Your Audio Is Never Re-encoded

The first question anyone with a lossless collection asks about Private Press is: “Will this touch my audio?”

For an artwork or supported metadata edit, Private Press changes the container metadata and preserves the encoded audio payload. It does not decode and re-encode audio as part of pressing artwork. Here is how the format-specific editors make that possible.

Audio files are structured containers

Metadata and encoded audio occupy different structures in these formats. The editor updates metadata while keeping the audio payload intact.

• MP3: ID3 tags sit in a header before the MPEG audio frames. Artwork is stored in an ID3 picture frame.
• M4A / ALAC: Artwork is held in an MP4 metadata atom. Sample tables describe where audio chunks live, so a metadata rewrite must preserve valid offsets.
• FLAC: Metadata blocks precede the audio frames. Artwork lives in a Picture block; pressing updates metadata and copies encoded frames through.
• AIFF: Metadata is stored in chunks separate from sound data. Chunk sizes and alignment have to remain valid.

The format-specific details

MP4 offsets. Artwork is stored in the covr atom inside ilst, which sits within moov. Audio samples live in mdat. If a metadata edit changes the size of moov, the stco and co64 sample-to-chunk tables may need new absolute byte offsets. Private Press rebuilds the metadata atom and recalculates those tables, checking for 32-bit offset overflow before writing the file.

FLAC blocks. The metadata section is a chain of block headers before the audio frames. Replacing a Picture block updates the block size and the final block's is-last flag, so a decoder can still find the start of audio frames.

AIFF chunks. AIFF chunks begin on word boundaries. When an ID3 chunk has an odd payload length, its padding byte and declared size must agree so later chunks, including SSND, remain correctly aligned.

These details let a file editor make a precise metadata change without handing the audio to a conversion pipeline.

Staged and journaled file changes

Private Press prepares and validates the edited file, records replacement steps in a journal, and can recover interrupted operations. Pre-write backups are on by default and configurable. See Support for backup settings and recovery guidance.

Native code and its scope

The shipped app has zero linked third-party package dependencies and uses Apple system frameworks. Its metadata writers are built in-house. The audio fingerprinting engine uses MIT-licensed Chromaprint classifier and encoding material, plus reference parameters for compatible resampling. The app also bundles DM Mono fonts under the SIL Open Font License 1.1; source and font notices ship with the app. That code reads audio for identification and is outside the file-writing path.

The separate pure Swift FLAC encoder and decoder are experimental. They are not used by the shipped FLAC metadata editor, which preserves encoded frames without decoding or re-encoding them.

The practical result

After a successful press, compatible players can read the artwork from the file's metadata. The encoded audio remains the same. You get a portable cover without asking an audio-conversion pipeline to rewrite the recording.