In FFmpeg, set the output audio encoder explicitly with -c:a aac. For one compatible video file, a basic command is ffmpeg -i input.mp4 -c:v copy -c:a aac -b:a 192k output.mp4. For a playlist, use FFmpeg’s concat demuxer when the files’ streams and formats are compatible; use the concat filter when they need re-encoding or normalization. The bitrate shown is an example, not a universal optimum.
Choose whether to copy or re-encode the video
The audio and video streams can be handled separately. -c:a aac tells FFmpeg to encode the output audio as AAC. The filename extension alone does not guarantee that codec choice. With -c:v copy, FFmpeg copies a compatible existing video stream without decoding and re-encoding it. That is fast and avoids another generation of video compression, but it cannot apply video filters and may fail if the target container cannot carry the source stream.
If the video must be filtered, normalized, or converted for the destination, remove -c:v copy and choose a video encoder deliberately. Otherwise FFmpeg selects a default encoder registered by the output muxer, which may not be the one you intend. The FFmpeg command-line and codec documentation, accessed October 3, 2026, describe these behaviors; available options can vary with the installed build.
Encode one video’s audio as AAC
- Check the source and destination. Confirm the input file’s streams and that the destination container and playback system support the intended video and AAC audio.
- Run the command:
ffmpeg -i input.mp4 -c:v copy -c:a aac -b:a 192k output.mp4 - Review the result. Check FFmpeg’s output for errors, then play the resulting file and verify that audio and video remain synchronized and that the destination accepts it.
-b:a 192k is an illustrative audio bitrate, not an official recommendation for every source, listener, or platform. Select settings for the actual destination and your quality and file-size needs. FFmpeg’s AAC documentation also lists VBR modes with approximate average rates per channel—32 kbps for mode 1, 40 kbps for mode 2, 48–56 kbps for mode 3, 64 kbps for mode 4, and about 80–96 kbps for mode 5. These are encoder reference values, not comparative quality measurements or universal targets.
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- Pro performance with great pre-amps - Achieve a brighter recording thanks to the high performing mic pre-amps of the Scarlett 3rd Gen. A switchable Air mode will add extra clarity to your acoustic instruments when recording with your Solo 3rd Gen
- Get the perfect guitar and vocal take with - With two high-headroom instrument inputs to plug in your guitar or bass so that they shine through. Capture your voice and instruments without any unwanted clipping or distortion thanks to our Gain Halos
- Studio quality recording for your music & podcasts - Achieve pro sounding recordings with Scarlett 3rd Gen’s high-performance converters enabling you to record and mix at up to 24-bit/192kHz. Your recordings will retain all of their sonic qualities
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- Everything in the box: Includes Pro Tools Intro+, Ableton Live Lite, Cubase LE, and Hitmaker Expansion: a suite of essential effects, powerful software instruments, and easy-to-use mastering tools
Why MP3 audio has to be encoded to become AAC
If the input audio is MP3 and the output must contain AAC, FFmpeg has to decode the MP3 and encode the audio as AAC. Streamcopy forwards compressed packets unchanged; it cannot turn MP3 packets into AAC. The FFmpeg Project documentation describes transcoding as decoding a stream and encoding it again, and notes that transcoding is often lossy. If the input already has AAC and the destination supports it, copying that audio with -c:a copy may avoid an unnecessary lossy generation—but that would preserve AAC, not convert MP3 to AAC.
Build the playlist input
Create a UTF-8 text file named playlist.txt using FFmpeg concat syntax, with one file entry per video in the order you want them to play:
Rank #2
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- Studio-quality sound: With a huge 120dB dynamic range, the newest generation of Scarlett uses the same converters as Focusrite’s flagship interfaces, found in the world's biggest studios
- Find your signature sound: Scarlett 4th Gen's improved Air mode lifts vocals and guitars to the front of the mix, adding musical presence and rich harmonic drive to your recordings
- All you need to record, mix and master your music: Includes industry-leading recording software and a full collection of record-making plugins
- Everything in the box: Includes Pro Tools Intro+, Ableton Live Lite, Cubase LE, and Hitmaker Expansion: a suite of essential effects, powerful software instruments, and easy-to-use mastering tools
file 'track1.mp4'
file 'track2.mp4'Paths are interpreted by FFmpeg’s concat demuxer. Use paths appropriate to your working directory, and take care with quotes or unusual characters in filenames.
