only change polarity. Swap left and right exchanges the channels. Keep the mid and Keep the side extract one of the two signals and play it on both sides. Remove the centre writes the side, so anything identical in both channels cancels. Encode left/right to mid/side stores the mid in the left channel and the side in the right, and Decode mid/side to left/right reverses that for a recording made with an M/S microphone and saved that way.

A worked example

Press Sample, choose Keep the mid (centre) only and press Apply. The result sample-song-mid.wav has two channels and 176,400 frames at 44,100 Hz, and its left and right samples are identical, as the browser test confirms by comparing them. Because the sample’s left channel plays the notes low and the right an octave higher, the mid contains both layers at half strength. Choose Keep the side only and you hear the difference of the two layers instead of their sum, which has the same notes but a different balance between the low and the high layer.

Edge cases

Centre removal works on identical content only; real vocals are often centred but are accompanied by reverb and doubling that stay. It also removes the kick and bass, which are centred too. Encoded mid/side files sound odd when played directly, with a loud centre on the left and a thin signal on the right, which is expected. Mono files accept only the inversion of both channels, and the page says so.

Limits

The tool has no phase-shifting, delay or width control; those belong to an editor. Output is WAV, Opus or AAC, and compressed formats lose a little detail on re-encoding. The work is done on your device.

Related tools

• Split Stereo to Mono Channels separates the two channels into files.
• Audio Volume Booster & Peak Normalizer restores level after removing the centre.
• Audio Converter folds a stereo file to mono.
• LUFS Loudness Meter shows how the change affects loudness.