In a BCD-to-seven-segment display system, why is a code converter needed between the 4-bit BCD input and the LED segment lines?

Introduction / Context:
Seven-segment displays require one control signal per segment (usually a–g). A BCD digit (0–9) is encoded with four bits, but to light the proper pattern, the system must translate that 4-bit value into seven on/off signals. This translation is the role of a BCD-to-seven-segment code converter/driver (e.g., 7447, 4511).

Given Data / Assumptions:

• Input: 4-bit BCD representing 0..9.
• Output: 7 control lines for segments a–g (plus optional decimal point).
• Display type: common-anode or common-cathode determines active polarity.

Concept / Approach:
The converter implements a truth table mapping each BCD code to a specific seven-bit pattern. For example, the digit 0 lights segments a,b,c,d,e,f and leaves g off; the digit 1 lights only b and c. Invalid BCD codes (10–15) are usually blanked or show a defined pattern depending on the IC.

Step-by-Step Solution:
Accept BCD input XXXX (4 bits).Use combinational logic/ROM to compute a–g outputs.Drive the LED segments with correct polarity and current limiting.Thus, a 4→7 code conversion is required.

Verification / Alternative check:
Datasheets for 7447/7448 or 4511 show explicit BCD input to segment output tables, confirming that seven control lines are derived from four input bits and that polarity differs by display type.

Why Other Options Are Wrong:

• No conversion: Without a mapping, segments cannot display the intended digit.
• BCD→gray or two's complement: Different code families, not needed for seven-seg control.
• BCD→10-bit: A decimal one-hot output is a different function (BCD-to-decimal decoder).

Common Pitfalls:

• Mixing common-anode with common-cathode drivers; logic polarity must match.
• Ignoring segment current limits, leading to dim or damaged displays.

Final Answer:
to convert the 4-bit BCD into 7-bit code