# Audio Crossfades, J and L in FFmpeg — effect skill for cuts & pacing

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Crossfade audio with acrossfade, or build J/L-cuts by delaying one track with adelay and mixing with amix.

- **Type:** [Effect](https://varg.ai/how-to/effects.md) · [Cuts & pacing](https://varg.ai/how-to/browse/skills/cuts.md)
- **Slug:** `ffmpeg-audio-crossfade-jl`
- **Status:** live
- **Publisher:** varg (varg, verified)
- **License:** MIT
- **Source:** https://github.com/vargHQ/skills/blob/main/varg-video-edit-library/references/cuts.md#ffmpeg-audio-crossfade-jl
- **Works in:** Claude Code, Claude.ai, Codex, Cursor, OpenClaw, Gemini CLI, Any MCP client, varg Studio
- **Requires:** ffmpeg
- **Cost:** Free — the take renders locally; generated footage, if any, bills by the model

## What it does

Sound changes smoothly or leads/trails the picture cut.

**When:** Every join with dialogue or music; J/L-cuts for interviews and scene changes.

**Watch out:** amix's default normalization halves levels per input; set `normalize=0` and control gain yourself.

**Timing:** Crossfade 0.05–1 s; J/L lead or tail 0.3–2 s

**Also called:** acrossfade, afade, adelay amix, audio split edit, audio overlap

### In FFmpeg

Crossfade (overlap by d, curves from afade: tri, qsin, hsin, esin, log, exp…):
```bash
[0:a][1:a]acrossfade=d=0.5:c1=tri:c2=tri[a]
```
J-cut (B's audio 1 s before B's picture, cut at 4 s):
```bash
[0:a]atrim=0:4,asetpts=PTS-STARTPTS,afade=t=out:st=3.4:d=0.6[aa];
[1:a]atrim=0:4,asetpts=PTS-STARTPTS,afade=t=in:d=0.3,adelay=3000:all=1[ab];
[aa][ab]amix=inputs=2:normalize=0:duration=longest[a]
```
`adelay` takes milliseconds per channel (`1500|500`), `S` suffix for samples, `s` for seconds; `all=1` applies one delay to all channels.

### Where this comes from

- [FFmpeg filters: acrossfade](https://ffmpeg.org/ffmpeg-filters.html#acrossfade)
- [FFmpeg filters: adelay](https://ffmpeg.org/ffmpeg-filters.html#adelay)
- [FFmpeg filters: amix](https://ffmpeg.org/ffmpeg-filters.html#amix)

## Use when

- Every join with dialogue or music; J/L-cuts for interviews and scene changes.

## Not for

- amix's default normalization halves levels per input; set `normalize=0` and control gain yourself.

## Install

- Claude Code: `claude plugin marketplace add vargHQ/skills && claude plugin install varg@varg`
- Codex: `npx skills add vargHQ/skills --skill varg-video-edit-library -a codex -y`
- Cursor: `npx skills add vargHQ/skills --skill varg-video-edit-library -a cursor -y`

## Example output

https://s3.varg.ai/how-to/previews/96126c63b093/take-ffmpeg-audio-crossfade-jl.mp4 — cut between two shots, with the timeline strip of the edit. Footage: Tears of Steel (2012), (CC) Blender Foundation | mango.blender.org, CC BY 3.0; Charge (2022), Blender Studio | studio.blender.org, CC BY 4.0. (credit: Rendered by varg · Tears of Steel (2012) + Charge (2022))

## More cuts & pacing skills

- [SmartCut (capcut-ai-editor)](https://varg.ai/how-to/skills/capcut-smartcut-mcp.md): MCP that reads CapCut auto-captions to cut silences and duplicate takes right inside your CapCut project.
- [Auto-Editor](https://varg.ai/how-to/skills/auto-editor.md): CLI that cuts dead space by loudness or motion and exports MP4 or editor timelines; now also an agent skill.
- [AutoCut](https://varg.ai/how-to/skills/autocut.md): Edit video in a text editor: Whisper transcribes, you delete lines in Markdown, AutoCut cuts the video.
- [Cut Video](https://varg.ai/how-to/skills/cut-video.md): Claude Code skill that tightens long recordings with forced-aligned cuts, keeping laughs and comedic pauses.
- [Videogrep](https://varg.ai/how-to/skills/videogrep.md): Search spoken words across videos and auto-build supercuts of every match.
- [PySceneDetect](https://varg.ai/how-to/skills/pyscenedetect.md): Find cuts, fades and scene changes in a video, then split it or export scene lists for the edit.