--- title: "Waveform/voice coding and GSM codec" subject: "Wireless Communication" chapter: 6 key: "semester_7/wireless_communication:waveform-voice-coding-gsm-codec" source: https://www.bctengineer.com/q/semester_7/wireless_communication/waveform-voice-coding-gsm-codec site: BCT Engineering --- # Waveform/voice coding and GSM codec Wireless Communication · Chapter 6: Speech and Channel Coding - Asked in: 2074 Magh, 4+2+2 marks - Video: [Speech coding](https://www.youtube.com/watch?v=QYcU8cfwR2U) — PiEdX - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/waveform-voice-coding-gsm-codec - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-6 ## Question Describe waveform and voice coding techniques. Mention characteristics of speech. Draw a suitable diagram of GSM CODEC. ## Short answer Waveform coders reproduce samples at 16–64 kbps; PCM is 64 kbps and ADPCM 32 kbps. Vocoders send model parameters at 1.2–4.8 kbps; hybrids like CELP/RPE-LTP work at 4–16 kbps. Speech is 300–3400 Hz, quasi-stationary, with silence. GSM full-rate RPE-LTP codes 260 bits per 20 ms frame at 13 kbps. ## Answer #### Waveform coding and voice (source) coding techniques **Waveform coders** try to reproduce the time waveform of speech sample by sample. They work for any signal (speech, music, tones), are robust, but need higher bit rates (16–64 kbps). - *Time domain:* PCM (64 kbps, μ/A-law companding), DPCM, ADPCM (32 kbps), delta modulation and CVSD. - *Frequency domain:* sub-band coding and adaptive transform coding, which give more bits to the important bands. **Voice coders (vocoders / source coders)** do not copy the waveform. They model speech production (excitation + vocal tract filter) and send only model parameters: spectral shape, pitch, voiced/unvoiced flag and gain. Bit rates are very low (1.2–4.8 kbps) but quality is synthetic. - Types: channel, formant, cepstrum, voice-excited and **linear predictive (LPC)** vocoders. **Hybrid coders** (multi-pulse LPC, RELP, **CELP**, RPE-LTP) combine both: LPC model plus a coded excitation chosen by analysis-by-synthesis, giving good quality at 4–16 kbps. These are used in cellular systems. | Point | Waveform coder | Vocoder | |---|---|---| | Sends | Waveform samples | Model parameters | | Bit rate | 16–64 kbps | 1.2–4.8 kbps | | Quality | High, natural | Synthetic | | Complexity | Low | High | #### Characteristics of speech - Band 300–3400 Hz; non-uniform amplitude pdf; strong sample correlation. - Non-flat spectrum with formants; voiced (pitch 80–350 Hz) and unvoiced sounds. - Quasi-stationary over 10–30 ms; about 40–60% silence. #### GSM CODEC GSM full-rate uses the **RPE-LTP** (Regular Pulse Excited – Long Term Prediction) coder at **13 kbps** (260 bits per 20 ms frame). ```text speech 8 kHz, 13-bit | [Pre-processing] (offset removal, pre-emphasis) | [STP: LPC analysis, 8 coefficients] --> LAR (36 bits) | [Short-term analysis filter] -> residual | [LTP: pitch lag & gain] --------------> (4 x 9 bits) | [RPE grid selection & coding] --------> (4 x 47 bits) | MUX: 36 + 36 + 188 = 260 bits / 20 ms = 13 kbps ``` The decoder reverses this: RPE decoding → LTP synthesis → short-term LPC synthesis filter → post-processing → speech. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which statement about waveform coders is right? A. They send only model parameters B. They reproduce the waveform sample by sample at 16-64 kbps C. They work only for speech D. They have synthetic quality Answer: B. Waveform coders like PCM and ADPCM are robust, natural but need higher rates. (Key point: Waveform coders) 2. What do hybrid coders such as CELP combine? A. PCM and delta modulation B. Sub-band and cepstrum C. Channel and formant vocoders D. An LPC model with a coded excitation chosen by analysis-by-synthesis Answer: D. Hybrids give good quality at 4-16 kbps. (Key point: Hybrid coders) 3. Which coder and bit rate does the GSM full-rate codec use? A. LPC-10 at 2.4 kbps B. ADPCM at 32 kbps C. PCM at 64 kbps D. RPE-LTP at 13 kbps Answer: D. RPE-LTP gives 260 bits per 20 ms = 13 kbps. (Key point: GSM coder) 4. How is the 260-bit frame made up? A. 36 + 36 + 188 B. 36 + 47 + 100 C. 64 + 64 + 132 D. 20 + 20 + 220 Answer: A. 36 bits LAR, 4 x 9 = 36 LTP, 4 x 47 = 188 RPE bits sum to 260. (Key point: GSM bit budget) ## Related in Wireless Communication - [Need for speech coding; vocoder concept](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspeech-coding-need-vocoder) — Wireless Communication, chapter 6, asked 2× - [GSM signal processing (speech to radio)](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-signal-processing) — Wireless Communication, chapter 8, asked 2× - [Source coders used in speech coding](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspeech-source-coders) — Wireless Communication, chapter 6 - [Speech characteristics and linear predictive coder](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspeech-lpc) — Wireless Communication, chapter 6, asked 5× - [Speech characteristics in coder design](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspeech-characteristics-coder-design) — Wireless Communication, chapter 6, asked 3× - [Speech characteristics; formant and channel vocoder](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspeech-formant-channel-vocoder) — Wireless Communication, chapter 6 ## Related in other subjects - [Dataword vs codeword; error coding techniques](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Adataword-codeword-error-coding) — Data Communication, chapter 8 - [List digital-to-digital line coding techniques](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aline-coding-techniques-list) — Data Communication, chapter 5 - [Mass haul diagram: characteristics and haul terms](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Amass-haul-diagram-characteristics) — Transportation Engineering II, chapter 3, asked 6× - [Compact data logger characteristics and block diagram](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Acompact-dl-block) — Instrumentation II, chapter 5