Companding in communication systems: Which statement about a compandor is FALSE?
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AIt compresses the higher-amplitude parts of a signal before modulation and expands them back to normal after demodulation.
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BIt gives preferential treatment to the weaker parts of the signal.
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CFor weaker signals it gives a poor ratio of signal strength to quantizing error.
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DWeaker signals traverse more quantum steps than otherwise, so quantizing error is reduced.
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ENone of these
Answer
Correct Answer: For weaker signals it gives a poor ratio of signal strength to quantizing error.
Explanation
Introduction / Context:Companding (compressing + expanding) is used in analog and digital telephony/audio to improve the perceived signal-to-noise and reduce quantization noise on low-level signals by reshaping dynamic range before and after transmission.
Given Data / Assumptions:
- Compandor performs pre-emphasis (compression) at the transmitter and de-emphasis (expansion) at the receiver.
- Goal: improve SNR for weak portions of the signal.
- Quantization noise is most objectionable for low-level signals in uniform quantization.
Concept / Approach:
Companding law (e.g., μ-law, A-law) increases effective resolution for small amplitudes by mapping them to a larger code range, thus reducing the quantization step relative to the weak signal amplitude and improving the signal-to-quantizing-noise ratio (SQNR) for soft passages.
Step-by-Step Solution:
Identify compandor action: compress highs, boost lows pre-transmission.At receiver, expand to restore original dynamic range while background/quantization noise is de-emphasized on weak segments.Therefore, weaker signals gain better (not poorer) SQNR.Verification / Alternative check:
Telephony standards (μ-law/A-law) show improved effective bits for small amplitudes compared to uniform PCM, confirming better SQNR for weak signals.
Why Other Options Are Wrong:
- (a), (b), (d) correctly describe compression, preferential treatment for weak parts, and increased quantum steps traversed for low-level signals after companding.
- (c) is false because companding specifically improves—not worsens—the SQNR of weak portions.
Common Pitfalls:
- Assuming companding benefits only strong signals; the key benefit is for low-level content.
Final Answer:
For weaker signals it gives a poor ratio of signal strength to quantizing error.