Basic Concepts
1. What is Delta Modulation (DM)?
A form of analog-to-digital conversion that encodes the difference (delta) between consecutive samples using 1-bit quantization.
2. What is the key difference between DM and PCM?
PCM encodes absolute sample values, while DM encodes the difference between consecutive samples.
3. What is the bit rate of a DM system with a sampling rate of 32 kHz?
32 kbps (since DM uses 1 bit per sample).
4. What is the main advantage of DM over PCM?
Simpler implementation and lower bit rate (1-bit quantization).
5. What is the main disadvantage of DM?
Slope overload distortion when the input signal changes too rapidly.
6. What is the function of the integrator in a DM system?
Reconstructs the staircase approximation of the input signal.
7. What is the step size (Δ) in DM?
The fixed quantization interval used to approximate signal differences.
8. What happens if the step size is too small in DM?
Granular noise occurs due to small signal variations not being captured.
9. What happens if the step size is too large in DM?
Slope overload distortion occurs because the signal changes too fast.
10. What is Adaptive Delta Modulation (ADM)?
A variant of DM where the step size (Δ) adjusts dynamically based on signal slope.
Mathematical Analysis
11. Write the DM encoding rule mathematically.
b[n] =
\begin{cases}
+1 & \text{if } x[n] > \hat{x}[n-1] \\
-1 & \text{if } x[n] < \hat{x}[n-1]
\end{cases}
12. What is the condition to avoid slope overload in DM?
Δ / T_s ≥ max |dx(t)/dt|
where Δ = step size, T_s = sampling interval.
13. Calculate the maximum input frequency a DM system can track without slope overload if Δ = 0.1V and f_s = 8 kHz.
f_{max} = Δ f_s / (2π A)
Assuming A = amplitude of a sinusoidal input, exact value depends on signal shape.
14. What is the SNR for DM under granular noise conditions?
SNR ≈ 6.02n + 1.76 dB (similar to PCM but with higher quantization noise)
15. How does Adaptive DM (ADM) improve performance?
By dynamically adjusting step size (Δ) to reduce slope overload and granular noise.
16. What is the main difference between DM and DPCM?
DPCM uses multi-bit quantization of prediction error, while DM uses 1-bit.
17. What is the role of the comparator in a DM encoder?
Compares input signal with the predicted (staircase) signal to generate +1/-1 output.
18. How does oversampling affect DM performance?
Reduces slope overload by allowing faster tracking of signal changes.
Applications & Comparisons
19. Where is Delta Modulation commonly used?
Voice coding (military comms, early digital telephony), simple ADC systems.
20. Why is DM not used for high-fidelity audio?
High quantization noise and distortion make it unsuitable for high-quality signals.
21. Compare DM and PCM in terms of bit rate.
DM uses 1 bit/sample, PCM uses 8-16 bits/sample → DM has lower bit rate.
22. What is Continuously Variable Slope Delta Modulation (CVSD)?
An ADM variant where step size varies continuously to reduce distortion.
23. What is the main advantage of ADM over DM?
Better dynamic range and reduced slope overload.
24. Why is DM rarely used in modern systems?
Modern PCM, DPCM, and ADPCM offer better quality and efficiency.
25. How does DM perform in noisy channels?
Poorly, because bit errors cause large staircase approximation errors.
26. What is the main trade-off in choosing step size (Δ)?
Small Δ → granular noise, large Δ → slope overload.
Advanced Topics
27. How does Differential PCM (DPCM) improve upon DM?
Uses multi-bit quantization of prediction error, reducing noise.
28. What is Sigma-Delta Modulation (ΣΔ)?
A high-resolution ADC technique using noise shaping and oversampling.
29. Why is ΣΔ modulation preferred over DM in modern ADCs?
Higher resolution, better noise shaping, and no slope overload.
30. What is the key idea behind noise shaping in ΣΔ modulators?
Pushing quantization noise to higher frequencies, which can be filtered out.
