Data is measured in the following units
- Bit
- Nibble
- Byte (1 byte = 2 nibbles)
- Kibibyte
- Mebibyte
- Gibibyte
- Tebibyte
- Pebibyte
- Exbibyte
Calculating File Sizes
- Calculating the size of an image file: resolution × colour depth (answer is in bits)
- Calculating the size of a sound file: sample rate (Hz) × sample resolution × length (seconds)
What is Compression?
- The reduction of FILE size
Why should we compress a file?
- Less bandwidth will be required
- Less storage will be required on the device
- Shorter transmission time (less time to download/upload)
- To make it small enough to attach to an email
Difference between Lossy and Lossless Compression
- Lossy compression is permanent, lossless can be recovered to its original form
- Lossy compression reduces the file size more than lossless compression
When to use Lossless or Lossy Compression?
Lossless
- Used when file quality is more important than file size
- Used when file needs to be recoverable to original form (eg. text file or source code)
Why would a musician choose to compress a file with lossless?
- They want to be able to edit the original sound file
- They want the highest sound quality for the file // They want the sound to be closest to the original recording
- …using lossy would reduce the sound quality
- …using lossy will permanently remove some of the data // no data will be permanently removed with lossless
Lossy
- Used when file size is more important than quality
- Considerably smaller file size than lossless, quality may be adequate
- Used for streaming as transmission of smaller files is faster
Explain why lossy compression is not suitable for compressing a text file
Data will be permanently removed
…and that could be important/necessary data
Lossy is not suitable for text files
…as it will damage/corrupt the file
Lossless Compression of Files
How is a text file compressed with lossless?
- Compression algorithm is used
- …such as RLE (run length encoding)
- Repeating words/characters/phrases are identified
- …and indexed
- …with the number of occurrences
- …with their position
How is a video file compressed with lossless?
- Compression algorithm is used
- …such as RLE (run length encoding)
- Repeating frames/pixels are identified
- …and indexed
- It records the changes between frames/pixels
- No data is permanently removed
How is a sound file compressed with lossless?
- Compression algorithm is used
- …such as RLE
- Repeating patterns in the music are identified (or repeated sounds/notes)
- …and indexed
- No data is permanently removed
How is an image file compressed with lossless?
- Compression algorithm is used
- …such as RLE
- Repeating colours are identified
- …and indexed
- …with the number of occurrences
- No data is permanently removed
Lossy Compression of Files
Video file
- Compression algorithm is used
- Reduce colour depth
- Reduce image resolution
- Reduce frame rate
Image file
- Compression algorithm is used
- Reduce colour depth
- Reduce image resolution
- Data is permanently removed
Sound file
- Compression algorithm is used
- Reduce sampling rate
- Reduce sound resolution
- Data is permanently removed