Display Systems & Architectures: Raster vs. Vector, Scan Types, and Hardware

TAKEAWAYS:

• Raster scan updates pixels row-by-row (like a TV), while random scan draws lines directly (like a CAD tool).
• Vector displays use beam deflection to draw lines (e.g., old CAD monitors), while raster displays use pixel grids (e.g., smartphones).
• CRTs use electron beams and phosphors; LCDs use backlight + liquid crystals; OLEDs emit their own light.
• Refresh rate (Hz) and resolution (DPI) directly impact flicker and sharpness—critical for apps like Pathao’s real-time maps.
• Frame buffers store pixel data; dual-port memories enable simultaneous read/write for smooth animations.
• Color models (RGB, CMYK) and bit depth determine color depth (e.g., 24-bit vs. 48-bit displays).

1. Introduction to Display Systems

Display systems convert digital data into visual images. They classify into two broad categories based on how they render graphics:

1.1 Raster Scan vs. Random Scan

Raster scan systems update pixels in a fixed order (left-to-right, top-to-bottom), like a TV or computer monitor. Random scan systems draw lines or shapes directly by moving a beam to specific coordinates, used in older CAD systems.

flowchart TD
    A["Raster Scan"] -->|"Updates pixels in fixed order (left-to-right, top-to-bottom)"| B["Fixed order (e.g., TV, smartphone)"]
    C["Random Scan"] -->|"Moves beam to dynamic coordinates (e.g., old CAD)"| D["Direct line drawing (e.g., CRT-based CAD)"]
    A -->|"Example: Horizontal line"| E["Pixel-by-pixel (e.g., CRT raster)"]
    C -->|"Example: Circle"| F["Mathematical calculations (e.g., parametric equations)"]

Key Difference:

2. Display Architectures

Display systems can be categorized based on how they generate images:

2.1 Vector Graphics Display Architecture

Uses a cathode-ray tube (CRT) to draw lines by deflecting an electron beam. The beam is controlled by voltages applied to deflection plates.

Block Diagram:

flowchart TD
    A["Computer"] --> B["Vector Generator"]
    B --> C["Deflection Circuits"]
    C --> D["CRT"]
    D --> E["Phosphor Screen"]

Advantages:

• High precision for line drawings.
• No aliasing for geometric shapes.

Disadvantages:

• Expensive and bulky.
• Limited to vector-based graphics.

Real-world Example: Old CAD workstations (e.g., AutoCAD on early terminals) used vector displays for precise engineering drawings.

2.2 Raster Graphics Display Architecture

Stores pixel data in a frame buffer and refreshes the screen row-by-row. Used in modern monitors, TVs, and smartphones.

Block Diagram:

flowchart TD
    A["Computer"] --> B["Frame Buffer"]
    B --> C["Raster Scan Generator"]
    C --> D["CRT/LCD/OLED"]
    D --> E["Phosphor/Liquid Crystal/OLED Screen"]

Advantages:

• Supports complex images (photos, animations).
• Cheaper and more compact.

Disadvantages:

• Aliasing (jagged edges) for non-rasterized shapes.
• Higher memory requirements.

Real-world Example: eSewa’s app UI renders on a raster display, where buttons and icons are stored as pixel data in the frame buffer.

3. Display Devices

3.1 Cathode-Ray Tube (CRT)

Uses an electron beam to excite phosphors on a screen. The beam is deflected by magnetic or electric fields.

How it Works:

1. Electron gun emits electrons.
2. Deflection coils/magnets steer the beam.
3. Beam hits phosphors, emitting light.

Advantages:

• High brightness and contrast.
• Good for large screens.

Disadvantages:

• Bulky and power-hungry.
• Limited refresh rates.

3.2 Liquid Crystal Display (LCD)

Uses liquid crystals to block or pass light from a backlight. Each pixel is controlled by a thin-film transistor (TFT).

How it Works:

1. Backlight illuminates the LCD panel.
2. Liquid crystals rotate light based on voltage.
3. Color filters (RGB) combine to form images.

Advantages:

• Thin, lightweight, and energy-efficient.
• Used in laptops and TVs.

Disadvantages:

• Viewing angle-dependent brightness.
• Lower peak brightness than CRTs.

Real-world Example: NTC’s broadcast monitors use LCDs for clear, high-resolution video feeds.

