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Output is the other half of the input → process → output chain: taking the processor's digital data and turning it back into something a human can see or hear, or into physical movement that changes the world. This lesson examines the operating principles of representative output devices, the criteria for choosing one for a scenario, and how they are deployed in a real-world system.
This lesson develops OCR H446 section 1.1.3 (input, output and storage) as it applies to output devices. It covers the operating principles of representative devices — laser and inkjet printers, 3D printers, monitor technologies (LCD and OLED), speakers, and actuators in control systems — explains how each converts digital data into a perceivable or physical output (often via a digital-to-analogue converter for sound, or a driver circuit for movement), and applies selection criteria to choose an appropriate device for a given scenario, anchored in a real application. It links forward to control/embedded systems and back to data representation (sound sampling and image resolution).
An output device is hardware that converts processed digital data from the computer into a form that humans can perceive (light, sound) or that acts on the physical world (movement, heat). The mirror image of an input device's "physical quantity → digital data" chain, an output device runs the chain in reverse: digital data → electrical signal → physical effect. Where the output must be a continuous quantity — most obviously sound — the digital values are first turned back into a smoothly varying voltage by a digital-to-analogue converter (DAC). Holding this "digital → signal → physical effect" chain in mind lets you explain any output device in an exam, exactly as the inverse chain explained input devices.
A monitor is the primary visual output device. The exam focuses on two dominant technologies — LCD and OLED — and the discriminator is almost always how each pixel produces its light, so understand the underlying mechanism rather than memorising a feature list.
An LCD (liquid crystal display) never makes its own light. Behind the panel sits an always-on backlight (today an array of white LEDs). In front of it, each pixel is a tiny cell of liquid crystal sandwiched between two polarising filters turned at 90° to each other and addressed by transparent electrodes. With no voltage applied, the liquid crystal molecules form a twisted helix that rotates the plane of polarisation of the light by 90°, so it passes cleanly through the second polariser — the pixel looks bright. Applying a voltage untwists the molecules; the light's polarisation is no longer rotated, the second polariser blocks it, and the pixel goes dark. Varying the voltage gives intermediate brightness. Colour comes from three sub-pixels per pixel behind fixed red, green and blue colour filters, mixed additively.
The crucial consequence is that an LCD produces "black" by trying to block the backlight — and the blocking is never perfect, so a little light always leaks through. That is why LCD blacks look dark grey, and why a dark scene on an LCD still draws the same backlight power as a bright one.
| Aspect | Detail |
|---|---|
| How it works | An always-on backlight shines through a liquid-crystal layer between crossed polarisers; per-pixel voltage twists/untwists the crystal to let more or less light through; RGB colour filters give colour |
| Advantages | Cheaper to manufacture, no burn-in, very bright, long lifespan, mature technology |
| Disadvantages | Imperfect blacks (backlight leaks through), lower contrast, viewing angle varies by panel type, the backlight wastes power on dark content |
| Variants | TN (fast but poor angles), IPS (better colour and viewing angles), VA (good contrast) |
An OLED (organic light-emitting diode) display has no backlight at all. Each sub-pixel is a microscopic diode made of an organic (carbon-based) compound that emits its own light when current flows through it — a process called electroluminescence. More current means more brightness; zero current means the pixel is genuinely off and emitting nothing.
This single difference drives every OLED advantage and disadvantage. Because an off pixel emits no light, OLED produces true black and therefore essentially infinite contrast; because only lit pixels draw power, a mostly-dark image uses less energy; and because there is no backlight layer the panel can be thinner and even flexible. The costs are equally direct: the organic compounds degrade with use, and a sub-pixel left showing the same bright image for a long time ages faster than its neighbours, leaving a faint permanent ghost — burn-in. (Blue organics degrade fastest, which is the engineering headache behind OLED lifespan.)
| Aspect | Detail |
|---|---|
| How it works | Each sub-pixel is an organic diode that emits its own light when energised; brightness set by current; no backlight |
| Advantages | True blacks (pixel fully off), near-infinite contrast, wide viewing angles, very thin/flexible, fast pixel response |
| Disadvantages | More expensive, organic materials degrade so shorter lifespan, risk of burn-in from static content |
| Common use | Smartphones, high-end TVs, smartwatches |
| Feature | LCD | OLED |
|---|---|---|
| Light source | Separate always-on backlight | Each pixel emits its own light |
| Black levels | Dark grey (backlight leaks) | True black (pixel off) |
| Contrast | Lower | Much higher |
| Power on dark content | Same as bright (backlight on) | Lower (dark pixels off) |
| Viewing angle | Varies (TN poor, IPS good) | Excellent |
| Burn-in risk | No | Yes (organics age unevenly) |
| Cost | Lower | Higher |
| Lifespan | Longer | Shorter (organics degrade) |
Three numbers describe a display's quality, and they connect directly to data representation.
| Term | Definition |
|---|---|
| Resolution | Pixel count, e.g. 1920×1080 (Full HD) or 3840×2160 (4K UHD). More pixels = sharper image but more data per frame |
| Refresh rate | Frames drawn per second, in Hz. 60 Hz is standard; 120/144/240 Hz give smoother motion for gaming |
| Response time | How fast a pixel can change state, in ms. Lower reduces motion blur; OLED is far faster than LCD here |
The link to module 1.4 is concrete: the data needed to drive one frame is (pixels × colour depth). A 4K frame at 24-bit colour is
3840×2160×24 bits≈1.99×108 bits≈23.7 MB per frame.
