I.8 Output Devices

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Exam pattern (2016–2026): 2024 MCQ Q2 (OLED gives the widest viewing angles) — full MCQ with explanation on I.11 Types of Hardware.

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1. Output Devices at a Glance

2. Device Notes

• Monitor: graphics card renders pixels; panel refreshes them many times a second. Soft copy only.
• Speaker: signal drives a coil on a cone → cone vibrates → sound. Not private.
• Headphones: a miniature speaker at the ear; private, one listener.
• Projector: lamp/LED light through an LCD or DLP panel → lens enlarges it. Needs a dim room.
• Plotter: motors move a pen along X/Y following vector data.
  • Draws continuous, exact lines at any scale.
  • Slow; poor for photos and bulk text.
  • Types: pen plotter, large-format inkjet plotter.
  • Pakistan use: Survey of Pakistan, WAPDA/NESPAK, development authorities.

3. Printers

3.1 Impact printers (strike the page)

• Dot-matrix: pins hit an inked ribbon; characters made of dots.
  • Only type that prints carbon copies (invoices, challans, manifests).
  • Cheap and durable; noisy, slow, low resolution.

3.2 Non-impact printers (no striking)

• Inkjet: nozzles spray ink droplets (thermal or piezo).
  • Great colour and photos; slow at volume, costly ink.
• Laser: laser charges a drum → toner sticks → heat fuses it.
  • Fast, sharp, cheap per page at volume.
• Thermal: heat darkens special paper.
  • Receipts, boarding passes, labels; fades over time.
• 3D printer: builds an object layer by layer; prototyping, implants.

3.3 Impact vs non-impact

3.4 Laser printer — six steps

1. Charge: roller gives the drum an even charge.
2. Write: laser (via spinning mirror) removes charge where the page has ink → hidden image.
3. Develop: toner powder sticks to that image.
4. Transfer: charged paper pulls toner off the drum.
5. Fuse: heated rollers (about 180–200 °C) melt toner into the paper.
6. Clean: drum wiped and discharged for the next page.

• Colour laser: repeats steps 3–4 for C, M, Y, K toners.
• Trap: laser uses dry toner, not liquid ink.

4. Monitor Technologies

4.1 CRT (Cathode Ray Tube)

• Electron gun fires a beam at a phosphor screen.
• Steps:
  1. Heated cathode emits electrons (three guns: R, G, B).
  2. Anode voltage (15–30 kV) accelerates them.
  3. Coils focus the beam.
  4. Yoke steers it left-right, top-bottom (raster scan).
  5. Shadow mask aims each gun at its own colour dot.
  6. Phosphor glows; beam strength sets brightness.
  7. Screen redrawn at 60–85 Hz (below about 70 Hz → flicker).
• Pros: accurate colour, no motion blur.
• Cons: bulky, power-hungry, hot.

4.2 LCD (Liquid Crystal Display)

• Backlight shines through liquid crystals that twist under voltage to let light through.
• Colour filters make R, G, B sub-pixels.
• Panel types:

• 🧠 IPS = colour, TN = speed, VA = contrast.

4.3 "LED" monitor

• Not a new technology: it is an LCD with an LED backlight instead of CCFL tubes.
• Gains: thinner, brighter, efficient, no mercury.
• Still a backlight: blacks look dark grey.
• Giant stadium screens are true LED displays — a different product.

4.4 OLED

• Each pixel makes its own light; no backlight.
• Black = pixel off → true black, near-infinite contrast.
• Pros: thin, flexible, wide angles, fast.
• Cons: costly, can burn in with static images.

4.5 Plasma (discontinued)

• Gas cells (neon/xenon) ionised by voltage → UV → phosphor glows R, G, B.
• Self-emissive like OLED, but uses gas.
• Strengths: deep blacks, wide angles, no blur.
• Why it died (about 2014–15): high power, heavy glass, burn-in, no small sizes, LCD/OLED became cheaper.

4.6 Evolution chain

• CRT (electron beam) → Plasma (gas) → LCD (crystal + backlight) → LED (LCD + LED backlight) → OLED (self-lit).

5. Pixels, Resolution and PPI

• Pixel: smallest addressable point; one colour at a time (three sub-pixels).
• Resolution: total pixels, width × height (1920 × 1080 = 2,073,600).
• PPI/DPI: pixels per physical inch — decides sharpness.

5.1 Worked numerical — same resolution, different PPI

• Diagonal pixels for 1920 × 1080:
  • $\sqrt{1920^2 + 1080^2} = \sqrt{4{,}852{,}800} \approx 2203$
• 24-inch monitor: $2203 \div 24 \approx 92 \text{ PPI}$ (pixels visible up close)
• 6-inch phone: $2203 \div 6 \approx 367 \text{ PPI}$ (very sharp)

5.2 Resolution standards (16:9)

• Key point: doubling width and height → 4× the pixels.
• Bandwidth: one raw 4K frame = $8{,}294{,}400 \times 3 \approx 24.9 \text{ MB}$; at 30 fps ≈ 747 MB/s → codecs (HEVC, AV1) are essential.
• Cinema (DCI): 2K = 2048 × 1080; 4K = 4096 × 2160.

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6. ⚠️ MCQ Traps

• Impact strikes the page; inkjet, laser, thermal do not.
• Plotter = precise line drawings, not general printing.
• LED monitor = LCD with LED backlight.
• OLED needs no backlight; that gives true black but also burn-in.
• Laser = dry toner, not ink.
• No LCD panel wins everything.
• Plasma is not LCD/LED.
• Resolution ≠ PPI.

7. ✅ Self-Test Questions (with Answers)

Q1. Impact vs non-impact printers, one example each.

• Impact: strikes a ribbon — dot-matrix (carbon copies).
• Non-impact: no contact — laser (toner fused by heat).
• Add the table in 3.3.

Q2. Plotter vs standard printer?

• Printer: raster (dots) — text, photos.
• Plotter: vector (continuous lines) — blueprints, CAD, maps.

Q3. How does a CRT form an image?

• Follow the seven steps in 4.1.

Q4. Why is "LED monitor" not a separate technology?

• Same liquid-crystal panel; only the backlight changed (CCFL → LED).
• OLED, by contrast, is truly different.

Q5. Steps of a laser printer?

• Charge → write → develop → transfer → fuse → clean (see 3.4).

Q6. Why does OLED give deeper blacks than LCD/LED?

• OLED pixel switches fully off; LCD crystals cannot fully block an always-on backlight.

Q7. Define pixel and resolution; how is PPI different?

• Pixel = smallest point; resolution = total count; PPI = density.
• Use the 24-inch vs 6-inch working in 5.1.

Q8. Three LCD panel types and a strength of each?

• TN = speed, IPS = colour and angles, VA = contrast.

Q9. How does plasma work, and why was it replaced?

• Gas → plasma → UV → phosphor glow.
• Replaced for power, weight, burn-in and cost reasons.

Q10. Resolution standards HD to 8K?

• Use the table in 5.2; each step ×4 pixels.