Compression — OCR GCSE Computer Science
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Compression explained
Compression covers 2 specification points, including The need for compression and Types of compression Lossy Lossless.
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Open a point below for its explanation and revision guidance.
The need for compression
Compression reduces the number of bits needed to store or transmit a file. It is needed because storage capacity and transmission bandwidth are finite, and because large files take longer to send and cost more to host. For example, a 12 MP photograph might occupy about 36 MB uncompressed; compressing it to 4 MB allows many more images on a 32 GB memory card and makes uploads faster. Compression also reduces network traffic, which matters when many users share a connection. The need applies to text, images, sound and video. Some compression methods remove data permanently, while others allow the original to be reconstructed exactly. Choosing a method depends on whether perfect fidelity is required and on how much reduction is acceptable.
Types of compression Lossy Lossless
Lossy compression permanently removes some data to achieve a smaller file. It is used where approximate reproduction is acceptable, such as JPEG images, MP3 audio and MP4 video. Lossless compression reduces file size without losing any data, so the original can be restored exactly. It is used for text, spreadsheets, executables and archives, for example PNG images, FLAC audio and ZIP files. Lossless methods find patterns and store them more efficiently, often using dictionary or run-length techniques. Lossy methods discard detail the human eye or ear is less likely to notice. The choice depends on whether exact reconstruction is required. A ZIP file of a document is lossless; converting a photograph to JPEG is usually lossy.
Your focus
- Describe why compression is needed for storage and transmission.
- Give examples of situations where compression is beneficial.
- Compare compressed and uncompressed file sizes in a given scenario.
Show all 6 objectives
- Define lossy and lossless compression.
- Classify given file formats or scenarios as lossy or lossless.
- Justify a choice of compression type for a stated purpose.
Compression exam tips
Marking Points
- Compression reduces file size by encoding the same information using fewer bits.
- Smaller files require less secondary storage, so more files fit on a given device.
- Smaller files transmit faster over a network because fewer bits must be sent.
- Reduced bandwidth use lowers congestion and can reduce data-transfer costs.
- Streaming and downloading become more practical when file sizes are smaller.
- Compression is needed for both storage and transmission, not only one of these.
- The need applies across text, images, sound and video files.
- Lossy compression permanently discards some data, so the original cannot be restored exactly.
- Lossless compression reduces file size while allowing exact reconstruction of the original.
- Lossy is suitable for images, audio and video where small quality losses are acceptable.
- Lossless is suitable for text, code, spreadsheets and archives where every bit matters.
- Lossless often works by replacing repeated patterns with shorter codes.
- Lossy can achieve greater size reduction than lossless for media files.
- Examples: JPEG, MP3 and MP4 are typically lossy; PNG, FLAC and ZIP are typically lossless.
Examiner Tips
- 💡Link each benefit to a concrete context such as a web page, email attachment or streaming service.
- 💡Use comparative language: 'fewer bits', 'less storage', 'faster transmission'.
- 💡If asked 'why compress?', give at least one storage reason and one transmission reason.
- 💡When comparing, state clearly whether the original can be recreated exactly.
- 💡Match each type to a named file format and a use case to show understanding.
- 💡Avoid saying 'lossy loses quality' without explaining that data is permanently removed.
Common Mistakes
- Error: stating that compression always makes a file smaller without limit. Correction: compression reduces size but cannot remove all redundancy; some files compress little.
- Error: claiming compression only helps storage. Correction: it also reduces transmission time and bandwidth use.
- Error: confusing compression with encryption. Correction: compression reduces size; encryption scrambles data for security, and the two can be combined.
- Error: saying lossless removes no data at all. Correction: it removes redundancy but keeps all original information so the file can be rebuilt exactly.
- Error: assuming lossy is always better because files are smaller. Correction: lossy is unsuitable when exact data is required, such as in a text document or program file.
- Error: labelling every image format as lossy or every audio format as lossless. Correction: format alone does not decide; the compression method used does, and some formats support both.