At 60 Hz that is over 1.4 GB/s of pixel data — which is why display links and GPUs need such high bandwidth, and why video is always compressed.
It helps to see where that pixel data comes from. The image to be shown is held in a region of memory called the frame buffer — one entry per pixel, holding its colour. The GPU renders into this buffer, and a display controller reads it out row by row, top to bottom, many times a second, sending each pixel's colour to the panel. The refresh rate is simply how often the whole buffer is scanned out; the resolution is how many entries the buffer has. This is the output-side mirror of the image-sensor read-out met in the input lesson: there, charge was read out of a pixel grid into memory; here, colour values are read out of memory into a pixel grid. It also explains why higher resolution and refresh rate cost bandwidth and power — there is simply more data to move per second, exactly as the calculation above shows.
A projector is a representative display device that produces a large image on an external surface rather than on its own panel, and the spec's "operating principles" remit covers the two common types:
Projectors are chosen when the output must be large and shared (a classroom, cinema or meeting room) rather than viewed up close by one person — the selection criterion of audience size and viewing distance rather than per-pixel quality.
An inkjet printer builds an image from millions of microscopic droplets of liquid ink fired onto the paper by a print head that sweeps across the page while the paper advances line by line. Colour is mixed from CMYK (cyan, magenta, yellow, key/black) inks. The two ways of ejecting a droplet are an exam favourite:
| Aspect | Detail |
|---|---|
| How it works | A moving head fires microscopic CMYK ink droplets onto paper; thermal nozzles boil ink to form a bubble, piezo nozzles flex a crystal to push ink |
| Advantages | Excellent colour blending (photos), cheap to buy, quiet, handles many paper types |
| Disadvantages | High running cost (ink), prints smudge when wet, slow for bulk, nozzles clog if idle |
| Best for | Home use, photo printing, low-volume colour |
A laser printer is a fundamentally different, electrostatic process built around a rotating photosensitive drum. The classic sequence is worth knowing as an ordered list because exam questions ask for it in order:
| Aspect | Detail |
|---|---|
| How it works | A laser writes a charge pattern onto a drum; toner sticks to it; the pattern transfers to paper; a hot fuser melts it on |
| Advantages | Fast, very low cost per page, crisp text, smudge-proof, reliable at high volume |
| Disadvantages | Dearer to buy (esp. colour), bulky, weaker at photo-quality colour, warm-up time |
| Best for | Offices, high-volume text documents |
A 3D printer performs additive manufacturing: rather than cutting material away, it builds a physical object up layer by layer from a digital model. Software first "slices" the 3D model into hundreds of thin horizontal cross-sections, and the printer reproduces them one on top of another. The dominant consumer method is FDM (fused deposition modelling):
Other methods include SLA (stereolithography), where a UV laser cures liquid resin layer by layer, and SLS (selective laser sintering), where a laser fuses powdered material — both give finer detail than FDM. Note how 3D printing leans on the stepper motors introduced in the actuators section: precise, repeatable steps are what place each bead of plastic accurately.
| Aspect | Detail |
|---|---|
| How it works | Slices a 3D model into layers and builds it up additively; FDM extrudes melted filament through a moving heated nozzle, each layer fusing to the last |
| Advantages | Makes complex/custom geometry, rapid prototyping, on-demand spare parts, many materials |
| Disadvantages | Slow (hours per object), limited part strength, rough surface (needs finishing), quality printers costly |
| Common uses | Prototyping, medical (prosthetics, dental), manufacturing jigs, education |
Print quality is quantified by DPI (dots per inch) — how many individual ink or toner dots the printer can place per inch. More dots per inch means finer detail and smoother gradients, which is why photo printing demands high DPI. The link to data representation is a quick calculation: a 6×4 inch photo printed at 300 DPI needs
(6×300)×(4×300)=1800×1200=2160000 dots,
so over two million dots for a small print — and at higher DPI the dot count, and hence the data the printer must process, rises with the square of the resolution. This mirrors the monitor's resolution maths: doubling linear resolution quadruples the data, whether the output is pixels on a screen or dots on paper.
| Feature | Inkjet | Laser | 3D printer |
|---|---|---|---|
| Output | 2D on paper | 2D on paper | 3D physical object |
| Speed | Slow–moderate | Fast | Very slow |
| Colour quality | Excellent (photo) | Good (text) | Depends on material |
| Cost to buy | Low | Moderate–high | Moderate–high |
| Running cost | High (ink) | Low per page (toner) | Moderate (filament/resin) |
| Resolution measure | DPI (high for photos) | DPI (high for crisp text) | Layer height (e.g. 0.1 mm) |
A speaker is the output device that most clearly needs a DAC, because sound is inherently a continuous quantity. The chain runs:
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