OCR H446 · A Level Computer Science · ~12 min read
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Video
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Worksheet
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Input Devices
An input device converts real-world data (sound, light, touch, text, position) into digital signals that the computer can process. Input devices transfer data from the user or environment into the CPU/memory.
Keyboard and Mouse
Keyboard: generates a scan code when a key is pressed; the keyboard controller maps scan codes to character codes (ASCII/Unicode). Uses a matrix of switches and a microcontroller.
Optical mouse: uses an optical sensor (small camera + LED) to detect surface movement by comparing frames; tracks displacement as relative X/Y coordinates. Resolution measured in DPI (dots per inch).
Barcode Reader and QR Code
Barcode reader (laser scanner): a laser beam is reflected off the barcode; dark bars absorb light, white spaces reflect. The sensor converts light intensity to a digital signal, decoded to a product number.
QR code: 2D barcode. Stores data in both horizontal and vertical directions using a matrix of black and white squares. Decoded using a camera and image processing algorithm. Can encode URLs, text, contact details.
Digital Camera and Scanner
Digital camera: light passes through the lens onto a CCD (Charge-Coupled Device) or CMOS sensor, which converts photons to electrical charge per pixel. Analogue signals are digitised by an ADC. Resolution measured in megapixels.
Flatbed scanner: a CCD array moves across the document; reflected light is captured and converted to digital pixel values. Resolution measured in DPI.
Touch Screen
Resistive touch screen: two conductive layers separated by a gap; pressing causes the layers to touch and complete a circuit at the contact point. Pressure required; less accurate.
Capacitive touch screen: a grid of transparent conductors carries an electrical charge. A finger (conductive) distorts the charge pattern; the controller detects the touch position from the change. Supports multi-touch; more accurate. Used in smartphones/tablets.
Infrared touch screen: a grid of IR LEDs and photodetectors; a finger interrupts IR beams, locating the touch point. Used for large public displays.
Microphone and ADC
A microphone converts sound waves (pressure variations) into an analogue electrical signal. An ADC (Analogue-to-Digital Converter) samples the analogue signal at a fixed sample rate (e.g. 44,100 Hz for CD quality) and converts each sample to a digital value of fixed bit depth (e.g. 16-bit). Higher sample rate and bit depth = better quality but larger file size.
Output Devices
An output device converts digital data from the computer into a form that humans (or other systems) can perceive or use.
Monitor/Display
LCD (Liquid Crystal Display): a backlight (LEDs) illuminates liquid crystal cells; the orientation of crystals is controlled by an electric field, blocking or transmitting light through colour filters to produce pixels. Low power, thin profile.
OLED (Organic LED): each pixel is a self-emitting organic compound that produces light when current flows through it. No backlight needed; true blacks, high contrast. Used in premium phones and TVs.
Resolution: number of pixels (width × height). Refresh rate: how often the screen redraws per second (Hz). Colour depth: bits per pixel (24-bit = 16.7 million colours).
Printers
Inkjet printer: tiny droplets of liquid ink are sprayed onto paper through nozzles (thermal or piezoelectric). Produces high-quality colour output. Ink cartridges can be expensive.
Laser printer: a laser beam charges a rotating drum in the pattern of the image; toner (dry powder) is attracted to charged areas; heat (fuser) bonds toner to paper. Fast for text/documents; cheaper per page than inkjet at high volume.
3D printer: builds objects layer by layer (additive manufacturing) by extruding melted filament (FDM) or curing resin with UV light (SLA/SLS). Used in prototyping, healthcare, manufacturing.
Speakers and DAC
Speakers convert electrical signals into sound. A DAC (Digital-to-Analogue Converter) converts digital audio data back into an analogue waveform that drives a speaker. The speaker uses an electromagnet to vibrate a cone, creating pressure waves (sound).
Actuators and Sensors
Actuator: an output device that causes physical movement (e.g. a motor, servo, solenoid). Used in robotics and embedded control systems.
Sensor: an input device that measures a physical quantity (temperature, pressure, light, humidity) and converts it to an electrical signal. Often requires an ADC to digitalise the measurement.
Exam tip: OCR H446 questions on I/O devices often ask you to explain the principle of operation of a specific device (e.g. how a capacitive touchscreen detects touch). Learn the mechanism, not just the name.
Exam tip: Remember the ADC/DAC chain: sound in → microphone → ADC → CPU/storage → DAC → speaker → sound out. Also remember that higher sample rate & bit depth = better quality AND larger file size.
⚠ Common Mistakes
Confusing resolution with refresh rate — resolution is pixel count (e.g. 1920×1080); refresh rate is frames per second (Hz).
Saying a capacitive screen uses pressure — capacitive screens detect the electrical conductivity of a finger, not pressure. Resistive screens use pressure.
Saying inkjet printers use powder — inkjet uses liquid ink; laser printers use dry toner powder.
Forgetting the ADC for microphone input — the microphone outputs an analogue signal; an ADC converts it to digital before the CPU can process it.
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Worksheet — 1.1.3a Input and Output Devices
8 questions · 20 marks · instantly marked
Q1Describe how a laser barcode scanner reads a barcode. Your answer should explain the role of light, the sensor, and how the digital signal is produced.[3 marks]
✓ Mark scheme
A laser beam is directed at the barcode [1]; dark bars absorb the light while white spaces reflect it back to a photodetector/sensor [1]; the sensor converts the varying light intensity into a varying electrical signal which is decoded into a binary product code [1].
