--- title: "DTMF vs pulse dialing and DTMF design" subject: "Telecommunications" chapter: 1 key: "semester_8/telecommunications:dtmf-vs-pulse-design" source: https://www.bctengineer.com/q/semester_8/telecommunications/dtmf-vs-pulse-design site: BCT Engineering --- # DTMF vs pulse dialing and DTMF design Telecommunications · Chapter 1: Telecommunication Networks - Asked in: 2068 Bhadra (old course), 4+12 marks; 2064 Poush (old course), 2+14 marks (asked 2 times) - Video: [Rotatory And DTMF dialing](https://www.youtube.com/watch?v=k-wT7QoDsGU) — manishvaryani53 - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/dtmf-vs-pulse-design - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-1 ## Question What are the advantages of DTMF (Dual Tone Multi-Frequency) telephone set over conventional pulse dialing telephone set? Explain the design features considered in DTMF telephone set. ## Short answer DTMF dialling is faster, allows end-to-end signalling, and uses a 4x4 keypad. Design features: frequencies in voice band (697-1633 Hz) with non-harmonic spacing, talk-off protection via duration/guard, frequency tolerance ±1.5%, level twist to compensate line loss, and electronic generation. ## Answer #### Advantages of DTMF over pulse (rotary) dialling **DTMF (Dual Tone Multi-Frequency)** dialling sends each digit as two audio tones played together, one from a low group and one from a high group, instead of loop-break pulses. | Point | Pulse dialling | DTMF dialling | |---|---|---| | Speed | 10 pps; digit 0 takes ~1 s + rotation | ~50 ms tone + 50 ms gap per digit (~10 times faster) | | Signal | Loop make/break (DC) | Two voice-band tones | | End-to-end use | Cannot pass through exchange | Tones pass over speech path (IVR, banking, voicemail) | | Keys | 10 digits | 12–16 keys (* , #, A–D) | | Errors | Dial wear, pulse distortion | Electronic, accurate | | Mechanism | Mechanical dial, needs repair | Push buttons + IC oscillator | | Exchange | Needs pulse counting relays | Suits electronic/SPC exchanges | | User effort | Slow rotation, misdialling | Easy keypad | Other advantages: shorter call set-up time so exchange registers are held less time (fewer receivers needed), and support for supplementary services. #### Keypad and frequencies ```text 1209 1336 1477 1633 Hz 697 Hz 1 2 3 A 770 Hz 4 5 6 B 852 Hz 7 8 9 C 941 Hz * 0 # D ``` Example: pressing **5** sends 770 Hz + 1336 Hz together. #### Design features of a DTMF telephone set 1. **Frequencies inside the voice band (697–1633 Hz)** so tones pass through the normal speech channel and filters. 2. **Two groups of four tones (2-out-of-8 code):** exactly one tone from each group. Receiver checks that one and only one tone of each group is present – a strong check against errors. 3. **Non-harmonic frequencies:** no frequency is a harmonic of another, and sum/difference (intermodulation) products of any two tones do not fall on another DTMF frequency. Spacing is about a constant ratio (~10% in low group, ~10% in high group), so harmonics of speech or of a tone are not mistaken for a digit. 4. **Protection against talk-off (speech imitation):** speech rarely contains two pure tones from both groups with steady amplitude; receiver also requires a **minimum tone duration** (about 40 ms) and a **minimum pause** (about 40 ms) between digits. 5. **Frequency tolerance:** generator accuracy within about ±1.5%; receiver accepts ±1.5% and rejects beyond ±3.5%. 6. **Level and twist:** each tone sent at about −6 to −8 dBm; high-group tone sent slightly louder (about 2 dB, "pre-emphasis/twist") to make up for greater line loss at high frequencies; receiver accepts a limited twist (e.g. up to 4–8 dB). 7. **Signal-to-noise and guard:** the energy of the two tones must be much larger than total energy in other frequencies (guard action), so music and speech are rejected. 8. **Electronic generation:** a crystal oscillator and divider IC generate tones accurately; the speech circuit is muted during tone sending so that no voice is mixed with tones, and loop current powers the set. 9. **Compatibility:** many sets have a switch for pulse/tone mode so they work with old exchanges. These features give fast, reliable signalling with very low probability of false digit detection. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does DTMF send a digit? A. Two voice-band tones, one from a low group and one from a high group, played together B. One tone from a single group of ten C. A series of loop make/break pulses D. Two tones from the same group Answer: A. DTMF uses a 2-out-of-8 code of simultaneous tones in the voice band. (Key point: DTMF signal) 2. Which tones does pressing 5 send? A. 697 Hz + 1209 Hz B. 770 Hz + 1336 Hz C. 852 Hz + 1477 Hz D. 941 Hz + 1336 Hz Answer: B. Key 5 lies in the 770 Hz row and 1336 Hz column. (Key point: Keypad) 3. Why can DTMF tones be used after the call is connected (e.g. IVR, banking)? A. They are DC pulses that the exchange absorbs B. They use a separate signalling cable C. They are voice-band tones that pass over the speech path D. They are at ultrasonic frequencies Answer: C. Pulses cannot pass end-to-end, but tones in the voice band travel over the speech path. (Key point: Speed advantage) 4. Why are DTMF frequencies chosen non-harmonically? A. So the tones are exactly multiples of each other B. So the keypad needs only one oscillator C. So the tones are outside the voice band D. So harmonics and intermodulation products of the tones or speech are not mistaken for a digit Answer: D. No tone is a harmonic of another, and intermod products avoid DTMF frequencies, reducing false detection. (Key point: Non-harmonic frequencies) 5. How does the receiver protect against talk-off (speech imitating digits)? A. It requires one tone from each group, a minimum tone duration (~40 ms) and a minimum pause B. It accepts any single tone of long duration C. It ignores the pause between digits D. It accepts tones only above 3 kHz Answer: A. Speech rarely holds two steady tones, and duration/pause checks reject it. (Key point: Talk-off) ## Related in Telecommunications - [Talk off in DTMF telephone sets](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adtmf-talk-off) — Telecommunications, chapter 1 - [TST vs STS; DTMF set pros and cons](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atst-sts-compare-dtmf) — Telecommunications, chapter 4 - [Advantages and disadvantages of DTMF set](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adtmf-advantages-disadvantages) — Telecommunications, chapter 1 - [In-channel multi-frequency signaling design](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ainchannel-mf-design) — Telecommunications, chapter 5 - [Pulse dialing waveform for number 401](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Apulse-dialing-waveform-401) — Telecommunications, chapter 1 ## Related in other subjects - [OO design cycle vs conventional cycle](https://www.bctengineer.com/question.md?key=semester_6%2Fobject_oriented_analysis_and_design%3Aoo-design-cycle-vs-conventional) — Object Oriented Analysis and Design, chapter 1 - [Conventional sources and features of a station](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aconventional-sources-features) — Power System Analysis I, chapter 1 - [Single tone DSB-FC AM in time and frequency domain](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Asingle-tone-dsbfc-time-freq) — Communication Systems, chapter 2 - [Angle modulation; single tone FM time and frequency domain](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aangle-mod-single-tone-fm-time-freq) — Communication Systems, chapter 3