Use the concat demuxer when streams are compatible
The concat demuxer reads files in sequence and adjusts timestamps so each follows the preceding file. It can avoid re-encoding where the inputs are suitable, but it is not a guarantee for arbitrary mixed files. Check that the inputs have compatible stream layouts and that their video streams can be copied into the output container.
Rank #3
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- TWO JACKS, TWO JOBS - The green jack is stereo OUT for headphones or powered speakers; the pink jack is mono microphone IN for a 3.5mm mic. It does NOT support 4-pole headsets on a single combo plug, it does NOT power passive speakers, and it does NOT add surround sound - it is a stereo 2-channel adapter.
- FOR LAPTOPS AND DESKTOPS THAT NEED AN AUDIO PORT BACK - Adds a headphone and mic port to a laptop, desktop, or mini PC whose onboard jack has failed or was never there. Managed and work-issued computers can block new USB audio devices by policy - check with your IT department before ordering for a company machine.
- SABRENT SUPPORT AND WARRANTY - What is in the box: one USB audio sound adapter. Backed by a 1-year limited warranty, extended to 2 years when you register within 90 days on the manufacturer's website.
ffmpeg -f concat -safe 0 -i playlist.txt -c:v copy -c:a aac -b:a 192k output.mp4-safe 0 allows paths outside the concat demuxer’s restricted safe subset. Use it only with a playlist file you control. The command is a template, not a tested command for a particular set of files; it may need explicit stream mapping, a video encoder, or different container settings. FFmpeg’s concat documentation also warns that differing stream lengths can create gaps when timestamps are adjusted.
Use the concat filter when the files need re-encoding
FFmpeg recommends the concat filter when re-encoding is needed. This route is more appropriate if sources have incompatible stream layouts or need normalization, but it requires decoding and encoding the streams you process. Unlike streamcopy, the filter approach can work with media that must be transformed into a consistent format before joining. The exact filter graph depends on the number of inputs, their streams, and the desired output, so inspect the files and build the graph for those inputs rather than assuming one command works for every playlist.
Rank #4
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| Method | When it fits | Re-encoding and trade-off |
|---|---|---|
| Concat demuxer | Inputs have compatible streams and can be joined in sequence. | Can avoid video re-encoding; timestamp adjustment can leave gaps if stream lengths differ. |
| Concat filter | Inputs need re-encoding or stream normalization. | Re-encoding takes more processing and may be lossy, but permits filtering and transformation. |
Troubleshoot common failures
- FFmpeg rejects the concat playlist: Check each file path and the concat syntax. Confirm that the inputs’ stream layouts and codecs are suitable for the selected method. If they are not compatible, use a concat-filter workflow and re-encode as needed.
- The output container rejects the copied video: Keep AAC audio encoding, but choose a compatible video encoder instead of
-c:v copy. Streamcopy cannot convert a video codec or apply filters. - The output audio is still MP3: Make sure the output command includes
-c:a aacand that you are examining the newly created output, not the original file. - There are pauses or gaps between playlist items: Compare stream lengths and timestamps. The concat demuxer adjusts timestamps, but unequal stream lengths can create gaps; consider re-encoding through the concat filter if the files need normalization.
- FFmpeg reports an unavailable encoder or option: Check the installed build and its enabled codecs. FFmpeg documentation is version-sensitive, and builds can differ in available options.
- Video processing takes longer than expected: If you omit
-c:v copy, FFmpeg may re-encode video using a muxer default. Set a suitable encoder explicitly when re-encoding is intended.
Or let it run in the cloud
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Quick Recap
Best Value
- The new generation of the artist's interface: Connect your mic to Scarlett's 4th Gen mic pres. Plug in your guitar. Fire up the included software. Start making your first big hit
- Studio-quality sound: With a huge 120dB dynamic range, the newest generation of Scarlett uses the same converters as Focusrite’s flagship interfaces, found in the world's biggest studios
- Never lose a great take: Scarlett 4th Gen's Auto Gain sets the perfect level for your mic or guitar, and Clip Safe prevents clipping, so you can focus on the music
- Find your signature sound: Air mode lifts vocals and guitars to the front of the mix, adding musical presence and rich harmonic drive to your recordings
- With Scarlett 4th Gen, you have all you need to record, mix and master your music: Includes industry-leading recording software and a full collection of record-making plugins
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
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