Digital Cameras, Keyboards, Microphones & Mice

🎯 Syllabus & Goals 3 min

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

By the end of this lesson you can:

1. Describe how a digital camera turns light into a digital image using a CCD and an ADC.
2. Describe how a keyboard and a microphone turn a key press or a sound into data.
3. Describe how an optical mouse tracks movement, and give its benefits.

Textbook: Chapter 3, §3.2.1 (pp. 92–96) — digital cameras, keyboards, microphones, optical mouse.

Recap / Warm-Up 5 min

In Lesson 4 a barcode scanner turned light and dark stripes into 0s and 1s. Every input device today does the same kind of job: it turns something from the real world — light, a press, a sound, a movement — into binary.

Quick starter

The real world is analogue; computers are digital. Which device converts one into the other?

🧠 Key Concept 14 min

1 · Digital cameras

Film cameras needed each film developed and printed before you saw the result. Unwanted shots still cost money. A digital camera shows the image at once and connects to a computer by USB or wirelessly by Bluetooth.

A digital camera is controlled by an embedded system (Lesson 3). It automatically adjusts the shutter speed, focuses the image, fires the flash, sets the aperture size, sets the image size and removes "red eye".

What happens when a photo is taken:

1. Light passes through the lens onto a light-sensitive cell, the CCD.
2. The CCD is made of millions of tiny sensors acting as photodiodes. Each one is a pixel (picture element).
3. The light becomes tiny electric charges. These pass through an ADC to form a digital image array.
4. The ADC turns each charge into a brightness level. An 8-bit ADC gives 2⁸ = 256 levels per pixel.
5. The sensors also measure colour. Most cameras use 24-bit RGB: 8 bits each for red, green and blue.

The number of pixels sets the file size. Image quality depends on the lens and sensor, the number of pixels, the light level and the storage format (JPEG, raw…). Phones now match cameras on pixel count, but often have smaller lenses and less storage.

2 · Keyboards

Keyboards are still the most common way to enter data, on computers, tablets and phones. They connect by USB or wirelessly. On a tablet the keyboard is often virtual — drawn on the touch screen.

Ergonomic keyboards reduce RSI. The keys are split and angled, and a palm rest supports the wrists.

3 · Microphones

A microphone is built in or plugged in by USB or Bluetooth. It turns sound waves into an electric current.

1. A sound makes the air vibrate. The diaphragm picks up the vibrations and vibrates too.
2. A copper coil is wrapped around the cone, which is joined to the diaphragm. The coil moves backwards and forwards.
3. The coil cuts through the field of a permanent magnet. This induces an electric current.
4. The current is analogue. It is amplified, or sent to a computer where the sound card's ADC turns it into digital values.

4 · The optical mouse

An optical mouse is a pointing device. It uses a tiny camera that takes about 1500 images a second, and it works on almost any surface.

1. A red LED in the base shines light onto the surface.
2. The reflection is picked up by a CMOS chip, which turns it into electric pulses.
3. The pulses go to a digital signal processor (DSP).
4. The DSP compares the changing images to work out the mouse's new coordinates.
5. The computer moves the on-screen cursor to those coordinates.

Worked Example 12 min

(a) One orange pixel, from sensor to binary

1. Brightness levels available: 2⁸ = 256 (0–255).an 8-bit ADC can output 256 different values.
2. Brightness 115 = 64 + 32 + 16 + 2 + 1 → 0111 0011 = 73 hex.split into place values 128 64 32 16 8 4 2 1; 0111 = 7, 0011 = 3.
3. Red 215 = 128 + 64 + 16 + 4 + 2 + 1 → 1101 0111 = D7.1101 = 13 = D, 0111 = 7.
4. Green 165 = 128 + 32 + 4 + 1 → 1010 0101 = A5.1010 = 10 = A, 0101 = 5.
5. Blue 40 = 32 + 8 → 0010 1000 = 28.0010 = 2, 1000 = 8.
6. Whole colour: 1101 0111 1010 0101 0010 1000 = #D7A528 (24 bits).24-bit RGB = three 8-bit values side by side.

(b) How big is one photo from a 12-megapixel camera?

1. Pixels: 4000 × 3000 = 12 000 000 (12 megapixels).width × height, one sensor per pixel on the CCD.
2. Bits: 12 000 000 × 24 = 288 000 000 bits.every pixel stores 24 bits of RGB.
3. Bytes: 288 000 000 ÷ 8 = 36 000 000 bytes.8 bits = 1 byte.
4. MiB: 36 000 000 ÷ 1024 ÷ 1024 ≈ 34.33 MiB.1 MiB = 1024 × 1024 bytes (IEC units, as in Unit 1).

So what? Doubling the pixels doubles the file size. That is why the textbook links the number of pixels to file size, and why cameras usually save as compressed JPEG.

(c) From key press to character

1. Sam presses H: the top conductive layer touches the bottom one through the insulating layer's hole.that completes one circuit in the key matrix.
2. The CPU detects which circuit closed, so it knows which key was pressed.each key sits at a unique place in the circuit.
3. The CPU looks the key up in an index file to find the character.the key position is not yet a letter — the index file maps it.
4. The character has an ASCII value: "H" = 72 = 0100 1000.the value is what gets stored and processed.

Try It Yourself 12 min

📝 Exam Practice 10 min

🗝️ Recap & Key Terms 3 min

A camera's CCD turns light into charges; an ADC turns them into brightness and RGB values. A key press closes a circuit, and the CPU maps it to an ASCII character. A microphone's moving coil induces an analogue current for an ADC. An optical mouse's LED, CMOS and DSP turn reflections into coordinates.

Charge-coupled device (CCD)

A light-sensitive cell made of millions of tiny sensors acting as photodiodes.

ADC (analogue-to-digital converter)

A device that converts analogue data (e.g. from a sensor or microphone) into digital data a computer understands.

Virtual keyboard

An on-screen keyboard that uses a touch screen to copy a physical keyboard.

Repetitive strain injury (RSI)

Pain in muscles, nerves and tendons caused by a repeated action, such as heavy typing or clicking.

Pointing device

An input device that controls an on-screen cursor or makes selections by clicking.

CMOS

Complementary metal oxide semiconductor: a chip that produces electric pulses when light falls on it.

Digital signal processor (DSP)

A processor that calculates, e.g., a pointing device's coordinates from the pulses it receives.

Homework 1 min

Task (≤ 15 min): A podcaster records speech with a USB microphone and then saves a 1920 × 1080 24-bit cover image from a digital camera. (a) Describe how the speech becomes digital data. [3] (b) Calculate the uncompressed image size in MiB, to 2 d.p. [2]