9-10 Digital Technologies: Data compression Hangman

The 16 key terms

1File size

How much storage a file needs, based on how much data (text, image, audio) it contains.

2KB, MB, GB, TB

Units of storage size built on the byte. Each unit is about 1000 times bigger than the one before: 1 KB, then 1 MB, then 1 GB, then 1 TB. Use them to check if files fit on a drive.

3bit

A bit is a single binary digit that is either 0 or 1, like a switch that is off (0) or on (1). A group of 8 bits is called a byte.

4character code

A number the computer uses to stand for one letter or symbol. It stores the number, then turns it back into the character on screen. In one A=1 code, A is 1 and B is 2.

5pixel

A pixel is one tiny square in a grid that makes up a digital image. Each pixel is stored as a number, such as 0 for white or 1 for black.

6redundancy

Repeated or predictable data, like a run of the same letter or colour, that can be stored in a shorter form without losing any information.

7Compression

Reducing file size by removing or simplifying data, sometimes losing quality.

8lossless compression

Shrinks a file so the exact original data can be rebuilt perfectly, with nothing lost. ZIP and PNG use it. Needed for text and programs.

9lossy compression

Shrinks a file by permanently throwing away detail people are unlikely to notice, so the original can't be rebuilt exactly. JPEG and MP3 use it.

113C2D = CCCDD

Decoding run-length data: read each count then its character, and write that character that many times. 3C means CCC and 2D means DD, so 3C2D expands back to CCCDD.

12run-length encoding

Compress a black-and-white image by writing each row as counts of same-coloured pixels in a run, starting with white. Row 00000 111 00 becomes 5,3,2. Counts can rebuild the grid.

13RLE expansion

When data has few repeats, run-length encoding adds a count to every character, so the result can be longer than the original. It only saves space when the data has long runs.

14dictionary compression

Give each repeated word or phrase a short code, store the code table once, then replace every repeat with its code so the data takes up fewer characters.

15variable-length coding

Gives common characters short bit codes and rare ones longer codes, so a whole message uses fewer bits than if every character used the same fixed number of bits.

16compression ratio = original size / compressed size

Original size divided by compressed size. Percentage saved = (original - compressed) / original x 100. Example: 10 MB down to 2 MB is a ratio of 5 to 1, saving 80%.

17Lossless vs lossy choice

Pick lossless (PNG, ZIP, FLAC) when every detail must be kept. Pick lossy (JPEG, MP3, MP4) when a smaller file matters more than perfect quality.