3.3 Organic Light-Emitting Diode (OLED)

Each pixel emits its own light when an electric current passes through organic material.

How it Works:

1. Voltage applied to organic layers.
2. Electrons and holes recombine, emitting light.
3. No backlight needed.

Advantages:

• Ultra-thin and flexible.
• Perfect blacks (no backlight bleed).
• Used in high-end smartphones (e.g., Samsung Galaxy).

Disadvantages:

• Expensive to produce.
• Burn-in risk with static images.

4. Refresh Rate and Resolution

4.1 Refresh Rate (Hz)

The number of times the screen is redrawn per second. Higher refresh rates reduce flicker.

Example:

• 60Hz: Standard for most monitors.
• 144Hz: Used in gaming monitors for smoother animations.

Worked Example: A Pathao driver’s app shows real-time traffic updates. If the display refreshes at 60Hz, the app updates 60 times per second, ensuring smooth navigation visuals.

4.2 Resolution (DPI)

Pixels per inch (PPI) determine sharpness. Higher DPI = sharper images.

Example:

• 72 PPI: Web graphics.
• 300 PPI: Print-quality images.

Worked Example: A Daraz product photo is rendered at 300 PPI for high-quality online listings. If displayed on a 72 PPI screen, the image will appear pixelated.

5. Frame Buffer and Memory

The frame buffer stores pixel data before sending it to the display. Dual-port memory allows simultaneous read/write for smooth animations.

How it Works:

1. Graphics processor writes pixel data to frame buffer.
2. Display reads pixel data row-by-row.
3. Dual-port memory enables parallel access.

Real-world Example: YouTube’s video streaming relies on frame buffers to render smooth video playback. The buffer stores each frame, and the display refreshes at 30Hz (or higher) for fluid motion.

6. Color Models

6.1 RGB (Additive)

Used in screens and projectors. Combines Red, Green, and Blue light.

Example:

• 24-bit color: 8 bits per channel (16.7 million colors).
• 48-bit color: 16 bits per channel (281 trillion colors).

6.2 CMYK (Subtractive)

Used in printing. Combines Cyan, Magenta, Yellow, and Key (Black).

Example:

• Printers use CMYK to mix inks for colors.

Comparison:

7. Worked Example: Displaying a Simple Image

Scenario: Render a 2x2 pixel image (green square on red background) on a raster display.

1. Frame Buffer Setup:

   • Allocate memory for 2x2 pixels.
   • Red background: Store (255, 0, 0) for each pixel.
   • Green square: Store (0, 255, 0) for the top-left pixel.

2. Pixel Data:

   [(255, 0, 0), (255, 0, 0)]
   [(0, 255, 0), (255, 0, 0)]
   

3. Display Output:

   • Raster scan generator reads row-by-row.
   • Screen updates to show the green square on red.

Visual Output:

[ (R,G,B) = (255,0,0) ] [ (R,G,B) = (255,0,0) ]
[ (R,G,B) = (0,255,0) ] [ (R,G,B) = (255,0,0) ]

(Green square at top-left corner.)

8. In the Real World

1. eSewa’s UI Rendering:

   • Uses raster scan to display buttons, icons, and transaction screens.
   • Frame buffer stores pixel data for smooth interactions (e.g., tapping "Pay Now").

2. NTC’s Broadcast Systems:

   • Relies on LCD/OLED displays for high-resolution news feeds.
   • Refresh rate of 60Hz ensures smooth video playback during broadcasts.

3. Pathao’s Real-Time Maps:

   • Vector graphics are used for route planning (lines, points).
   • Raster images show satellite views and traffic icons.
   • Dual-port memory enables fast updates as the driver’s location changes.

Exam Tip

• Compare raster vs. random scan: Focus on update methods, use cases, and examples (e.g., TV vs. CAD).
• Draw block diagrams: Always include the frame buffer, deflection circuits, and display device.
• Discuss trade-offs: Highlight pros/cons of CRT vs. LCD vs. OLED (e.g., brightness, power, cost).
• Calculate refresh rates: If given a flicker frequency, compute the refresh rate (e.g., 50Hz = 20ms per frame).
• Relate to real apps: Mention how eSewa’s UI or Pathao’s maps use these concepts in 1-2 sentences.
• Avoid vague answers: Specify whether you’re discussing hardware (CRT) or software (frame buffer).