Stereo / Mono & Differential Audio Mixer

This mixer handles practical channel math without opening a DAW. In one place you can fold a stereo file to a safe 0.5/0.5 mono sum, upmix a mono file to a dual‑mono stereo track, solo the left or right channel on its own, swap left and right, derive the mono difference (L−R) for phase inspection, or encode and decode Mid/Side pairs for mastering and microphone work. Every operation exports a clean audio file in a format you choose.

Stereo → mono with a 0.5/0.5 average is the default because it’s the safest general‑purpose downmix. When left and right are largely in phase (typical for pop mixes and speech recorded with a mono mic duplicated to both sides), simply adding them would raise the level by up to 6 dB and cause clipping. Averaging keeps the perceived level close to the original, preserves mono compatibility for broadcast and mono playback devices, and is the correct default for 90% of downmix tasks.

When 0.5/0.5 isn’t what you want: if a stereo file was mixed in M/S and the sides were phase‑inverted on purpose for a wide effect, averaging can cancel some material (classic “vocal disappears in mono” issue). In those rare cases you either want to keep the file stereo, or pre‑process in a DAW with phase correction. For everyday speech and music, the 0.5/0.5 sum is almost always the right call.

Mono → stereo duplicates the single channel to both sides, producing what engineers call “dual mono.” This is not the same as actually stereo: the result still sounds centered and narrow, because left and right are identical. It’s useful when a platform or player insists on a stereo input but your source is mono (for example, publishing a mono voice recording to a podcast host that rejects mono uploads). If you want a believably wide sound from a mono source, you need actual processing (stereo widener, reverb, Haas delay) — dual mono is strictly a container‑level accommodation.

Left‑only and right‑only soloing are helpful for inspection and rescue work. If an interview was recorded with the guest on one channel and the host on the other, soloing a single channel lets you extract just one speaker — useful for transcription, for isolating a cleaner take, or for diagnosing which microphone has a problem. Channel swap flips the two: handy for fixing a recording where cables were plugged in reversed, or for matching the orientation of a video with a different panning convention.

Differential modes (L−R and R−L) subtract one channel from the other. The mathematical effect is that anything identical in both channels (centered material — kick drum, lead vocal, centered speech) cancels to silence, and anything different between channels (reverb, stereo widening, side‑positioned instruments) is left behind. This is a diagnostic tool: if L−R has a lot of vocal, your mix isn’t truly mono‑compatible; if L−R is nearly silent, your file is essentially mono already. It’s also the basis of karaoke extraction, though results depend heavily on how the source was mixed.

Mid/Side encoding and decoding is the pro‑audio tool for working with stereo width separately from content. Mid/Side encode produces a file where the left channel carries the centered (“mid”) content and the right channel carries the stereo difference (“side”) information. Mastering engineers use this split to, for example, compress just the center of a mix without touching the stereo width, or to boost side information for a wider chorus without affecting the vocal. Decode reverses the process and returns a normal L/R stereo file.

The tool applies 0.5 gain on sums and differences during M/S conversion, which keeps peak levels near the source and reduces clipping risk when the encoded file is processed further. If you enable the optional soft limiter, a gentle ceiling around −0.2 dBTP is applied after the pan filter so that downmixes of very hot sources cannot clip. Disable the limiter if you want raw mathematical output — useful for measurement, QC, or when you’re going to apply your own true‑peak limiting downstream.

Output format choice matters depending on where the file is going. WAV and FLAC are lossless and preserve every bit of the channel math — ideal for further editing, mastering, archival work, or QC measurements. MP3 with a VBR preset is the right choice for broad playback compatibility and sharing, especially for speech content where MP3 artifacts are minimal. M4A (AAC) at a chosen bitrate is the most efficient lossy format and plays natively on Apple devices and modern Android; it’s a good pick for compact delivery when both sides of the connection are modern.

Input flexibility: the tool accepts common audio containers (WAV, MP3, M4A/AAC, FLAC, OGG, OPUS) and common video containers with an audio track (MP4, MOV, MKV, WEBM). For video inputs, the first audio stream is processed and the video is discarded — so you can drop a movie clip directly to extract and downmix its audio in one step. If your video has multiple audio streams (different languages, commentary tracks), export the specific stream you want in a video editor first and then run it through this tool.

Workflow patterns this tool fits into: preparing a stereo podcast for a mono radio broadcast, checking mono‑compatibility before uploading to a streaming service, rescuing a badly‑routed recording where hosts are on different channels, encoding a project to M/S for mastering in a stereo‑aware DAW, extracting a single channel of a dual‑mono interview for transcription, and verifying that a “stereo” file actually contains distinct information rather than being dual‑mono dressed up as stereo. For each of these, the math is simple, the expected output is predictable, and the tool saves you from a round‑trip through a full DAW.

Common pitfalls: do not assume that a file with two channels is true stereo — many “stereo” exports are dual mono, and the differential mode will reveal this immediately. Do not disable the limiter and then push very hot material through a sum; you may produce peaks above 0 dBFS that clip on export. And if you hear material disappear after a mono sum, check phase: an out‑of‑phase stereo file will cancel the common content when summed, which is a signal of a mixing issue rather than a bug in the tool.