Output Devices

CSS Computer Science (Optional) — Paper-I, Section-A, Topic I, p.16-18. Covers: Output Devices.

Detailed Output Device Table

Device	Function
Monitor (VDU)	Visual display of processed output on screen
Printer	Produces a physical (hard copy) output
Speaker	Audio output
Plotter	High-precision output for large drawings/blueprints (used in engineering/CAD)
Projector	Displays output onto a large external surface
Headphones	Private/personal audio output

Device-by-Device Detail

Monitor (VDU)
- Definition: main visual output device — shows text/images/video on screen.
- How it works: graphics card renders pixels → panel refreshes each pixel's colour many times/sec.
- Variants: CRT, LCD, LED, OLED (see Monitor Types below).
- Note: soft-copy only — output is lost once powered off, unlike a printer's hard copy.
Printer
- Definition: produces a permanent hard-copy of output on paper.
- Variants: impact vs non-impact (see Printer Types below).
- Use: documents, reports, photographs.
Speaker
- Definition: converts an electrical audio signal into audible sound.
- How it works: signal drives a coil on a diaphragm/cone → magnetic field vibrates cone → sound.
- Use: music/video, system alerts, text-to-speech output.
- Con: not private — can disturb others (unlike headphones).
Plotter
- Definition: draws high-precision, line-based images on large sheets.
- How it works: motors move a pen/marking head along X/Y axes per vector instructions.
- Variants: pen plotters, large-format inkjet plotters.
- Use: architectural blueprints, engineering drawings, CAD output.
- Con: slow; unsuited to photos or general text/images.
Projector
- Definition: displays output as an enlarged image on an external surface.
- How it works: lamp/LED light shines through an LCD/DLP panel → lens enlarges image onto the surface.
- Use: lectures, boardroom meetings, home theatre.
- Con: quality depends on ambient light and screen distance.
Headphones
- Definition: personal audio output device worn at the ear.
- How it works: same principle as a speaker, miniaturised.
- Use: private listening in shared/quiet spaces.
- Pro: private, portable — Con: one listener only.

Printer Types — Impact vs Non-Impact

Impact Printers: strike an inked ribbon against paper (like a typewriter).
- Dot-Matrix Printer
  - How it works: grid of pins fires against a ribbon → builds characters from dots.
  - Use: multi-part carbon-copy forms (invoices, manifests).
  - Pros/Cons: cheap, durable, only type that prints carbon copies — but noisy, low-res, slow.
Non-Impact Printers: form output without striking the page.
- Inkjet Printer
  - How it works: nozzles eject ink droplets — thermally or piezoelectrically.
  - Use: home/office colour and photo printing.
  - Pros/Cons: great colour quality, cheap upfront — but slow at volume, costly ink.
- Laser Printer
  - How it works: laser charges a drum → toner sticks to charged areas → heated fuser bonds toner to page.
  - Use: high-volume office/business printing.
  - Pros/Cons: fast, sharp, cheap per-page at scale — but costly toner/machine.
- Thermal Printer
  - How it works: heating elements darken heat-sensitive paper (or melt a ribbon onto plain paper).
  - Use: POS receipts, boarding passes, shipping labels.
  - Pros/Cons: fast, quiet, no ink/toner — but receipts fade, needs special paper.
3D Printer (emerging category)
- How it works: builds a physical object layer by layer (molten filament or fused resin/powder).
- Use: prototyping, engineering models, medical implants.
- Con: slow per object.

Monitor Types — Display Technologies

CRT (Cathode Ray Tube)
- Definition: old, bulky tech — fires an electron beam at a phosphor screen.
- How it works: electron gun fires beam → magnetic coils steer it row-by-row (raster scan) → hits phosphor dots → glows RGB → redrawn at the refresh rate.
- Pros/Cons: accurate colour, no motion blur — but bulky, high power/heat, can flicker.
LCD (Liquid Crystal Display)
- Definition: slim flat-panel display, controls light via liquid crystals.
- How it works: backlight shines through crystal cells → voltage twists crystals to vary light → colour filters form RGB sub-pixels.
- Pros/Cons: slim, energy-efficient — older panels: narrower viewing angle, slower response.
- Panel sub-types:
  - TN (Twisted Nematic): fastest response time, cheapest — but narrow viewing angles, weaker colour.
  - IPS (In-Plane Switching): best colour accuracy and wide viewing angles — but slower response, costlier.
  - VA (Vertical Alignment): best contrast/deep blacks, middle ground on speed and viewing angle.
LED (LED-backlit LCD)
- Definition: not a separate tech — an LCD with an LED backlight instead of a fluorescent tube.
- Pros/Cons: thinner, brighter, more efficient than old CCFL-LCD — but still needs a backlight, so blacks are weaker than OLED.
OLED (Organic Light-Emitting Diode)
- Definition: each pixel emits its own light — no backlight at all.
- Pros/Cons: true blacks, high contrast, thin/flexible — but costlier, can suffer burn-in.
Plasma Display (older, now mostly discontinued)
- Definition: flat-panel tech that lights tiny gas-filled cells into a plasma state to glow.
- How it works: voltage ionises gas in each cell → ionised gas (plasma) emits UV light → excites a phosphor coating to glow RGB, one cell per pixel.
- Pros/Cons: excellent contrast/colour, wide viewing angles — but heavy, high power use, screen burn-in risk; replaced in the market by LED/OLED.