--- title: "Impairments to voice channel transmission" subject: "Telecommunications" chapter: 2 key: "semester_8/telecommunications:voice-channel-impairments" source: https://www.bctengineer.com/q/semester_8/telecommunications/voice-channel-impairments site: BCT Engineering --- # Impairments to voice channel transmission Telecommunications · Chapter 2: Transmission Media - Asked in: 2079 Bhadra, 2+6 marks - Video: [Telecommunication Systems (08 - Transmission Impairments: Attenuation Distortion)](https://www.youtube.com/watch?v=hpq-sUsv6vk) — Dr. Samy S. Soliman - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/voice-channel-impairments - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-2 ## Question What do you mean by impairments to voice channel transmission? Explain the following impairments: Attenuation Distortion, Phase Distortion and Noise. ## Short answer Attenuation distortion: loss varies with frequency, greatest at band edges, causing muffled speech and pulse errors, corrected by equalisers. Phase (delay) distortion: non-constant group delay leads to intersymbol interference in data, corrected by delay equalisers. Noise: thermal, intermodulation, crosstalk and impulse noise reduce channel SNR. ## Answer #### Impairments to voice channel transmission **Impairments** are the changes that a voice-frequency channel (nominally 300–3400 Hz) makes to a signal so that the received signal is not an exact copy of the transmitted one. They reduce speech quality and data accuracy. Main impairments: attenuation and attenuation distortion, phase (delay) distortion, noise, echo, crosstalk, frequency offset and jitter. #### Attenuation distortion - The loss of a channel is **not the same at all frequencies** in the band; different frequency components of the signal are attenuated by different amounts. - Caused by cable capacitance (more loss at high frequency), transformers and filters (more loss near band edges). - Specified as loss at each frequency relative to the loss at **1004 Hz (reference)**, e.g. −1 to +3 dB from 500–2500 Hz. ```text Relative loss(dB) + ┐ ┌ │ \ / │ 0 │ ───────────────────── │ └───┴────────┴──────────┴──┴── f 300 1004 3400 Hz ``` - Effect: speech sounds muffled; data pulses distorted. Corrected by **equalisers** and loading coils. #### Phase (delay) distortion - Different frequencies **travel with different delays** because the phase shift is not linear with frequency. Measured as **envelope delay distortion** – group delay $\tau = d\phi/d\omega$ relative to the minimum delay (usually near 1700–1800 Hz). - Delay rises sharply near band edges due to filters. - Little effect on speech (ear is insensitive to phase), but serious for **data**: causes intersymbol interference. Corrected by **delay equalisers**. #### Noise Unwanted signals added to the channel: - **Thermal noise** ($kTB$, white) – sets the noise floor. - **Intermodulation noise** – from non-linear devices (sum/difference frequencies). - **Crosstalk** – coupling from adjacent pairs (NEXT, FEXT). - **Impulse noise** – spikes from switching, lightning; causes burst errors in data. - **Quantisation noise** in PCM systems. Noise is measured with weighting (dBrnC, psophometric dBmp) and limits the signal-to-noise ratio of the channel (a voice channel needs SNR of about 30 dB or more). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is meant by impairments to voice channel transmission? A. Only the noise added in the exchange B. Changes made by the channel so the received signal is not an exact copy of the transmitted one C. The total length of the channel D. Failures of the subscriber telephone set Answer: B. Impairments include attenuation distortion, phase distortion, noise, echo and crosstalk, degrading speech and data. (Key point: Impairment meaning) 2. Attenuation distortion in a voice channel means: A. Different frequencies are delayed by different times B. Noise is added to the signal C. Loss differs at different frequencies, measured relative to loss at 1004 Hz D. Signal power rises with distance Answer: C. Attenuation is frequency dependent and specified relative to the 1004 Hz reference; equalisers correct it. (Key point: Attenuation distortion) 3. Why is phase (envelope delay) distortion more serious for data than for speech? A. Speech needs a higher SNR than data B. It causes intersymbol interference in data, while the ear is insensitive to phase C. It adds thermal noise to data D. It attenuates speech to inaudible levels Answer: B. Non-linear phase spreads pulses into neighbours (ISI); the ear largely ignores phase. (Key point: Phase distortion effect) 4. Which is the noise floor of a channel, being white noise with power kTB? A. Thermal noise B. Impulse noise C. Crosstalk D. Quantisation noise Answer: A. Thermal noise (kTB) is always present and sets the noise floor. (Key point: Noise in channel) ## Related in Telecommunications - [Sources of attenuation and distortion](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aattenuation-distortion-sources) — Telecommunications, chapter 2 - [Analog vs digital multiplexing; 24-channel system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aanalog-digital-mux-24-channel) — Telecommunications, chapter 3 - [TDM frame, time slot and 24-channel system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atdm-frame-24-channel) — Telecommunications, chapter 3 ## Related in other subjects - [Transmission impairments; attenuation of cable](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aattenuation-cable-numeric) — Data Communication, chapter 1 - [Transmission impairments](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Atransmission-impairments) — Data Communication, chapter 1, asked 6× - [Noise, thermal noise, delay distortion](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Anoise-types-thermal) — Data Communication, chapter 1 - [Noise vs interference vs distortion](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Anoise-interference-distortion) — Communication Systems, chapter 1