Side-by-side comparison
| Parameter | PCM | Delta Modulation |
|---|---|---|
| Bits per Sample | n bits (typically 8 for telephony) | 1 bit (direction of change only) |
| Bit Rate (voice, 8 kHz) | 64 kbps (G.711) | 16–32 kbps typical |
| Quantisation Noise | Uniform or non-uniform (µ-law, A-law) | Granular noise at low signal levels |
| Slope Overload | Not applicable | Occurs when signal slope > step size/Ts |
| Bandwidth Required | Higher — proportional to n×fs | Lower — proportional to fs only |
| Companding | Yes — µ-law (North America), A-law (Europe) | Not standard — adaptive DM (ADM) used instead |
| Reconstruction Complexity | Higher — n-bit DAC per sample | Simple — integrator and comparator |
| Standard | ITU-T G.711, G.726 | No major ITU standard; used in proprietary voice ICs |
| Signal Quality (SNR at same channel) | Higher — ~6n dB rule | Lower — limited by step size |
Key differences
PCM quantises each sample independently to n bits, giving an SNR of approximately (6n + 1.76) dB — for 8-bit PCM that is about 50 dB, well above voice-quality threshold. Delta modulation uses a single comparator and a step integrator, so its hardware is trivially simple, but slope overload distortion kicks in whenever the input signal changes faster than the step size allows. Adaptive Delta Modulation (ADM) varies the step size to fight this, but still cannot match PCM's SNR at equivalent bit rates. G.711 PCM at 64 kbps remains the gold standard for voice quality in PSTN and VoIP (G.711 passthrough mode).
When to use PCM
Use PCM when high audio fidelity and interoperability with standards like G.711 are required. The PSTN, VoIP gateways, and CD-quality audio all use 8-bit or 16-bit PCM without exception.
When to use Delta Modulation
Use Delta Modulation when hardware simplicity and very low bit rate matter more than SNR. Older speech-processing ICs like the MC3417 used DM for voice storage in toys and simple intercom systems.
Recommendation
For any exam or interview question involving telephony, choose PCM — it is the ITU standard and the SNR formula (6n + 1.76 dB) is directly examinable. Choose Delta Modulation only if the question explicitly asks about 1-bit encoding or hardware simplicity.
Exam tip: Examiners frequently ask you to derive the bit rate of PCM — write fs × n = 8000 × 8 = 64 kbps — and then compare slope overload condition for DM: Aω ≤ δ/Ts, where δ is step size and Ts is sample period.
Interview tip: An interviewer at a DSP or telecom company expects you to name µ-law and A-law companding standards for PCM and explain why they improve SNR for low-amplitude signals, not just recite that "PCM has better quality."