LED, LCD & OLED Screens and Loudspeakers

🎯 Syllabus & Goals 3 min

Cambridge 3.2 · Input and output devices Paper 1 · Computer Systems

By the end of this lesson you can:

1. Explain the difference between an LED screen and an LED-backlit LCD screen.
2. Describe how an OLED screen works and give its advantages over LCD and LED.
3. Describe how digital sound is played through a loudspeaker using a DAC and an amplifier.

Textbook: Chapter 3, §3.2.2 (pp. 107–110) — LED and LCD screens, OLED, loudspeakers.

Recap / Warm-Up 5 min

Last lesson, a DLP projector made colour by mixing red, green and blue. Screens use the same trick. In Unit 1, a 24-bit colour gave each pixel 8 bits for red, 8 for green and 8 for blue.

Quick starter

How many different colours can a 24-bit pixel show?

🧠 Key Concept 14 min

1 · LED screens

An LED screen is built from tiny light-emitting diodes. Each LED is red, green or blue. Changing the current through each LED changes its brightness. Mixing the three gives a huge range of colours. The colours are brilliant, so true LED screens suit large outdoor displays.

2 · LCD screens — they need a backlight

An LCD screen uses tiny liquid crystals arranged as pixels. An electric field changes each crystal so it lets light through or blocks it. The crystals make no light of their own. So every LCD needs backlighting.

• Older LCDs used CCFL — cold cathode fluorescent lamps (tubes) behind the screen.
• Modern LCDs use a matrix of tiny blue-white LEDs behind the screen.

3 · OLED — organic LEDs

An OLED uses organic (carbon-based) materials. Thin organic films sit between two electrodes: a metallic cathode (negative) and a glass anode (positive). Apply an electric field and the films give off light. No backlight is needed, so the screen can be extremely thin — and even bendable.

4 · Loudspeakers

Sound stored in a file is digital. A loudspeaker needs a varying analogue current. So three things happen:

1. The digital data passes through a DAC (digital-to-analogue converter), becoming an electric current.
2. That current is tiny, so an amplifier makes it big enough to drive the speaker.
3. The loudspeaker converts the current into sound.

Inside the speaker, current flows through a coil wrapped round an iron core. The core becomes a temporary electromagnet near a permanent magnet. As the current varies, the attraction varies, and the core vibrates. The core is fixed to a paper or plastic cone, which vibrates the air and makes sound waves.

Worked Example 12 min

(a) How many LEDs in a true LED panel?

1. Pixels = 1920 × 1080 = 2 073 600.resolution is width × height, as in Unit 1.
2. LEDs per pixel = 3 (red, green, blue).a true LED screen lights each colour with its own diode.
3. Total LEDs = 2 073 600 × 3 = 6 220 800.multiply pixels by LEDs per pixel.
4. Compare a 3840 × 2160 (4K) panel: 8 294 400 pixels × 3 = 24 883 200 LEDs.twice the width and height gives four times the pixels.

That is over six million tiny lights — which is why true LED screens are costly and are mostly used for big outdoor displays.

(b) Playing a song from a file — step by step

1. The music is stored as digital (binary) data, e.g. sample values like 1001 0101.state the starting form of the data.
2. The data is sent to a DAC, which converts it into an analogue electric current.speakers need a smoothly varying current, not bits.
3. An amplifier increases the current.the DAC's output is too small to drive a speaker.
4. The current flows through the coil round the iron core, making a varying electromagnet.this is where electricity becomes magnetism.
5. Attraction to the permanent magnet varies, so the core and the attached cone vibrate, producing sound waves.vibrating cone → vibrating air → sound.

How the marks are earned: one per step above. Missing the amplifier is the commonest lost mark.

(c) Judging an advertising claim

1. With 8 bits per colour channel, a pixel has 2²⁴ = 16 777 216 colours.recall 24-bit RGB from Unit 1.
2. So "over 1 million" is true — but it greatly understates the real figure.16.7 million is more than 16 times larger.
3. "Vivid" is fair: OLEDs give bright light and make their own light, with no backlight bleeding through.link the claim to a real OLED feature.
4. "True-to-life" is marketing: the human eye and the source image limit what is seen.a comment question rewards a reasoned judgement, not just facts.

Try It Yourself 12 min

📝 Exam Practice 10 min

🗝️ Recap & Key Terms 3 min

True LED screens light every pixel with coloured LEDs. LCDs only filter light, so they need a backlight — now LEDs rather than CCFL tubes. OLEDs make their own light, so they are thin, flexible and low-power. A loudspeaker needs a DAC and an amplifier before its coil and cone turn current into sound.

LED screen

A screen whose pixels are tiny red, green and blue light-emitting diodes that produce the light.

LCD screen

A screen of liquid crystals that block or pass light; it needs backlighting (usually LEDs).

Organic LED (OLED)

A light-emitting diode using organic layers between a cathode and an anode; it generates its own light, so no backlighting is needed.

Cathode / anode

The negative / positive electrode in an OLED.

DAC (digital-to-analogue converter)

A device that converts digital data into an electric current that can drive a speaker, motor or actuator.

Loudspeaker

An output device that converts an electric current into sound.

Homework 1 min

Task (≤ 15 min): A shop sells two TVs. TV A is advertised as an "LED TV"; TV B is an "OLED TV". Explain to a customer how each one produces its picture, and give two advantages of TV B. [5]