Audio Channel Tool

Why Do This in Your Browser?

Splitting, merging and downmixing channels are small jobs that still send people to the command line, because each needs a different FFmpeg filter and the syntax is easy to get wrong. A merge written the obvious way, for example, silently keeps the first file's own left and right channels and drops the second file whenever the first one is stereo.

This tool covers the three common cases. Pick the mode, drop the file (or two files for Merge), and the filter chain runs underneath. Split outputs a zip with both mono files; Merge and Downmix output a single file. MP3, WAV, OGG, FLAC, M4A and AAC keep their format (MP3 and AAC at 192 kbps, OGG at 160 kbps); anything else, such as Opus, is written as MP3. Everything runs in your browser via FFmpeg.wasm.

How It Works

Split takes a stereo file and writes its left and right channels as two mono files in a zip; a mono file is refused, since it has only one channel. Merge takes two files, the first for the left channel and the second for the right. Each is mixed to mono first, so a stereo file contributes both of its sides rather than just its left one; files with different sample rates are resampled to the first file's rate; and the shorter file is padded with silence so nothing is cut off. Downmix folds a surround file (5.1 or 7.1) to stereo, and also turns a mono file into stereo with the same sound on both sides.

Under the hood: Split runs pan=mono|c0=c0 and pan=mono|c0=c1. Merge runs aformat=channel_layouts=mono and apad on each input, then join into a stereo layout. Downmix uses -ac 2, FFmpeg's default fold-down. We measured it on a 5.1 test file with a different tone on each channel: front left and right go to their own side at 0.41 of their original level; centre and the two surrounds go in at 0.29 (3 dB below the fronts); and the LFE (subwoofer) channel is dropped. FFmpeg scales the whole mix down so it can't clip, which is why the result plays about 7.7 dB quieter; run the Audio Normalizer afterwards if you need the level back. A mono file comes out with the same signal on both sides at 0.71 (−3 dB).

Tip: For straight concatenation of audio clips, use the Audio Merger — channel-merge here is for combining mono left + mono right into stereo. To change the sample rate after channel work, the Audio Sample Rate Converter handles that step.

Common Use Cases

Split Stereo for Single-Channel Editing

Extract just the host's microphone from a stereo podcast recording where each speaker was on a separate channel. Edit only the channel you need.

Merge Dual-Mono Recordings

Combine separate mic recordings (e.g., a Zoom recording's audio + a phone recording's audio) into a single stereo file for editing.

Downmix Film Audio to Stereo

Convert a 5.1 film soundtrack to stereo for headphone listening or for systems that don't decode surround properly.

Mono-to-Stereo for Compatibility

Some upload systems require stereo input. Duplicating a mono recording into both channels (effectively mono-but-stereo) prevents rejection.

Karaoke Track Channel Isolation

Some karaoke tracks have the vocals on one channel and the instrumental on the other. Split to isolate the side you want.

Multichannel Test File Reduction

Reduce a 7.1 reference test file to stereo for quick playback in any environment.

How We Compare

Honest read on free, paid, and self-hosted options for this kind of job:

UDT Audio Channel Tool (this tool): Free, browser-based, FFmpeg.wasm pan / join / -ac filters. Three modes (split, merge, downmix) in one page; downmix uses FFmpeg's fixed coefficients, with no controls for them. No upload, no watermark, no signup.

Audacity (desktop): Free, full audio editor with Tracks > Mix > Mix Stereo Down to Mono and Split Stereo to Mono menu options. This browser tool is the focused alternative — no install.

FFmpeg CLI: Free, command-line; same engine as this tool. The syntax differs per operation, which is exactly the friction this tool eliminates.

Adobe Audition: Paid, via an Adobe Creative Cloud subscription. Multitrack channel editing with control over how each channel is mixed.

Reaper (DAW): Paid licence. Flexible per-track channel routing for production work.

Online channel splitters (cloud): Free web tools exist for individual operations; check whether a site processes the file in your browser or uploads it.

Frequently Asked Questions

Is this audio channel tool really free?+

Yes — completely free, no watermark, no time limits, no signup.

What's the difference between split, merge, and downmix?+

Split takes one stereo file and produces two mono files, one per side. Merge takes two files and makes one stereo file, with the first on the left and the second on the right. Downmix folds a surround file (5.1, 7.1) to stereo, or turns a mono file into stereo with the same sound on both sides.

Why does split mode output a zip file?+

Because split produces two files (one mono left, one mono right), the tool packages them into a single zip download for convenience. The zip contains both files named with -left and -right suffixes.

Will downmix preserve the dialogue from a 5.1 file?+

Yes. The centre channel, where film dialogue usually sits, goes into both stereo sides at 3 dB below the front channels, and the surrounds go to their own sides at the same level. The LFE (subwoofer) channel is dropped, and the result plays about 7.7 dB quieter than the source's fronts because FFmpeg scales the mix down to prevent clipping. These are FFmpeg's default -ac 2 coefficients, measured on a 5.1 test file; the tool has no controls to change them.

Can I merge two stereo files instead of two mono files?+

Yes, but each stereo file is mixed down to mono first, so the result is the first file (as mono) on the left and the second (as mono) on the right. To join two files end to end instead, use the Audio Merger.

Are my audio files uploaded anywhere?+

No. FFmpeg.wasm runs entirely in your browser. The 32MB engine downloads the first time and your browser usually caches it. Once it has loaded, processing needs no network connection while the page stays open.

What's the maximum file size?+

Limited by browser memory. The whole file and the result are held in memory at once, so very large files, hundreds of MB and up, can fail, and phones run out sooner than laptops. Split mode handles two output files in memory simultaneously, so it has slightly less headroom than single-output modes.

What's the underlying engine and license?+

FFmpeg.wasm v0.12.x with @ffmpeg/core v0.12.6 (pan, aformat, apad and join filters). JSZip handles the zip in Split mode. The wasm wrapper is MIT and JSZip is available under MIT; the FFmpeg core build this tool loads includes x264, which makes that build GPL.

Related Tools on UDT

Why Do This in Your Browser?

How It Works

Common Use Cases

How We Compare

Frequently Asked Questions

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