Character Sets, Sound & Images

Syllabus & Goals 3 min

Cambridge 1.2 · Text, sound and images Paper 1 · Computer Systems

By the end of this lesson you can:

1. Describe how and why text is stored using the ASCII and Unicode character sets.
2. Describe how sound is sampled, and the effect of sample rate and sample resolution.
3. Describe how bitmap images are stored, and the effect of resolution and colour depth.

Textbook: Chapter 1, §1.2.1–1.2.3 (pp. 25–31)

Recap / Warm-Up 5 min

You can store numbers in binary. But a computer also stores letters, music and photos — and every one of them must become binary too.

Quick starter

If each character used 7 bits, how many different characters could you represent?

Key Concept 14 min

1 · Text — character sets

A keyboard sends a code, not a letter. A character set is the agreed list that gives every character a unique binary code. Sender and receiver must use the same set, or the text turns to nonsense.

ASCII groups characters in sequence: A = 65, B = 66 … and a = 97. Lower case is always 32 higher, so only one bit — the 32s bit — differs. That makes changing case a fast, simple operation.

2 · Sound

Sound waves are vibrations in the air. Each wave has a frequency and an amplitude (its loudness). Sound varies continuously, so it is analogue. A computer can only store digital values.

So the wave is sampled: an analogue-to-digital converter (ADC) measures the amplitude at regular time intervals. The amplitude cannot be measured exactly, so each sample is an approximate value, stored in binary.

A higher sampling rate captures the wave's shape more often. A larger resolution gives more amplitude levels, so each sample is closer to the real value. Both give a more faithful recording but a bigger file.

3 · Bitmap images

A bitmap image is a two-dimensional grid (matrix) of pixels (picture elements). Each pixel's colour is stored as a binary number. The number of bits per pixel is the colour depth; n bits give 2ⁿ colours.

Image resolution is the number of pixels, written width × height. A 4K screen is 3840 × 2160 = 8 294 400 pixels. With fewer pixels an image has less detail, and when enlarged it looks pixelated — the square pixels become visible.

Worked Example 12 min

Worked example 1 · store the text "Hi!" in ASCII

1. Look up each character. H is the 8th capital letter: 65 + 7 = 72.capitals run in order from A = 65.
2. i is lower case: its capital I is 65 + 8 = 73, and 73 + 32 = 105.
3. ! is a symbol; its code is 33 (from the table).
4. Convert each code to 7 bits. The whole text is stored as 1001000 1101001 0100001 — 3 characters × 7 bits = 21 bits (3 bytes if each is stored in a whole byte).

Beyond ASCII: these Unicode characters have codes far above 127, so ASCII cannot store them.

Worked example 2 · how many bits per sample?

Question: a sensor records amplitudes from 0 to 100. State the smallest sampling resolution that can store every level.

1. Count the levels: 0 to 100 is 101 different values.the 0 counts as a level too.
2. Find the smallest power of 2 that is at least 101: 2⁶ = 64 (too small), 2⁷ = 128 ✓.
3. Answer: 7 bits. For comparison, a 0–10 range needs 4 bits (2⁴ = 16 ≥ 11).

Worked example 3 · colour depth for a palette

Question: a logo uses 50 different colours. What colour depth is needed, and how many colours does that allow?

1. Try powers of 2: 2⁵ = 32 is fewer than 50; 2⁶ = 64 is enough.
2. Answer: a colour depth of 6 bits, allowing 64 colours (14 spare).a pixel cannot use part of a bit, so always round up.

See ASCII codes in code

# Every character has a code
for letter in "Hi!":
    print(letter, ord(letter), bin(ord(letter)))
print("Code 97 is", chr(97))

H 72 0b1001000
i 105 0b1101001
! 33 0b100001
Code 97 is a

Try It Yourself 12 min

📝 Exam Practice 10 min

Recap & Key Terms 3 min

Text is stored with a character set: ASCII (7-bit, 128 characters) or Unicode (up to 32 bits, every language). Sound is sampled: rate = samples per second, resolution = bits per sample. Images are grids of pixels set by resolution and colour depth. More of any of these means higher quality but a larger file.

ASCII code

A character set for all the characters on a standard keyboard and control codes (7-bit; extended ASCII is 8-bit).

Unicode

A character set which represents all the languages of the world; its first 128 characters are the same as ASCII.

Sampling rate

The number of sound samples taken per second in digital sound recording.

Sampling resolution (bit depth)

The number of bits used to represent the amplitude of each sound sample.

Pixel

Picture element: the smallest element used to make up an image on a display.

Colour depth

The number of bits used to represent the colour of each pixel.

Image resolution

The number of pixels that make up an image, e.g. 4096 × 3072 pixels.

Homework 1 min

Task (≤ 15 min): (a) Write the word Cab in 7-bit ASCII binary (A = 65, a = 97). (b) An image has a colour depth of 5 bits. How many colours can it show? (c) Explain one drawback of increasing the sampling rate of a recording. [5]