---
title: "Short note: Adaptive delta modulation"
subject: "Communication Systems"
chapter: 4
key: "semester_6/communication_systems:short-note-adm"
source: https://www.bctengineer.com/q/semester_6/communication_systems/short-note-adm
site: BCT Engineering
---

# Short note: Adaptive delta modulation

Communication Systems · Chapter 4: Pulse Modulation

- Asked in: 2072 Kartik (CS II), 4 marks
- Video: [Communication Engineering - Adaptive Delta Modulation (ADM)](https://www.youtube.com/watch?v=Sd0EhSCpcP4) — Dr. Sapna Katiyar
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/communication_systems/short-note-adm
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-4

## Question

Write a short note on adaptive delta modulation.

## Short answer

Adaptive delta modulation varies step size based on signal slope to avoid trade-off between granular noise and slope overload. It increases step size for successive same bits and decreases for alternating bits, improving dynamic range.

## Answer

**Adaptive Delta Modulation (ADM)** is a delta modulator in which the **step size $\Delta$ is varied automatically** according to the slope of the input signal, instead of being fixed.

**Why needed:** linear DM with a fixed step faces a trade-off. A large $\Delta$ avoids slope overload but gives large granular noise; a small $\Delta$ gives low granular noise but causes slope overload. ADM removes this conflict.

```text
m(t)->(+)->[1-bit Q]-+-------------> out
      ^-             |
      |              v
      +-[Accum]<-[x]<+--[Step-size logic]
                  ^         |
                  +---------+
```

**Working**
- The output bits are watched by a step-size control logic.
- If several successive bits are the **same** (e.g. 111 or 000), the signal is rising/falling steeply, so the step is **increased** (e.g. multiplied by 1.5 or doubled).
- If the bits **alternate** (1010), the signal is flat, so the step is **reduced** (e.g. halved), down to a minimum $\Delta_{min}$.
- The receiver uses the same logic on the received bits, so no extra side information is sent.
- A common rule (Song algorithm): $\Delta_k = |\Delta_{k-1}|\left(e_k + \tfrac{1}{2}e_{k-1}\right)$; another is CVSD (continuously variable slope DM).

**Advantages**
- Reduced slope overload and granular noise together.
- Wider dynamic range and better SQNR than linear DM at the same bit rate.
- Good speech quality at 16–32 kbps (CVSD used in military and Bluetooth voice).

**Drawback:** more complex circuitry; bit errors affect step size for a few samples.

## Check yourself

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

1. What problem of fixed-step DM does ADM remove?
   A. Aliasing
   B. The trade-off between slope overload (needs large step) and granular noise (needs small step)
   C. Aperture effect
   D. Crosstalk
   Answer: B. A large step causes granular noise and a small step causes slope overload; varying the step fixes both. (Key point: Reason for ADM)

2. In ADM, when several successive output bits are the same (111 or 000):
   A. Step is reduced
   B. Step is kept fixed
   C. Step is increased because the signal is changing steeply
   D. Sampling stops
   Answer: C. Repeated equal bits show the staircase is lagging a steep slope, so the step grows. (Key point: Step increase rule)

3. Alternating bits (1010) in ADM mean:
   A. The signal is flat, so the step is reduced
   B. Slope overload, so the step is doubled
   C. Channel error
   D. Aliasing
   Answer: A. Alternation shows the staircase hunts around a slowly varying signal, so a smaller step cuts granular noise. (Key point: Step decrease rule)

4. How does the ADM receiver know the step size?
   A. Extra side information is sent
   B. It uses the same logic on the received bits
   C. It uses a fixed step
   D. A pilot tone
   Answer: B. Step logic is driven only by the bit stream, so no side information is needed. (Key point: Receiver)

## Related in Communication Systems

- [T1 digital carrier system; adaptive delta modulation](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3At1-hierarchy-adm) — Communication Systems, chapter 5
- [Delta and adaptive delta modulation](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adm-adm-encoder-decoder) — Communication Systems, chapter 4
- [SQNR of delta modulation](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adm-sqnr-derive) — Communication Systems, chapter 4, asked 2×
- [SQNR of delta modulation for a single-tone signal](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adm-sqnr-single-tone) — Communication Systems, chapter 4
- [DPCM vs delta modulation; E1 hierarchy](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adpcm-vs-dm-e1) — Communication Systems, chapter 4
- [Delta modulation encoder/decoder; PCM vs DM](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adm-encoder-decoder-pcm-vs-dm) — Communication Systems, chapter 4