[appendix]
== User Data, Bitstream, Fluxes and Tags

This appendix explains the relationships between user data, bitstream data, flux data, and both sector and media tags in the context of digital imaging and data preservation.

=== 🧑‍💻 User Data

User data represents the information a user interacts with—such as a document or file.
This data is typically split into discrete units called _sectors_.

- A sector (also known as a block) is the smallest unit a medium can read or write.
- Most media divide user data into independent sectors.

=== 🏷️ Sector Tags

A sector may include metadata not visible to the user, but accessible to the operating environment.

Examples:

- Apple Lisa filesystem tags
- CompactDisc subchannel data

These metadata elements are referred to as _sector tags_ and are stored alongside user data.

=== 📀 Media Tags

Media tags relate to the storage medium as a whole rather than individual sectors.

Examples:

- CompactDisc Absolute Time In Pregroove (ATIP)
- DVD Disc Manufacturing Information (DMI)

Media tags may or may not be accessible to end users but are often essential for authentication, playback, or archival purposes.

=== 💾 Bitstream Encoding

User data and associated sector tags must be encoded into a binary format before being stored physically.
This encoded data is called the _bitstream_.

Common encoding formats include:

- FM (Frequency Modulation): used in early floppies
- MFM (Modified FM): used in most floppy formats
- GCR (Group Code Recording): used by Apple and Commodore
- EFM (Eight-to-Fourteen Modulation): used in CompactDiscs

The bitstream is composed of sequences of 0s and 1s derived from the digital content.

=== ⏱️ Flux Data

For physical media (e.g., magnetic or optical), the bitstream must be translated into _flux data_, which captures physical transitions over time.

- A flux transition is a change in magnetic polarity or optical reflectivity.
- Flux data represents the time elapsed since the last transition.
- In FM encoding, for instance:
* Every 4μs there's a guaranteed transition.
* A transition at 2μs represents a `1`.
* Absence of transition at 2μs represents a `0`.

These transitions are what ultimately get recorded onto physical media.

=== 🔁 Data Conversion Path

[source]
----
User Data → Sector Tags → Bitstream → Flux Data → Physical Media
----

Reverse path (during reading or imaging):

[source]
----
Physical Media → Flux Data → Bitstream → User Data + Sector Tags + Media Tags
----

=== 📦 Image Composition

Digital images can contain different combinations of data types:

- Flux data only
- Bitstream data only
- User data only
- Any combination of the above

Each format has specific use cases depending on the accuracy, fidelity, and target preservation needs.

