--- title: "Advantages and disadvantages of DTMF set" subject: "Telecommunications" chapter: 1 key: "semester_8/telecommunications:dtmf-advantages-disadvantages" source: https://www.bctengineer.com/q/semester_8/telecommunications/dtmf-advantages-disadvantages site: BCT Engineering --- # Advantages and disadvantages of DTMF set Telecommunications · Chapter 1: Telecommunication Networks - Asked in: 2070 Chaitra, 8 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/dtmf-advantages-disadvantages - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-1 ## Question What are the advantage and disadvantage of DTMF telephone set? Explain. ## Short answer DTMF offers fast dialling (~100 ms/digit), end-to-end signalling for IVR, 16 signals, and reliable 2-of-8 error checking. Disadvantages include the need for DTMF receivers, susceptibility to talk-off (false digit detection from speech), level problems on long lines, and audible tones posing a security risk. ## Answer **DTMF (Dual Tone Multi-Frequency)** telephone sets send each digit as the sum of two sine tones – one from a low group (697, 770, 852, 941 Hz) and one from a high group (1209, 1336, 1477, 1633 Hz). ```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: key **8** = 852 Hz + 1336 Hz. #### Advantages 1. **Fast dialling:** each digit takes about 50 ms of tone plus 50 ms pause (~100 ms) against up to ~1 s per digit for a rotary dial. Call set-up time and register holding time in the exchange are reduced. 2. **End-to-end signalling:** tones are in the voice band, so they pass through the connected call – used for IVR, telebanking, voicemail, remote control. 3. **More signals:** 16 combinations including * and # (and A–D) for supplementary services. 4. **Reliability:** electronic generation with crystal accuracy; no mechanical governor that wears out or drifts. 5. **Error checking:** exactly one tone from each group must be present (2-of-8 code), so most faults are detected. 6. **Convenient:** push buttons are easy, fewer misdialled numbers; suitable for redial and memory features. 7. **Suited to electronic/digital exchanges** – no DC pulse counting relays. #### Disadvantages 1. **Needs DTMF receivers** at the exchange (digital filters/DSP), adding cost; old Strowger/crossbar exchanges cannot accept it without converters. 2. **Talk-off:** speech, music or noise may imitate a DTMF pair and cause false digits, so receivers need guard circuits. 3. **Talk-down/level problems:** on long lines the high-group tone is attenuated more (twist), and noise or echo can stop detection. 4. **Needs power and electronics** in the set; more complex than a simple dial. 5. **Tones are audible** and can be recorded or overheard (e.g. PINs) – a security issue. 6. **Compatibility:** sets often need a pulse/tone switch in areas with old exchanges. Overall, the advantages far outweigh the disadvantages, so DTMF has replaced pulse dialling in modern networks. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is key 8 sent in DTMF? A. 852 Hz plus 1336 Hz B. 770 Hz plus 1209 Hz C. 941 Hz plus 1477 Hz D. 852 Hz plus 1633 Hz Answer: A. Row 852 Hz and column 1336 Hz give key 8; each digit is one low-group plus one high-group tone. (Key point: DTMF tone generation) 2. Why does DTMF reduce call set-up time? A. Each digit takes about 100 ms against up to about 1 s for rotary dial, shortening register holding time B. It uses higher voltage C. It bypasses the exchange D. It needs no receiver Answer: A. About 50 ms tone plus 50 ms pause per digit is much faster than pulse dialling. (Key point: Speed advantage) 3. How does the 2-of-8 code help error detection? A. Exactly one tone from each group must be present, so most faults are detected B. Any two tones are accepted C. It uses parity bits D. It repeats digits twice Answer: A. Requiring exactly one low and one high tone rejects invalid combinations. (Key point: Error checking) 4. Which is a disadvantage of DTMF? A. Cannot send * and # B. Speech or noise may imitate a tone pair (talk-off) causing false digits C. Slower dialling D. Cannot be used end to end Answer: B. Talk-off is false detection of digits, and receivers need guard circuits; DTMF also needs receivers at the exchange. (Key point: Disadvantage: talk-off) ## Related in Telecommunications - [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 - [Talk off in DTMF telephone sets](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adtmf-talk-off) — Telecommunications, chapter 1 - [DTMF vs pulse dialing and DTMF design](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adtmf-vs-pulse-design) — Telecommunications, chapter 1, asked 2× ## Related in other subjects - [FP-Growth and Apriori disadvantage](https://www.bctengineer.com/question.md?key=semester_7%2Fdata_mining%3Afp-growth-vs-apriori) — Data Mining, chapter 4