Analogue Television

1. Learning Objectives

By the end of this study guide, the student should be able to:

1. Describe the complete analogue television chain from camera and microphone to display and loudspeaker.
2. Specify the monochrome composite video signal: picture levels, blanking, horizontal and vertical sync, and the timing of the 625-line/50-field system.
3. Explain how the video signal modulates the carrier (negative AM, vestigial sideband) and how the sound is carried (FM), and compute channel occupation.
4. Explain the principles and derive parameters of interlaced scanning: line and field frequencies, blanking intervals, Kell factor and video bandwidth.
5. Describe the TV transmitter (vision modulator, VSB filter, aural chain, combining network) and the superheterodyne TV receiver (tuner, IF, video detector, sync separator, deflection, AGC, AFC).
6. Apply antenna and propagation principles to analogue TV: coverage prediction, field-strength planning values, ghosting mechanisms, receiving antenna specifications.
7. Solve numerical problems on scanning frequencies, video bandwidth, channel plans, ERP/link budgets and field strength for analogue television.

2. The Analogue TV Chain — Overview

Analogue TV conveys a continuously varying voltage that represents image brightness (and, in colour sets, colour) plus sound. Every link in the chain processes that waveform:

The sound is always handled by a separate FM carrier, so picture and sound can be processed and received independently (a key difference from the single digital multiplex).

3. The Composite Video Signal (Monochrome)

3.1 Composition

• Picture (video) information: voltage between black level and white level, proportional to scene brightness; transmitted with negative modulation (white = low carrier power) so that impulse noise produces dark specks, which are far less visible than bright ones, and so that weak-sync effects (over-modulation) stay harmless.
• Blanking pulses: at blacker-than-black level, applied during line and field flyback so retrace lines are invisible.
• Synchronizing pulses: sit on top of the blanking level (sync tips = maximum carrier), precisely timed, so the receiver's scanning generators lock to the transmitter's.

3.2 Timing of the 625-Line System

3.3 Video Bandwidth

The highest baseband frequency occurs for a fine vertical chequerboard alternating black/white on every successive element along a line:

4. The TV Broadcast Channel

4.1 Vision Carrier — AM with Vestigial Sideband

• The composite video (0 – 5 MHz) amplitude-modulates the vision carrier with negative modulation.
• Full double sideband would need ≈ 10–11 MHz. One sideband is transmitted almost complete (up to ≈ 5 MHz above carrier); the other is cut off by a vestigial-sideband filter about 1.25 MHz below the carrier, leaving a controlled slope.
• The receiver uses a complementary response (IF filtering with a Nyquist slope at vision-carrier frequency) so the detected video amplitude response is flat.

4.2 Sound Carrier — FM

• Audio (50 Hz – 15 kHz) frequency-modulates the sound carrier, offset above the vision carrier (commonly +5.5 MHz, +6.0 MHz or +6.5 MHz depending on the system).
• Maximum deviation ≈ ±50 kHz (System G) with pre-emphasis (50 µs) and, in satellite/older links, additional energy-dispersal.
• FM gives the sound a capture effect and constant amplitude — hence good noise performance — and lets receivers use a simple ratio detector or PLL discriminator.

4.3 Channel Occupation (8 MHz plan example)

4.4 Band Plans

5. Synchronization and Interlace in Detail

5.1 Horizontal (Line) Sync

• Sync tip ≈ 4.7 µs wide at maximum carrier, occurring 12 µs after the previous line's sync (64 µs line period).
• Front porch (≈ 1.5 µs) before the tip lets the video settle; back porch (≈ 4.7 µs) after the tip holds black level while the flyback completes (and carries the colour burst in colour systems).

5.2 Vertical (Field) Sync — Keeping Interlace Exact

Two fields of 312.5 lines must interlace with half-line accuracy. During the field-blanking interval (≈ 25 lines) a special pulse train is sent:

• Pre-equalizing pulses: 5 (or 6) narrow pulses at twice line rate — set the field divider to the correct half-line state before sync.
• Vertical sync (broad/serrated) pulses: 5 long pulses with serrations at line rate so the horizontal oscillator keeps running during field sync.
• Post-equalizing pulses: mirror the pre-equalizing set, guaranteeing identical charge conditions for both field types — this is what preserves the exact half-line offset.

5.3 AFC and AGC

• AGC: DC voltage derived from sync-tip level keeps video amplitude constant despite fading.
• AFC: the line oscillator free-runs near 15 625 Hz and is phase-corrected each line by integrating the sync pulse position — a classic phase-locked loop teaching example.

6. The Analogue TV Transmitter

6.1 Vision Chain

1. Composite video input → clamping/DC restoration → AM modulator (usually a high-level class-B push–stage modulated PA for efficiency, or low-level modulation + linear PA).
2. VSB filter / sideband shaping (SAW filter in modern designs) with the complementary Nyquist slope.
3. Harmonic filtering, then the vision power amplifier (klystron, IOT or solid-state).

6.2 Aural (Sound) Chain

1. Audio → pre-emphasis (50 µs) → FM modulator (crystal oscillator + varactor, deviation ±50 kHz).
2. Frequency multiplication to the sound carrier frequency; small PA.

6.3 Combining and Antenna

• Vision and sound are combined in a diplexer before the feeder so one antenna radiates both without the sound PA loading the vision PA.
• The transmitting antenna is a high-gain, vertically stacked array (e.g. superturnstile for omnidirectional service, or panel arrays for shaped patterns), horizontally polarized, mounted as high as possible.
• ERP = transmitter power × antenna gain (relative to dipole). Typical UHF TV ERP: 10 kW – 100 kW for main stations.