Q2Explain how a capacitive touch screen detects the position of a finger touch. Why must the user use a finger (or a special stylus) rather than a gloved finger?[4 marks]
✓ Mark scheme
A grid of transparent conductors across the screen carries an electrical charge [1]; when a finger touches the screen, the finger's own electrical conductivity distorts the electric field at that location [1]; the controller measures the change in charge at multiple grid points to calculate the precise touch coordinates [1]; a gloved finger is non-conductive and does not distort the field, so no touch is registered — a capacitive stylus is needed [1].
Q3A sound clip is recorded at a sample rate of 44,100 Hz with a bit depth of 16 bits. Explain the terms 'sample rate' and 'bit depth' and calculate the file size of a 10-second mono recording in kilobytes.[4 marks]
✓ Mark scheme
Sample rate: the number of times per second the analogue signal is measured and converted to a digital value [1]; Bit depth: the number of bits used to store each sample (determines precision/amplitude resolution) [1]; File size = 44,100 × 16 × 10 = 7,056,000 bits ÷ 8 = 882,000 bytes ÷ 1024 ≈ 861 KB [1] (accept any correct working) [1 for correct calculation process].
Q4Compare inkjet and laser printers, explaining the differences in how they produce output and giving one scenario where each would be preferred.[4 marks]
✓ Mark scheme
Inkjet: sprays tiny droplets of liquid ink through nozzles onto paper; laser: uses a laser to charge a drum attracting dry toner powder, which is then fused onto paper by heat [1]; inkjet is preferred for high-quality photo printing or low-volume colour output at home [1]; laser is preferred for high-volume text/document printing in offices, as it is faster and cheaper per page at high volumes [1]; any valid comparison of quality/speed/cost [1].
Q5Explain the role of an ADC (Analogue-to-Digital Converter) in the recording of sound using a microphone.[3 marks]
✓ Mark scheme
The microphone converts sound pressure waves into a continuous analogue electrical signal [1]; the ADC samples this signal at regular time intervals (the sample rate) [1]; each sample's amplitude is converted to a binary number with a fixed number of bits (bit depth), producing digital data the CPU can process and store [1].
Q6Describe one advantage of an OLED display over an LCD display.[2 marks]
✓ Mark scheme
Any one of: OLED pixels emit their own light (no backlight needed), so when a pixel is black it consumes no power, reducing energy use [1]; this also gives OLED true blacks and a much higher contrast ratio than LCD, which always has some backlight leakage through pixels [1] (award 1 for a correct advantage, 1 for a supporting reason or explanation).
Q7State two advantages of a QR code over a traditional 1D barcode.[2 marks]
✓ Mark scheme
Any two of: QR codes store more data (data in both horizontal and vertical directions) [1]; QR codes can store different types of data such as URLs, text, contact information [1]; QR codes include error correction so they can still be read if partially damaged [1]; QR codes can be scanned with any camera, not a specialist laser scanner [1].
Q8A robotic arm in a factory uses sensors and actuators. Explain the role of each in the context of this system.[2 marks]
✓ Mark scheme
Sensors measure physical quantities (e.g. the position/angle of the arm, force applied to an object, presence of a component) and convert them to electrical signals for the controller [1]; actuators receive electrical signals from the controller and produce physical movement (e.g. a motor rotates a joint, a pneumatic actuator extends an arm) [1].
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Mini Test — 1.1.3a I/O Devices
Timed exam conditions.
10 questions · 10 marks · 10 minutes
5 MCQ + 5 short answer
⏱10:00
10 marks
Section A — Multiple Choice
Q1A capacitive touchscreen works by:
Q2In audio recording, increasing the bit depth from 8-bit to 16-bit:
Q3A laser printer uses which process to produce output?
Q4An ADC (Analogue-to-Digital Converter) is required when using a microphone because:
Q5Which type of display has true black pixels because each pixel produces its own light?
Section B — Short Answer
Q6State one advantage of a QR code over a traditional 1D barcode.
Mark schemeAny one of: stores more data [1] / stores different types of data (URLs, text) [1] / includes error correction [1] / can be scanned with any camera [1].
Q7State what CCD stands for and in which input device it is commonly found.
Mark schemeCharge-Coupled Device [1]; found in digital cameras / scanners / photocopiers [1].
Q8State the difference between a resistive and a capacitive touch screen in terms of what triggers a touch detection.
Mark schemeResistive: detects physical pressure (pressing causes two conductive layers to make contact) [1]; Capacitive: detects the change in electrical charge caused by a conductive object (e.g. a finger) touching the screen [1].
Q9State what a DAC does and give one example of when it is needed.
Mark schemeA DAC (Digital-to-Analogue Converter) converts digital data into an analogue signal [1]; for example, converting digital audio data to an analogue waveform to drive a speaker / headphones [1].
Q10State what is meant by 'DPI' in the context of an optical mouse or a scanner.
Mark schemeDPI = Dots Per Inch; a measure of resolution indicating how many distinct points can be detected/captured per inch of movement/surface. Higher DPI = greater precision / finer detail. [1 mark for definition, 1 mark for relating to resolution/precision].