Question 1

What is the difference between lossy and lossless compression?

Accepted Answer

**Lossless** compression can reconstruct the original data exactly. **Lossy** compression discards information permanently to achieve much smaller files.

**Lossless** works by finding and encoding redundancy more efficiently. If a file contains a long run of identical bytes, store the byte and the count. If certain patterns recur, assign them shorter codes. Decompression reverses the process **perfectly** — the output is bit-for-bit identical to the input.

Used where any change is unacceptable: **ZIP, PNG, FLAC, ALAC**, and general file archiving. You cannot lossy-compress a spreadsheet or an executable, because a single altered bit breaks it.

The limit is that redundancy is finite. Typical lossless ratios are perhaps 2:1 for text and far less for already-compressed data.

**Lossy** achieves dramatically better ratios — often 10:1 or more — by exploiting the limits of human perception. It does not just discard data randomly; it discards what you are least likely to notice.

**JPEG** exploits the eye's greater sensitivity to brightness than to colour detail, so colour information is stored at lower resolution (**chroma subsampling**), and fine high-frequency detail is coarsely quantised.

**MP3 and AAC** use **psychoacoustic models**: sounds masked by louder simultaneous sounds are removed entirely, as are frequencies beyond hearing.

**Video codecs** additionally exploit similarity between consecutive frames, storing differences rather than whole images.

**The critical practical point: lossy loss is cumulative.** Every save re-applies the compression, so repeatedly editing and re-saving a JPEG degrades it progressively — **generation loss**. Always keep a lossless master and export lossy copies. Converting an MP3 to FLAC does not restore anything; the discarded data is gone.

**Modern formats do both** — WebP, AVIF and HEIC offer lossless and lossy modes, and newer codecs achieve better quality at the same size than older ones.

Question 2

What is the difference between a file format and a file extension?

Accepted Answer

A file format is the actual structure of the data inside the file. A file extension is the few characters after the dot in the filename — a label, and nothing more. The crucial point is that the

Question 3

Why do old files become unreadable, and what is digital preservation?

Accepted Answer

Because a file is only meaningful to software that understands its format, and both the software and the medium it sits on decay — on timescales far shorter than people assume. The three separate

Question 4

What is a checksum, and why do downloads have hashes?

Accepted Answer

A checksum or hash is a fixed-length string calculated from a file's contents. Recalculating it after downloading and comparing against a published value confirms the file is exactly what was

Question 5

What counts as a data breach, and when must it be reported?

Accepted Answer

A personal data breach is any security incident leading to accidental or unlawful destruction, loss, alteration, unauthorised disclosure of, or access to personal data — which is considerably broader

Question 6

How do QR codes encode data?

Accepted Answer

As a two-dimensional pattern of black and white squares read as binary, with substantial built-in error correction — which is why a code still scans when damaged, partially covered or printed over a

Question 7

What metadata does a file contain, and what can it reveal?

Accepted Answer

Metadata is data about the file rather than its visible content. Almost every file carries some, and it frequently reveals considerably more than people intend to share. Photographs — EXIF data: GPS

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