7. The Analogue TV Receiver (Superheterodyne)

1. Tuner: RF amplifier + mixer; channel selection by local oscillator. Double conversion common (first IF 38.9 MHz System G vision IF).
2. IF strip: flat over the channel with a Nyquist slope at the vision carrier (complementing the transmitter VSB filter) and a sound trap at the sound IF to limit sound-in-vision.
3. Video detector: diode envelope detector → composite video (0–5 MHz) + the 5.5 MHz inter-carrier sound beat between vision and sound carriers.
4. Sound path: 5.5 MHz bandpass → FM limiter → discriminator → de-emphasis → audio amp. Because the beat is generated after detection, sound stays locked even if the RF drifts slightly.
5. Sync separator: clips at sync level → horizontal sync to AFC'd line oscillator; integrates → field sync to field oscillator → drives deflection coils (and EHT generation by flyback).
6. Video path: amplification, AGC keyed off sync tips → CRT cathode (brightness modulation). Colour sets add the chroma bandpass, burst gate, reference oscillator, demodulators and matrix (PAL/SECAM/NTSC per country).
7. Scanning: sawtooth currents in deflection yoke; line flyback at 15 625 Hz, field flyback at 50 Hz; EHT (≈ 25 kV for colour CRT) from the line flyback transformer.

8. Propagation and Antennas for Analogue TV (ECE 523 Context)

8.1 Coverage Model

Real planning uses ITU-R P.1546 curves with clutter corrections; analogue PAL Grade-1 planning field strengths are ≈ 64–74 dBµV/m (UHF) — much higher than digital thresholds.

8.2 Impairments Visible on the Analogue Picture

8.3 Receiving Antenna Requirements

• Horizontally polarized (Kenyan convention): cross-polarization costs 15–25 dB.
• Flat response across the 8 MHz channel (especially to 4.43 MHz + 1.5 MHz for colour).
• High front-to-back ratio (≥ 15 dB) to reject rearward echoes that cause ghosts.
• 75 Ω low-loss coax; every 1 dB of feeder loss raises the visible snow in weak-signal areas.
• Types: Yagi–Uda (high gain, narrowband), log-periodic (wideband), phased arrays for fringe areas.

9. Worked Examples

Example 1 — Line, field and frame frequencies

For a 625-line, 50 Hz system, determine f_L, f_F, line period and the time of one frame.

Example 2 — Maximum video frequency

Compute the theoretical maximum video frequency for the 625-line system (active lines 575, active line time 52 µs, aspect ratio 4:3), and the practical value with Kell factor 0.7.

Example 3 — Channel plan check

Channel E21 in an 8 MHz plan starts at 470 MHz. Locate the vision carrier, the sound carrier (System G, +5.5 MHz) and check the channel fits.

Example 4 — Ghost delay

A ghost appears 1/5 of the screen width to the right of the main image. Estimate the echo's excess path length.

Example 5 — ERP and field strength

A transmitter runs 5 kW into an antenna with 10 dBd gain. Find the ERP, EIRP, and the free-space field strength at 30 km.

Example 6 — Link budget to receiver input

For the 50 kW ERP station of Example 5 at 650 MHz, 30 km, with a 12 dBi antenna, 3 dB cable loss: find received power and C/N if the receiver noise figure is 8 dB and vision bandwidth is 5 MHz.

Example 7 — Sound IF (inter-carrier)

A receiver tunes channel E21 (vision IF 38.9 MHz after first mixer). Find the sound IF by the inter-carrier method.

10. Interactive Calculators

11. Summary of Key Points

1. Analogue TV transmits a continuous waveform: AM (negative, VSB) vision + FM sound on a common channel of 7–8 MHz.
2. The 625/50 system: f_L = 15 625 Hz, 64 µs lines, 25 frames of 2 interlaced fields; ≈ 575 active lines, ≈ 5 MHz luminance bandwidth (Kell factor).
3. Sync structure (line 4.7 µs; field with pre/post-equalizing and serrated pulses) preserves exact half-line interlace; failures show as tearing or line pairing.
4. Channel plan: vision carrier 1.25 MHz above lower edge, upper sideband to ≈ +5 MHz, sound carrier at +5.5 MHz; VSB + Nyquist-slope IF filtering gives flat video response.
5. Transmitters use high-level AM vision PAs + VSB filtering, FM sound chains and diplexed high-gain antennas; ERP set by coverage planning (height ≈ range).
6. Receivers are superheterodynes with Nyquist-slope IF strips, envelope video detection, inter-carrier 5.5 MHz sound, sync separators and AFC'd deflection.
7. Propagation: snow from low C/N, ghosts from multipath (delay = offset × 64 µs); coverage via ITU-R P.1546 field-strength curves; planning field ≈ 64–74 dBµV/m.
8. Receiving antennas: horizontal polarization, flat 8 MHz response, high F/B ratio, 75 Ω low-loss feed — the same antenna physics later demanded (more strictly) by DTV.

12. Self-Test Quiz (12 Questions)

Attempt each question, then click "Check Answers".

13. References & Further Reading

1. Gulati, R. R. — Monochrome and Colour Television, New Age International.
2. Dhake, A. M. — Television Engineering, Tata McGraw-Hill.
3. Roddy, D. & Coolen, J. — Electronic Communications, Pearson.
4. ITU-R BT.470 — Conventional analogue television systems (B/G, PAL parameters).
5. ITU-R P.1546 — Point-to-area propagation prediction for TV planning.
6. Kraus, J. D. & Marhefka, R. J. — Antennas for All Applications.
7. Communications Authority of Kenya — Analogue switch-off & digital migration records.

Prepared for ECE 523E — Antenna & Radio Wave Propagation, Department of Electrical & Communication Engineering, Masinde Muliro University of Science and Technology.