Look for the copy note before changing anything
M4A is an audio form of the MP4 container family; it is not another name for the AAC codec. PrivateMP3 can write M4A in two ways: copy the existing compressed packets into a new container, or decode the selected track and encode new AAC. The output extension is the same, but the quality history is not.
The packet-copy route is offered only when all of these are true:
- The selected audio track is identified as AAC, AC-3 or E-AC-3.
- M4A is the selected output.
- The complete audio timeline is kept.
- There are no cuts, volume changes or fades, and on the ringtone page the selection is not the output.
When those conditions hold, the controls show Copied without re-encoding — same audio, no quality loss.
That visible note, rather than the page title or the M4A button alone, confirms the copy route.
Packet copy and AAC encoding are different jobs
| State | M4A result |
|---|---|
| Unedited AAC, AC-3 or E-AC-3 | Copy packets; no audio decode or encode. |
| Edited AAC | Decode; encode new 192 kbps AAC. |
| Other supported audio | Decode; encode new 192 kbps AAC. |
| no decoder | Copy only if it is unchanged AAC, AC-3 or E-AC-3. |
Packet copy is also called remuxing: the encoded audio is moved into a newly written container without changing the compressed audio packets. The source video is not modified. The result contains one selected audio track; the video and subtitle tracks are not carried into the M4A.
A selection is not an edit until you apply it
Dragging over the waveform creates a selection for inspection. On this page it does not by itself shorten the output. Pressing Remove selection or Keep only selection changes the kept timeline and disables packet copy. A volume adjustment also changes decoded sample values, so it requires re-encoding.
To preserve eligible AAC, use this order:
- Open the local video and choose the intended Audio track if more than one appears.
- Leave the complete timeline and volume unchanged.
- Select M4A and confirm that the copy note is visible.
- Convert and save the new audio-only file.
If you need a cut, make it deliberately and treat the result as newly encoded AAC. The old claim that a copied file could still be cut on an AAC frame boundary did not match this implementation: any applied cut switches to the decode-and-encode path.
Measured: what the copy keeps
These copies were made in Chrome on Windows 11 between 5 and 10 October 2026 from FFmpeg-generated files, and each saved M4A was read back with ffprobe. The method and the rest of the results are in the screen recording test.
| Source | Copy took | Saved M4A | What ffprobe found |
|---|---|---|---|
| 60 min MP4, 4.39 GB, AAC 160 kbps | 6.1 s | 72,676,105 bytes | AAC, 48 kHz stereo, 159.9 kbps, 3,600.021 s |
| 10 min MOV, AAC 128 kbps | 1.2 s | 9,739,957 bytes | AAC, 48 kHz stereo, 128.3 kbps, 600.021 s |
| 10 min MKV, AAC 160 kbps on track 1 of 2 | 1.3 s | 12,263,586 bytes | AAC, 48 kHz stereo, 159.9 kbps |
| 30 s MKV, AAC 5.1 | 0.05 s | 710,176 bytes | AAC, 48 kHz, all 6 channels kept |
| 3 min M4A, AAC 219 kbps | 0.09 s | 4,965,348 bytes | AAC, 44.1 kHz stereo, 219.2 kbps |
In every case the bitrate, sample rate and channel count came out as they went in, which is what an unchanged copy should do. Two details show up in the numbers. Each copy runs 1,024 samples longer than the source, which is 21.3 ms at 48 kHz and 23.2 ms at 44.1 kHz: AAC begins with that much encoder priming, which the original hides, and the copy keeps it at the start rather than altering any packet. And the M4A is about 1 percent larger than bitrate times duration, because an M4A indexes every audio packet; the File check panel and the size calculator both include that index. A 5.1 track copies with all six channels, unlike an MP3, which can hold only two.
What happens when the source is not AAC
An M4A can still be produced from another audio codec when the current browser can both decode that track and encode AAC. PrivateMP3 then makes new 192 kbps AAC. This is a conversion, not a lossless container change. It may still be the right output when the destination requires M4A, but it does not preserve the original compressed packets.
If the selected track says no decoder, a newly encoded M4A cannot start. Try another audio track in the same file. Unchanged AAC is the exception because its packets can be read and written without decoding samples. PrivateMP3 does not claim that every codec combination works in every browser.
Copy failures have their own messages
The copy path can fail while reading compressed packets or while finishing the new M4A container. The compressed audio packets could not be copied from this file
points to packet reading and may indicate an incomplete source. The output file could not be finished
occurs later and tells you to check available storage; it does not mean the original was changed.
Before 5 October 2026 the packet-copy message appeared for every MP4 and MOV, whatever their condition. That was a defect here, not a problem with those files: the encoder priming described above gives the first AAC packet a negative timestamp, and the M4A writer refused it. It is fixed. If you saw that message on an MP4 or MOV before then, the same file should now copy.
A missing track, unreadable duration or waveform failure happens before conversion and is a different problem. Use the waveform and Convert diagnostic guide to identify the stage instead of assuming that every M4A failure is a codec problem.