Compression

What you'll learn

• Why computers compress files to save storage space and speed up transmission.
• The difference between lossless and lossy compression.
• The advantages, disadvantages, and effects of each type.
• How to choose the right compression type for a scenario in an exam.

Starting point: files and file size

A file is a named collection of data stored on a computer, such as an image, song, video, document, or program.

A bit is the smallest unit of data, either 0 or 1. A byte is 8 bits. A file’s file size is the amount of storage space it takes up, usually measured in bytes, kilobytes, megabytes, gigabytes, and so on.

For OCR J277, storage prefixes are normally treated as decimal powers of ten, so 1 kB = 1,000 bytes and 1 MB = 1,000 kB. Using 1,024 as the multiplier is also accepted, but follow the question if it gives a value.

To decompress a file means to turn compressed data back into a usable form. A compression algorithm is the set of rules or software method used to compress data, but for this topic you do not need to carry out any specific compression algorithm.

Why compression is needed

Compression is useful because large files can cause practical problems. They take longer to send, use more storage, and may cost more to transfer or store.

Common scenarios where compression may be needed include:

• Sending attachments by email when there is a file size limit.
• Uploading and downloading files more quickly over the internet.
• Streaming audio or video, where data must arrive fast enough to play smoothly.
• Storing photos and videos on a phone, laptop, or cloud storage account.
• Backing up large amounts of data while using less storage space.
• Reducing website loading times, especially for images and video.

Bandwidth means the amount of data that can be transmitted across a network in a given time. If a file is smaller, it usually needs less bandwidth and takes less time to transfer.

Calculating storage saved

You may be asked to compare an original file size with a compressed file size.

$$\begin{array}{rl}space\; saved& =original\; size-compressed\; size\\ percentage\; reduction& =\frac{space\; saved}{original\; size}\times 100\end{array}$$

Two main types of compression

There are two types of compression you need to know for OCR J277:

• Lossless compression
• Lossy compression

The key difference is whether the original data can be restored exactly.

Lossless compression

With lossless compression, no data is permanently removed. After decompression, the file should be identical to the original, bit-for-bit.

Effects on the file

Lossless compression affects the file like this:

• The compressed file is smaller than the original.
• The original file can be recreated exactly.
• There is no loss of quality.
• The amount of compression may be limited, especially if the data cannot be reduced much.

Advantages of lossless compression

Lossless compression is useful because:

• It preserves all original data.
• It is suitable for files where accuracy matters.
• It avoids quality loss.
• It can be used for text, programs, spreadsheets, databases, and important images.

For example, a program file must not lose data. If even one important bit changed, the program might not run correctly.

Disadvantages of lossless compression

Lossless compression also has limitations:

• It usually does not reduce file size as much as lossy compression.
• The compressed file may still be too large for some uses.
• The file may need to be decompressed before it can be used.

Lossy compression

Lossy compression is often used for media files where humans may not notice small changes, such as images, audio, and video. The compressed version is an approximation of the original.

Effects on the file

Lossy compression affects the file like this:

• The compressed file is usually much smaller than the original.
• Some data is permanently lost.
• The original file cannot be recreated exactly.
• Quality may be reduced.
• Repeated lossy compression can make quality worse over time.

In images, quality loss might appear as blur, blocky areas, or less detail. In audio, quality loss might sound less clear. In video, quality loss might appear as blur, blockiness, or reduced sharpness.

Advantages of lossy compression

Lossy compression is useful because:

• It can create much smaller files than lossless compression.
• It reduces storage space significantly.
• It makes files quicker to upload, download, and stream.
• It is suitable when perfect accuracy is not required.

For example, a music streaming service may use lossy compression so songs load quickly and use less mobile data.

Disadvantages of lossy compression

Lossy compression has important drawbacks:

• Data is permanently removed.
• The original file cannot be recovered exactly.
• Quality can decrease.
• It is unsuitable for files where every bit matters, such as program files, databases, or legal documents.

Choosing the right type

In an exam, the best compression type depends on the purpose of the file. Ask yourself: does the file need to be restored exactly, or is a slight quality loss acceptable?

Quick comparison

What OCR expects you to focus on

For this sub-topic, you need to understand:

• why compression is needed;
• common scenarios where compression is useful;
• the difference between lossy and lossless compression;
• the advantages and disadvantages of each type;
• the effect each type has on the file.

You do not need to perform specific compression algorithms. So do not spend revision time memorising algorithm steps for this topic. Focus on explaining the choice and its consequences.