Audio Channel Tool

Why Do This in Your Browser?

Channel routing — splitting, merging, downmixing — looks like the most basic audio task imaginable, and yet it's the one most likely to send people back to the command line. Stereo-to-mono is a single FFmpeg flag, but the syntax is different for split-to-files versus downmix-to-mono. Stereo merging needs the `amerge` filter. Downmixing 5.1 surround to stereo requires the right channel coefficients or the result loses dialogue clarity.

This tool covers the three common cases in one UI. Pick the mode, drop the file (or two files for stereo-merge), and the right filter chain runs underneath. Split mode outputs a zip with both mono files; merge and downmix output a single file. All three modes run entirely in your browser via FFmpeg.wasm.

How It Works

Pick a mode: Split (stereo → 2× mono files), Merge (2× mono → stereo), or Downmix (multichannel surround → stereo). For Split mode, drop a stereo audio file; the tool extracts each channel as a separate mono file and packages both as a zip. For Merge mode, drop two mono files in the order you want as left and right channels. For Downmix mode, drop a 5.1 or 7.1 file; the tool applies industry-standard downmix coefficients to preserve dialogue while folding the surround channels back into stereo.

Under the hood: Split uses `-map_channel 0.0.0` and `-map_channel 0.0.1` to extract left and right. Merge uses `amerge=inputs=2` with `pan=stereo|c0=c0|c1=c1` to specify the channel layout. Downmix uses `-ac 2` with FFmpeg's default `pan=stereo|FL<FL+0.707*FC+0.707*BL|FR<FR+0.707*FC+0.707*BR` coefficients for the standard center-equal downmix. Auto-downmix uses FFmpeg's built-in matrix when channel layout is detected automatically.

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 map_channel / amerge / pan filters. Three modes (split, merge, downmix) in one UI. 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: Creative Cloud $20+/mo. Multitrack channel editing with full control over downmix coefficients.

Reaper (DAW): $60–$225. Industry-standard channel routing for production work.

Online channel splitters (cloud): Several free options for individual operations. Most cover only one of split/merge/downmix. UDT covers all three in one UI.

Frequently Asked Questions

Is this audio channel tool really free?+

Yes — completely free, no watermark, no time limits, no signup. The site is supported by ads elsewhere; the tool is unrestricted.

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

Split takes one stereo file and produces two mono files (left and right separately). Merge takes two mono files and combines them into one stereo file with left/right assignment. Downmix takes a multichannel surround file (5.1, 7.1) and folds it down to stereo using standard mixing coefficients.

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 standard downmix coefficients (the same ones used by Blu-ray players and AV receivers) include the center channel in both stereo outputs at -3dB. Dialogue, which lives on the center channel, stays clearly audible in the stereo result.

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

No — merge mode expects two mono files. To combine two stereo files into a single stereo or multichannel file, the Audio Merger handles concatenation. For more complex layouts, FFmpeg CLI is the right tool.

Are my audio files uploaded anywhere?+

No. FFmpeg.wasm runs entirely in your browser. The 32MB engine downloads once and caches; after that, the tool works fully offline.

What's the maximum file size?+

Limited by browser memory. Most laptops handle 1–2GB audio files comfortably; phones cap around 500MB. 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 (map_channel, amerge, pan filters built in). JSZip handles the multi-file packaging. FFmpeg is LGPL-licensed; the wasm wrapper and JSZip are MIT.

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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