A screen recording is not an MP3
The capture is a video container — MP4, MOV, MKV or WebM — with a picture track and at least one audio track. Extracting audio writes a new sound file. It does not convert the recording itself into an MP3, and it does not fetch a file from a website. Open a file already on this device, then Convert and Save file.
The input file is not copied into memory
A long screen capture is handled differently from a conventional whole-file converter. PrivateMP3 asks the local file for ranges as they are needed. It does not decode the picture frames. For the waveform, decoded audio is reduced to minimum and maximum peaks for time buckets rather than retained as full PCM. This makes the page useful for a lecture you recorded, a meeting captured with permission, a tutorial you made or another local screen recording where only the sound is needed.
The output can be the memory problem
The completed audio still has to be stored somewhere. PrivateMP3 normally streams it into the Origin Private File System, browser-managed storage on the same device, and exposes it through Save file when finished. If that interface is unavailable, the worker falls back to an in-memory output buffer. A long result is more demanding on that fallback even though the source continues to be read in ranges.
Duration still costs time and storage
The browser must decode the audio timeline, build waveform peaks and encode every kept sample. Device speed, free storage, audio codec and output format therefore matter. The picture resolution is not sent to the audio encoder, but the container still has to be read; two recordings of equal duration should not be promised equal processing times.
Keep the tab open and the device awake. The page requests a screen wake lock while converting when the browser allows it, but system power controls can still suspend a background tab.
Check the tracks before converting
A capture may store microphone and system audio as separate tracks, or mix them into one. When several audio tracks exist, a picker appears with codec and channel information. PrivateMP3 can select one track. It cannot combine tracks, separate speakers from a mix or recover audio that the recorder never captured. An interrupted recording may also be missing packets or final container metadata.
Choose the output for the job
- MP3 uses 192 kbps. It favors compatibility, not the smallest possible speech file; no lower bitrate is available here.
- M4A can copy a complete, unchanged AAC track from an MP4 or MOV. Cuts and volume changes force a new AAC encode. Opus from WebM cannot use this copy path.
- WAV writes 16-bit PCM for editing or analysis and grows directly with duration, sample rate and channel count.
Remove an empty introduction, long break or unused ending before export. The output-duration estimate updates to reflect what remains.