Why Files Are Compressed

Compression reduces file size. A smaller file requires less secondary storage and can allow more files to fit on the same device. It also contains fewer bits to transmit, so it can use less bandwidth and take less time to upload or download under the same network conditions.

Compression is useful for backups, websites, streaming, email attachments and data transfer. The benefit must be balanced against processing time, compatibility and, for lossy methods, reduced quality.

Compression does not automatically make the content higher quality or more secure. Its main purpose in this syllabus is to reduce the number of bits.

Lossless Compression

Lossless compression represents the data more efficiently without permanently discarding information. After decompression, every bit of the original data can be reconstructed exactly.

Exact reconstruction is necessary for program files, source code, executable instructions, text where every character matters and database records where a changed value could alter meaning. Some images can also be stored losslessly when exact pixels are required.

Lossless compression often exploits repetition or other redundancy. It may achieve less size reduction than a lossy method because it must retain enough information to rebuild the original exactly.

Run-Length Encoding

Run-length encoding is a lossless method suited to data containing long sequences of the same value. Instead of storing every repeated item, the compressed form stores the value and the number of repetitions.

For example, a row of twelve white pixels may be represented conceptually as white x 12. Decompression repeats the stored value the stated number of times, recreating the original run exactly.

RLE is effective for simple images with large areas of one colour and other data containing long runs. It may be ineffective or even larger than the original when values change frequently, because every short run needs both a value and a count.

Lossy Compression

Lossy compression permanently removes selected information. Decompression produces an approximation rather than an exact copy. The method is suitable when some loss is acceptable and a much smaller file is valuable.

For images, lossy compression may remove fine detail or reduce effective colour information. For sound, it may remove or simplify audio detail. The syllabus examples also include reducing image resolution or colour depth and reducing sound sample rate or sample resolution.

Lossy compression is commonly appropriate for photographs, music and video intended for viewing or listening. It is not suitable for executable programs, source code or records that must be reproduced exactly.

Choosing The Correct Compression Type

The best choice depends on purpose. Ask whether exact reconstruction is essential. If any changed bit could make the file incorrect or unusable, choose lossless compression. If the data is perceptual media and a controlled quality reduction is acceptable, lossy compression may provide greater size reduction.

A complete examination explanation links the file type and purpose to the consequence. For example, a program file needs lossless compression because every instruction must be recovered exactly; a website photograph may use lossy compression because a small reduction in visual detail is acceptable and faster download is useful.

Do not state that one method is always better. Lossless protects exact data; lossy often provides smaller files. The suitability depends on the scenario.

Compression And Transmission

When a compressed file contains fewer bits, it normally needs less bandwidth for a given transfer time or less time for a given bandwidth. This can reduce network congestion and make streaming practical.

Compression and decompression require processing, but unless a question focuses on that trade-off, the central syllabus impacts are reduced storage space, reduced bandwidth requirement and shorter transmission time.

Lossless And Lossy Compression

Compression Benefits

When RLE Works Well

Worked Examples

Examination Guidance

• Define compression as reducing the number of bits, not merely putting files into a folder.
• For a benefit, state a measurable consequence such as less storage, less bandwidth or shorter transfer time.
• Use the phrase exact reconstruction when explaining lossless compression.
• When recommending a method, connect the choice to the file purpose.
• For RLE, explain value plus run length and mention that long repeated runs are needed.

Knowledge Check

1. Why can compression reduce transmission time?

2. Why is lossless compression suitable for source code?

3. Give one way lossy compression can reduce an image file.

4. When can RLE increase file size?

5. What is the main difference between lossy and lossless compression?