Character sets (ASCII and Unicode), how text size is worked out, and bitmap images with resolution, colour depth and file size.
What you will learn
- Find character codes from a known one
- Calculate the size of text and image files
- Link colour depth to the number of colours
- Read a black and white bitmap as binary
Character sets
A character set gives every character its own binary code.
- ASCII uses 7 bits: 128 characters, enough for English letters, digits and symbols. Extended ASCII uses 8 bits for 256.
- Unicode uses up to 32 bits, so it covers the characters of almost every language, plus symbols and emoji. Its first 128 codes match ASCII.
- Codes are in order: if "A" is 65, then "B" is 66 and "G" is 71. Lower case starts at "a" = 97.
Text size in bits = number of characters × bits per character. For 250 characters of 7-bit ASCII: 250 × 7 = 1750 bits, and 1750 ÷ 8 = 218.75, so 219 bytes when rounded up to a whole byte.
The rest of this lesson and its 9 practice questions are locked.
In this lesson
- Character sets
- Bitmap images
- Try it: How big is the image?
9 practice questions, marked as you go; about 25 minutes in all.
Key terms
Character set, ASCII, Unicode, Bitmap, Pixels, Resolution, Colour depth, Metadata
In this unit: Data representation
Binary, denary and hexadecimal, binary addition and shifts, two's complement, and how characters, images and sound are stored and compressed.