• Digital signals: Represent information using discrete levels (commonly two: 0 and 1).
• Analog signals: Represent information using continuous variation over time.

My understanding

• In practice, digital is only relatively discrete and analog is only relatively continuous — perfect separation doesn’t exist due to physical limitations like noise, quantization errors, and bandwidth limits.
• We can treat digital signals as discrete and analog signals as continuous for design purposes.
• Digital signals are threshold-controlled:
  • Any value above the high threshold is treated as logic 1.
  • Any value below the low threshold is treated as logic 0.
• This makes digital signals not easily affected by small noise.

Analog noise susceptibility

• Analog signals record the exact amplitude at every moment, so even tiny changes (from electrical noise, interference, or distortion) directly change the output.
• No threshold protection — a small variation is still a real change in the analog value.
• Over long distances or processing steps, noise accumulates and degrades quality.

Codes

Binary-Coded Decimal (BCD)

• Definition: Each decimal digit (0–9) is represented separately in binary using 4 bits.
• My understanding:
  • Instead of converting the whole number to binary, each digit is converted individually.
  • Requires 4 bits because the highest decimal digit 9 is 1001 in binary.
  • The nearest power of 2 less than 9 is 8 (2³), so we need an extra bit to represent 9 → total of 4 bits.
  • This makes conversion and decimal arithmetic simpler in hardware at the expense of more storage.

Carry adjustment rule in addition

• After 1001 (9), the next binary value is 1010 (10), which is invalid in BCD.
• The difference between 2⁴ (16) and the next valid decimal after 9 is 6.
• In BCD arithmetic, if a 4-bit group exceeds 1001, we add 6 to reset the count and carry over.

Serial vs. Parallel Transmission

My understanding

Serial transmission

• Data is transmitted within the same line, one after another.
• (+) Easy to ensure predictable ordered arrival.
• (-) Slower per clock cycle since all bits use the same “highway”.
• (+) Simpler single-cable setup → easier long-range maintenance.

Parallel transmission

• More than one channel for transmission.
• (-) Less predictable timing due to multiple channels.
• (++) Faster by multiples of total channels used.
• (-) Requires reassembly and authentication at the end.
• (-) Maintenance cost scales with cable count.

Where each is preferred

• Serial: Long-distance, high-speed links (PCIe, SATA, USB, DisplayPort) where multiple synchronized wires would be too costly.
• Parallel: Short-distance, high-speed internal connections like DDR RAM between CPU and memory controller, where skew is manageable.
• Parallel (low-speed): Cheap way to boost speed where complexity is easy to handle (e.g., old printer ports).

Clock Cycles / Oscillations

• A clock signal is a repeating electrical pulse that synchronizes data transfer and operations.
• Oscillations = clock cycles in this context — they keep time and rhythm so transistors know when to read/send bits.
• Synchronous systems operate on specific clock edges (rising/falling).
• Higher clock = more ops/sec but harder timing design.

Historical Shift Table