Reducing File Sizes

File Formats In this lesson, we will be looking at: How do we measure file size? Why are some files bigger than others? Why should we make files smaller? Linking and embedding Lossless and lossy compression Examples

How Is File Size Measured? The smallest unit of storage is a byte A byte could store: one character of text one number between one pixel (dot) in a photo 1/44,000 of a second of music on a CD A kilobyte is 1024 bytes A megabyte is 1024 kilobytes (1,048,576 bytes) A gigabyte is 1024 megabytes (1,073,741,824 bytes)

What Affects File Size? All information is stored and processed in the form of numbers Pictures, sounds and text can be converted into numbers using different methods – called coding – e.g. vector/bitmap, wave/midi Some types of coding are more efficient, or can store a greater level of detail – e.g..txt files vs Word/RTF files

What Affects File Size? The RTF and Word documents are bigger then the.txt file because they store extra information… …but why is the HTML file so much smaller?

Why Make Files Smaller? If all of the files you made or downloaded were very large, your computer would quickly fill up Large files would also take more memory (RAM) to process, which could slow the computer down. A large file will take longer to transfer – either from disc to memory to processor, or through a USB cable, or across the internet.

Linking and Embedding When you insert an object (e.g. a picture, sound, etc) into a document, it can stored in one of two ways… It can be embedded into the document, so that all of the data for the inserted object is stored inside the document – e.g. Word The document can store a link to the object, with the linked object stored in a separate file – e.g. web-pages

Compression You can also use a technique called compression to make files smaller. Lossless compression uses sequences and pattern recognition to squeeze the file – it can then be decompressed to reproduce the original data exactly (a bit like vacuum- packing a pillow or duvet for storage) Lossy compression removes bits of data that it doesn’t consider to be important, and then sometimes uses lossless compression as well.

Examples Zip files and compressed folders use lossless compression – you can get all of your data back when you open the zip file.jpegs, MP3 and Windows Media use lossy compression – you can never get all the detail back – ask Gabrielle! Sometimes you can tell where lossy compression has reduced quality – e.g. the metallic quality of streamed audio, or the fuzziness on digital television – these are called compression artefacts

Examples Some compression techniques are better suited to certain types of information Notice how the Lossless Windows Media format does a much better job of compressing sound than the.zip file. FLAC is another good lossless compression technique for sound files.