--- title: "Characteristics of speech signal" subject: "Wireless Communication" chapter: 6 key: "semester_7/wireless_communication:speech-characteristics" source: https://www.bctengineer.com/q/semester_7/wireless_communication/speech-characteristics site: BCT Engineering --- # Characteristics of speech signal Wireless Communication · Chapter 6: Speech and Channel Coding - Asked in: 2077 Chaitra, 6 marks - Video: [Characteristics of speech signals Chapter-6 part-1](https://www.youtube.com/watch?v=t5_394QJE9g) — IOE Electronics and communication engineering - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/speech-characteristics - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-6 ## Question Discuss the different characteristics of speech signal. ## Short answer Speech has 300–3400 Hz bandwidth, non-uniform amplitude pdf, high sample-to-sample correlation, non-flat spectrum with formants, voiced/unvoiced segments, short-term stationarity over 10–30 ms, long-term pitch periodicity, silence about 40–60% of conversation, and perceptual insensitivity to phase/masking. ## Answer Speech signals have special statistical and spectral properties. Speech coders use these to cut the bit rate below 64 kbps PCM. 1. **Limited bandwidth** – Speech covers about 50 Hz–10 kHz, but almost all intelligibility lies in **300–3400 Hz**, so telephone speech is sampled at 8 kHz. 2. **Non-uniform amplitude distribution** – The pdf of speech amplitude is peaked at zero (Laplacian/gamma-like): small amplitudes are much more frequent than large ones. This leads to non-uniform (μ-law, A-law) and adaptive quantization. 3. **High sample-to-sample correlation** – At 8 kHz sampling, adjacent samples are strongly correlated (coefficient often above 0.85). The future sample can be predicted from past ones, which is used by DPCM, ADPCM and LPC. 4. **Non-flat (non-white) spectrum** – Energy falls with frequency, and there are resonance peaks called **formants** due to the vocal tract. Sub-band and transform coders exploit this by allocating bits to strong bands. 5. **Voiced and unvoiced sounds** – Voiced sounds (vowels) are quasi-periodic, produced by vibrating vocal cords; the repetition rate is the **pitch** (about 80–160 Hz for men, 160–350 Hz for women). Unvoiced sounds (s, f, sh) are random, noise-like. Vocoders use a pulse or noise excitation accordingly. 6. **Short-term stationarity** – The vocal tract changes slowly, so speech is nearly stationary over 10–30 ms. Coders process speech in frames of about 20 ms. 7. **Long-term periodicity** – Successive pitch periods look similar; long-term (pitch) predictors remove this redundancy. 8. **Silence/pauses** – In a conversation each speaker is active only about 40% of the time. Voice activity detection and discontinuous transmission use this to save power and reduce interference. 9. **Perceptual properties** – The ear is fairly insensitive to phase and to noise masked by nearby strong components; perceptual weighting in CELP uses this. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is telephone speech sampled at 8 kHz? A. Intelligibility lies mostly within 300-3400 Hz B. Speech has no energy above 1 kHz C. Pitch is 8 kHz D. To match silence periods Answer: A. Nearly all intelligibility is in 300-3400 Hz, so 8 kHz sampling suffices. (Key point: Sampling from bandwidth) 2. Which pitch ranges apply to male and female voiced speech? A. 160-350 Hz men, 80-160 Hz women B. 300-3400 Hz for both C. About 80-160 Hz men and 160-350 Hz women D. 3-12 kHz for both Answer: C. Men have roughly 80-160 Hz pitch, women 160-350 Hz. (Key point: Pitch range) 3. What does high correlation between adjacent samples allow? A. Companding B. Predicting future samples from past ones (DPCM, ADPCM, LPC) C. Voice activity detection D. Frame processing Answer: B. Predictability lets coders send only prediction error or model parameters. (Key point: Correlation) 4. About how long is each speaker active in a conversation? A. About 100% B. About 90% C. About 40% D. About 5% Answer: C. Speakers are active about 40%, enabling DTX and voice activity detection. (Key point: Activity factor) 5. Over what duration is speech nearly stationary? A. 1 microsecond B. 10-30 ms C. 1-2 seconds D. Minutes Answer: B. The vocal tract changes slowly, so coders use frames of about 20 ms. 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