How to Convert MOV to MP4 by Re-encoding Only the Audio

bill lee — Tue, 10 Nov 2026 10:02:01 +0000

Your upload pipeline expects an MP4 with H.264 video and AAC audio. The MOV you received already has H.264 video, but its audio is PCM. You can copy the video into the MP4 and encode just the audio. In this test, all 72 compressed video packets matched the source.

AI disclosure — This article was generated with AI. The commands and results come from a local FFmpeg 8.1.2 test.

Make two MOV files with the same video

Run the commands in a fresh directory with ffmpeg and ffprobe installed. The first command needs an FFmpeg build with libx264. It creates a three-second test pattern with AAC audio. The second takes the same video stream and gives it a PCM audio track.

ffmpeg -f lavfi -i 'testsrc2=size=320x180:rate=24:duration=3' \
  -f lavfi -i 'sine=frequency=440:sample_rate=48000:duration=3' \
  -c:v libx264 -pix_fmt yuv420p -c:a aac -b:a 128k demo-aac.mov

ffmpeg -i demo-aac.mov \
  -f lavfi -i 'sine=frequency=440:sample_rate=48000:duration=3' \
  -map 0:v:0 -map 1:a:0 -c:v copy -c:a pcm_s16le demo-pcm.mov

Inspect the streams before choosing a command. Both inputs end in .mov, but the audio codecs differ.

codecs() {
  ffprobe -v error -show_entries stream=codec_type,codec_name -of csv=p=0 "$1"
}

codecs demo-aac.mov
codecs demo-pcm.mov

Input	Video codec	Audio codec
`demo-aac.mov`	`h264`	`aac`
`demo-pcm.mov`	`h264`	`pcm_s16le`

Change only what your destination needs

If both tracks already meet the destination requirements, copy both. If the video qualifies and the destination requires AAC, copy the video and encode the audio.

ffmpeg -i demo-aac.mov \
  -map 0:v:0 -map 0:a:0 -c copy -movflags +faststart copied.mp4

ffmpeg -i demo-pcm.mov \
  -map 0:v:0 -map 0:a:0 -c:v copy -c:a aac -b:a 128k \
  -movflags +faststart audio-converted.mp4

codecs copied.mp4
codecs audio-converted.mp4

Both outputs report H.264 and AAC. The -map options select the first video track and the first audio track. If your file has alternate audio or subtitles, decide which tracks the output needs before adapting the command. +faststart moves the MP4 index to the front of the file. FFmpeg stream handling · MP4 muxer options

Diagram based on the local PCM-to-AAC test.

Verify the video packets

Seeing h264 on both sides tells you the codec. An H.264-to-H.264 re-encode would show the same name. To check what survived the conversion, hash the compressed payload of each video packet and compare the lists in order. FFprobe packet and data hash options

video_packets() {
  ffprobe -v error -select_streams v:0 -show_packets \
    -show_entries packet=data_hash -show_data_hash sha256 \
    -of csv=p=0 "$1"
}

video_packets demo-pcm.mov > before.sha256
video_packets audio-converted.mp4 > after.sha256
wc -l before.sha256 after.sha256
diff -u before.sha256 after.sha256

Each list had 72 hashes, and diff exited successfully with no output. That checks video packet payloads in this sample. The container, timestamps and audio are outside that comparison. For a production upload, also open the output in the destination player and check playback and sync.

Stop Installing Software for One-Off Tasks: A Developer's Web Tool List

bill lee — Thu, 15 Oct 2026 09:14:04 +0000

The browser-based tools I reach for when I need to compress a video, convert a file, trim audio, or shrink an image or PDF without installing anything.

My laptop is full of tools I used once. A video converter for a single project. A PDF utility I needed for one contract. An audio editor I opened twice.

These days, for one-off media tasks, I try a browser tool first. It's faster than installing something, and nothing is left behind to update or uninstall.

Here's how I decide when to use a web tool, when to write a script, and which tools I use by category.

Disclosure: I work on VideoBee, which appears in the video section. I've tried to be fair about where it fits and where other tools are better.

How I choose a tool

Before I use any online tool, I check:

Does it handle my file size? Many tools silently cap uploads. This matters most for video.
What happens to my file? For anything sensitive (client work, private recordings), I prefer local tools.
How many steps? If it takes more than a few clicks, a script is probably better.
Does it need an account? For a one-off task, I'd rather not.
Is the output predictable? Re-encoding, metadata changes, and quality loss should be controllable.

When to use a web tool vs. a script

Situation	My choice
One-off task, one or two files	Web tool
Same task every week	Script (FFmpeg, ImageMagick, etc.)
Batch of dozens or hundreds of files	Script
Sensitive or confidential files	Local tool or script
Quick check on a machine that isn't mine	Web tool
Needs to run in CI or a pipeline	Script

Video

Video is where installing software feels the most disproportionate. Full desktop editors are huge, and FFmpeg's flags are easy to forget.

Compressing a video: when a file is too big to email, upload, or attach, I use an online compressor. VideoBee's video compressor handles large single files (up to 10 GB), which is the main reason I use it for files that other tools reject.
Converting formats: MOV to MP4, MP4 to WebM, and similar jobs are easy to do with a video converter when you don't want to dig up the right FFmpeg command.
Trimming a clip: cutting the first and last few seconds off a screen recording is a perfect web-tool job. A video cutter does it in the browser.
For scripting or full control: FFmpeg remains my default. If you want a local GUI, HandBrake is a long-standing open-source option. Audio
Quick trims and format conversions: a browser-based audio tool is enough for most one-off jobs.
Real editing: Audacity is the free desktop standard.
Scripting: FFmpeg again, for extracting or converting audio in bulk. [Add your own picks here, ideally tools you've actually used and can describe in a sentence.]

Images

Compressing and converting for the web: Squoosh is a browser-based tool that lets you compare formats and quality settings side by side.
Batch processing: ImageMagick or Sharp (Node) in a script.

PDFs

Merging, splitting, compressing: browser tools cover the common cases.
Self-hosted option: if you handle sensitive PDFs, look at open-source tools you can run yourself. [Add the PDF tools you actually use here.]

A small FFmpeg safety net

Even if you prefer web tools, keep these two commands handy for when you're offline or processing many files:

# Compress a video
ffmpeg -i input.mp4 -c:v libx264 -crf 26 -preset medium -c:a aac -b:a 128k output.mp4

# Convert MOV to MP4
ffmpeg -i input.mov -c:v libx264 -crf 23 -c:a aac output.mp4

What I don't use web tools for

Anything confidential, such as unreleased product footage or client documents
Jobs I'll repeat, since a script pays off quickly
Very large batches
Workflows that need to be reproducible in CI

The bottom line

Install software for work you do every day. For everything else, a good browser tool saves time, disk space, and mental overhead.

What's in your no-install toolbox? Drop your favorites in the comments, especially for audio and PDFs, where I'm still looking for better options.

DEV Community: bill lee