--- title: "Analog vs digital multiplexing; 24-channel system" subject: "Telecommunications" chapter: 3 key: "semester_8/telecommunications:analog-digital-mux-24-channel" source: https://www.bctengineer.com/q/semester_8/telecommunications/analog-digital-mux-24-channel site: BCT Engineering --- # Analog vs digital multiplexing; 24-channel system Telecommunications · Chapter 3: Signal Multiplexing - Asked in: 2080 Bhadra, 3+5 marks - Video: [DS1 (T1) Fundamentals](https://www.youtube.com/watch?v=eKMjMFbxx4E) — VeEX Inc. - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/analog-digital-mux-24-channel - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-3 ## Question Difference between Analog and Digital Multiplexing. Explain the mechanism involved in American 24 voice channel multiplex system. ## Short answer Analog multiplexing (FDM) separates channels in frequency, while digital multiplexing (TDM) separates them in time using sampled and coded signals. The American T1 system band‑limits 24 voice channels, samples each at 8 kHz, compresses with µ‑law to 8‑bit PCM, adds one framing bit per 125 µs frame, and transmits at 1.544 Mbps using bipolar AMI line coding. ## Answer #### Analog versus digital multiplexing | Point | Analog multiplexing (FDM) | Digital multiplexing (TDM/PCM) | |---|---|---| | Principle | Channels separated in frequency | Channels separated in time slots | | Signal | Continuous analogue | Sampled, quantised, coded bits | | Equipment | Modulators, band-pass filters, oscillators | Sampler, coder (codec), clock, digital logic | | Noise | Accumulates over every repeater | Regenerated at each repeater; no accumulation | | Crosstalk | From filter overlap, non-linearity | Very low | | Synchronisation | Not needed | Needed (frame and bit sync) | | Cost/size | Bulky filters | Cheap, IC-based | | Example | 12-channel group, 60-channel supergroup | T1 (24 ch), E1 (30 ch) | #### American 24-voice-channel PCM multiplex (T1 / DS1) **Mechanism:** 1. **Filtering:** each voice signal is band-limited to 300–3400 Hz by a low-pass filter. 2. **Sampling:** each channel is sampled at **8000 samples/s** (> 2 × 3.4 kHz) by a PAM gate controlled by the commutator; samples of the 24 channels are interleaved in time. 3. **Compression and coding:** samples are compressed by **µ-law** ($\mu = 255$) and coded into **8-bit** words by a shared codec (256 levels). 4. **Framing:** the 24 words (192 bits) plus **one framing bit** form a frame of 193 bits, repeated every 125 µs. ```text 1 bit 8 bits 8 bits 8 bits +-----+-------+-------+-----+--------+ | F | Ch1 | Ch2 | ... | Ch24 | +-----+-------+-------+-----+--------+ <--------- 193 bits in 125 us --------> F = framing bit ``` $$ \begin{aligned} \text{Bits per frame} &= 24 \times 8 + 1 = 193\ \text{bits} \\ \text{Frame rate} &= 8000\ \text{frames/s}\ (T = 125\ \mu\text{s}) \\ R_{T1} &= 193 \times 8000 = 1.544\ \text{Mbps} \end{aligned} $$ 5. **Superframe and signalling:** 12 frames form a superframe (D4); the F-bits form the pattern 100011011100 for frame and superframe alignment. In frames 6 and 12 the LSB of every channel is "robbed" to carry signalling bits A and B (on-hook/off-hook, dialling). 6. **Line coding:** the stream is sent in bipolar AMI (later B8ZS) on twisted pair, with regenerative repeaters about every 6000 ft (1.8 km). 7. **Receiver:** the decoder finds frame alignment from the F-bits, distributes each 8-bit word to its channel, decodes and expands it, and a low-pass filter recovers the voice. ```text ch1 -[LPF]-\ /-[LPF]- ch1 ch2 -[LPF]-- sampler -> encoder -> - decoder ... ch24-[LPF]-/ (8 kHz) (8 bit, u-law) \-[LPF]- ch24 + F bit -> AMI line 1.544 Mbps ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does noise behave in digital multiplexing compared with analog FDM? A. It accumulates at every repeater in digital systems B. It is regenerated at each repeater, so it does not accumulate C. Digital has more crosstalk D. There is no difference Answer: B. Digital repeaters regenerate pulses, while analog repeaters amplify noise along with signal. (Key point: Noise behaviour) 2. Why is each voice channel sampled at 8000 samples/s in the 24-channel system? A. To fit 24 channels in 1 MHz B. To give 193 samples per frame C. Because voice is band-limited to 300-3400 Hz and the rate must exceed twice 3.4 kHz D. Because 8 bits are used Answer: C. Nyquist requires fs > 2 x 3.4 kHz; 8 kHz is the standard. (Key point: Sampling rate) 3. How is the 193-bit frame formed? A. 24 x 8 = 192 bits plus one framing bit B. 32 x 8 bits C. 24 x 8 bits plus 8 framing bits D. 24 x 7 bits plus 25 bits of signalling Answer: A. 24 words of 8 bits plus one F bit per 125 us gives 1.544 Mbps. (Key point: Frame composition) 4. What does the receiver do first with the incoming stream? A. Applies mu-law compression B. Samples it at 8 kHz C. Adds a framing bit D. Finds frame alignment from the F-bits, then distributes each 8-bit word to its channel Answer: D. Frame sync lets words be routed to the right channel, then decoded, expanded and low-pass filtered. (Key point: Receiver action) ## Related in Telecommunications - [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 - [Digital carrier system and multiplexing techniques](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adigital-carrier-multiplexing) — Telecommunications, chapter 3 - [Impairments to voice channel transmission](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Avoice-channel-impairments) — Telecommunications, chapter 2 - [North American T1 vs European E1 hierarchy](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3At1-e1-hierarchy-compare) — Telecommunications, chapter 3, asked 2× ## Related in other subjects - [Digital transmission system; analog vs digital](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aanalog-digital-transmission-compare) — Instrumentation II, chapter 5 - [Duplex digital communication system vs analog](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adigital-comm-duplex-vs-analog) — Communication Systems, chapter 1 - [Difference between analog and digital signals](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Aanalog-vs-digital-signals) — Digital Logic, chapter 1 - [Short note: Analog vs digital mux hierarchy](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aanalog-vs-digital-mux-hierarchy) — Data Communication, chapter 6