---
title: "Digital system, logic level, BCD vs binary"
subject: "Digital Logic"
chapter: 1
key: "semester_3/digital_logic:digital-system-logic-level-bcd"
source: https://www.bctengineer.com/q/semester_3/digital_logic/digital-system-logic-level-bcd
site: BCT Engineering
---

# Digital system, logic level, BCD vs binary

Digital Logic · Chapter 1: Introduction

- Asked in: 2082 Baisakh, 1+1+3 marks
- Video: [Lec-16: Introduction to Binary code | ASCII, UNICODE, EBCDIC, BCD | Number System](https://www.youtube.com/watch?v=0UVSPwzGw4U) — Gate Smashers
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/digital_logic/digital-system-logic-level-bcd
- Whole chapter: https://www.bctengineer.com/md/semester_3/digital_logic/chapter-1

## Question

Define digital system and logic level. What is the difference between BCD and binary codes?

## Short answer

A digital system processes discrete binary signals. Logic levels are voltage states representing 0 and 1. Binary code converts the whole number to base 2; BCD codes each decimal digit in 4 bits, using more bits but enabling easy decimal display.

## Answer

#### Digital system
A **digital system** is a system that processes information in discrete form, usually as binary signals 0 and 1, e.g. a computer, calculator or digital watch.

#### Logic level
A **logic level** is one of the discrete voltage states used to represent a binary value: HIGH (logic 1) and LOW (logic 0). For TTL, an input of 2.0–5 V is HIGH and 0–0.8 V is LOW.

#### BCD vs binary code

| Point | Binary code | BCD code |
|---|---|---|
| Method | Whole number converted to base 2 | Each decimal digit coded separately in 4 bits |
| Example: 25 | 11001 | 0010 0101 |
| Bits needed | Fewer (efficient) | More (e.g. 99 needs 8 bits vs 7) |
| Unused codes | None | 1010–1111 invalid |
| Arithmetic | Simple | Needs correction (add 0110) |
| Conversion to decimal | Lengthy | Very easy |
| Use | Computation, memory | Displays, calculators, clocks |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is a digital system?
   A. A system with continuous voltage
   B. A system processing information in discrete form, usually binary 0 and 1
   C. A system using only analog amplifiers
   D. A system with no memory
   Answer: B. Computers and calculators are examples. (Key point: Digital system)

2. What are logic levels?
   A. Different supply voltages
   B. Clock frequencies
   C. Discrete voltage states for HIGH (1) and LOW (0)
   D. Gate counts
   Answer: C. For TTL, 2-5 V is HIGH and 0-0.8 V is LOW. (Key point: Logic levels)

3. How is 25 coded in BCD versus binary?
   A. BCD 11001; binary 0010 0101
   B. Both 11001
   C. BCD 0011 0010; binary 11001
   D. BCD 0010 0101; binary 11001
   Answer: D. BCD codes digits separately. (Key point: BCD vs binary)

4. Which needs more bits for the same number?
   A. BCD
   B. Binary
   C. Both equal
   D. Neither
   Answer: A. 99 needs 8 bits in BCD and 7 in binary. (Key point: Bit count)

## Related in Digital Logic

- [Digital system advantages; BCD and Excess-3](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Adigital-advantages-bcd-excess3) — Digital Logic, chapter 1
- [BCD code and 3-bit binary-to-Gray converter](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Abcd-code-gray-converter) — Digital Logic, chapter 1
- [CMOS logic level profile](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Acmos-logic-level-profile) — Digital Logic, chapter 10
- [Difference between analog and digital signals](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Aanalog-vs-digital-signals) — Digital Logic, chapter 1
- [Digital/analog signals, Gray and Excess-3 codes](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Adigital-analog-gray-excess3) — Digital Logic, chapter 1
- [Combinational vs sequential; BCD-to-decimal decoder](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Abcd-to-decimal-decoder) — Digital Logic, chapter 4

## Related in other subjects

- [8085 ALP: BCD to binary conversion](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Aalp-8085-bcd-to-binary) — Microprocessor, chapter 2
- [Analog vs digital multiplexing; 24-channel system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aanalog-digital-mux-24-channel) — Telecommunications, chapter 3
- [Digital transmission system; analog vs digital](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aanalog-digital-transmission-compare) — Instrumentation II, chapter 5
- [Duplex digital communication system vs analog](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adigital-comm-duplex-vs-analog) — Communication Systems, chapter 1