Data Compression (Lossy and Lossless)
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Computer Science (A Level) discusses data compression techniques. Compression reduces the number of bits required to represent data to save disk space and increase transfer speeds. There are two main types of compression: lossy compression, which permanently removes non-essential data and can reduce quality, and lossless compression, which identifies patterns to compress data without any loss. Common lossy techniques are JPEG, MPEG, and MP3, while common lossless techniques are run length encoding and dictionary encoding.
Data Compression (Lossy and Lossless)
- 1. Computer Science (A Level) Data Compression
- 2. A process of reducing the number of bits required to represent data (Definitions)
- 3. Compression A file can be too large to store data and can take up too much memory. Therefore, compression allows you to reduce the volume of data, the result is known as compressed data. Reasons to Compress Data o Reduce file size o Save disk space o Increase transfer speed at a given data rate o Allow real time transfer at a given data rate
- 5. Data is constantly being moved around computer systems and networks Transfer is usually high speed and accurate As distances get longer, transfer is slower and more susceptible to interference Storage space can be limited Data Storage and Transfer
- 6. Lossy Non-essential data is permanently removed, there is a degradation of data. Lossy compression loses data You cannot get back the original file Cheaper Requires less time and space Lossy compression techniques work by identifying data that can be removed, while still creating an acceptable representation. Lossy compressions reduce the file size but can also reduce the quality of an image.
- 7. Lossy - JPEG ➔Joint Photographic Experts Group ➔Images and Graphics ➔Removes unnecessary data to reduce file size ➔Tries to reconstruct an image without the missing data ➔JPEG analyses pixel data within each 8x8 block (the real image is broken down into 8x8 blocks) and removes data that is least likely to affect the human perception of the image
- 8. Lossy - MPEG ➔Motion Picture Experts Group ➔Video ➔Removes unnecessary data in a video file. Lossy – MP3 ➔Removes sounds in frequency range that cannot be easily heard or has the least affect ➔E.g. quieter notes played at the same time as louder notes, are also removed
- 9. Lossless Patterns in data are spotted and summarised in a shorter format with no data loss, the compressed file is as accurate as it was before compression. Lossless compression records patterns in data and this allows it to replicate the file exactly as it is without losing data.
- 10. Lossless – Run Length Encoding ➔Basic method of compression which summarises consecutive patterns of the same data ➔Works well with sounds and images where data is repeated ➔E.g. B,B,B,B,B,B,B,B (B=Blue) - is written as 8B when compressed. A method of compressing data by eliminating repeated data
- 11. Lossless – Dictionary Encoding Data Token tion 1 sta 2 na 3 crea 4 A method of compressing text files by spotting common strings of characters and storing them separately in a dictionary e.g. “Station”, “Nation”, “Creation” = 21,31,41