Laboratory Objectives
By the end of this virtual laboratory session, the student should be able to:
- Explain the principle of raster scanning and distinguish between progressive (sequential) and interlaced scanning used in television systems.
- Illustrate the vertical and horizontal retrace intervals, blanking pulses, and synchronising pulses in a TV waveform.
- Determine the vertical and horizontal resolution of a television picture and apply the Kell factor to practical systems.
- Derive and compute the video (picture) bandwidth of a television signal from the number of lines, frame rate, aspect ratio, and active ratio.
- Relate the TV channel bandwidth to the transmission standards (e.g. 625-line / 50-field CCIR systems and 525-line / 60-field NTSC systems) relevant to antenna and propagation engineering.
- Present experimental observations and conclusions in a well-structured laboratory report.
General Theory
1. The Television Problem
Television converts a two-dimensional, time-varying optical scene into an electrical signal suitable for transmission over a channel, and reconstructs it at the receiver. Since a single communication channel can only carry a one-dimensional function of time, the scene must be sampled in an orderly, repetitive manner. This sampling process is called scanning: a spot of light or electrons traverses the image in a fixed pattern called a raster, and the brightness of each sampled point is converted into a proportional voltage by a camera sensor (photodiode / CCD / CMOS).
2. Scanning
In the standard raster pattern the spot moves from left to right along a line, then rapidly returns to the left (the horizontal retrace) while stepping down to the next line. When the last line is completed the spot returns to the top (the vertical retrace) and the pattern repeats. Each complete pattern of lines is a frame.
- Progressive (sequential) scanning: all lines of a frame are traced in one sweep (lines 1, 2, 3, …, N). Simple, but produces visible flicker for frame rates below ~50 frames/s.
- Interlaced scanning: each frame is divided into two fields. The first (odd) field scans lines 1, 3, 5, …; the second (even) field scans lines 2, 4, 6, …. The eye integrates the two fields into one frame, so the flicker rate is doubled without doubling the transmission bandwidth. All analogue TV standards (405, 525, 625 lines) use interlacing.
Typical standards: CCIR System B/G (Kenya/Uganda/Europe) — 625 lines, 50 fields/s, 25 frames/s, video bandwidth 5 MHz, channel spacing 8 MHz, VHF/UHF bands III–V. NTSC (USA/Japan) — 525 lines, 60 fields/s, 30 frames/s, 4.2 MHz video bandwidth, 6 MHz channels.
3. Resolution
Resolution is the ability of the TV system to reproduce fine detail, measured in TV lines or equivalently as a bandwidth.
- Vertical resolution (Nv) is limited by the number of active (visible) scan lines. Because the scanning spot straddles picture elements that fall between lines, the effective resolution is reduced by the Kell factor (K ≈ 0.7):
Nv = K × Nvisible
- Horizontal resolution (Nh) depends on the video bandwidth: the highest sinusoidal frequency the channel passes determines how many brightness alternations fit across one line duration.
The aspect ratio (A = width/height, 4/3 in conventional TV, 16/9 in HDTV) links the two: to make picture elements square, the system must pass Nv × A brightness changes per picture height, i.e. per frame time.
4. Video (Picture) Bandwidth
Consider the worst-case picture: alternating black-and-white elements as fine as the resolution limit. If there are Nv elements vertically and A·Nv horizontally, each frame contains Nv × A × Nv transitions per second. Each transition requires half a cycle of the highest transmitted frequency (maximum rate = 2B transitions/s). Accounting for the fact that only a fraction of each line/field is active (active ratio α ≈ 0.84 of lines, retrace losses), the standard result is:
Worked check (CCIR 625-line system): B = 0.5 × 0.7 × 0.84 × 6252 × 25 × 1.33 ≈ 4.8 MHz (standardised at 5 MHz). The full channel is wider (8 MHz) because it must additionally carry the FM sound carrier and vestigial-sideband guard bands — a key link to the antenna/channel-planning topics of ECE 523E.
5. Relevance to Antennas & Radiowave Propagation
- The TV picture bandwidth directly sets the occupancy of the 8 MHz VHF/UHF channel whose propagation (line-of-sight, diffraction over terrain, tropospheric effects) is studied in this course.
- The 625-line raster rate fixes the line frequency fh = N × fframe = 15 625 Hz, from which harmonic interference patterns near transmitting antennas and in receiver front-ends can be analysed.
- Antenna bandwidth must accommodate the full 8 MHz channel — a design constraint for TV receiving antennas (Yagi-Uda, log-periodic) treated in ECE 523E.
Experiment 1: Raster Scanning (Progressive vs Interlaced)
Aim: To demonstrate progressive and interlaced raster scanning and to measure line and field timing parameters.
Apparatus (virtual): Raster-scan simulator, dual-trace timing display, number-of-lines selector, field/frame counter.
Theory Recap
In the simulator below, a bright spot traces the raster. A faint "phosphor persistence" trace shows the path history so that the full raster is visible while the beam moves. A line trace shows the instantaneous scanning spot amplitude (brightness vs. time) as would be seen at the output of a camera tube or the input of a picture tube.
Procedure
- Set the number of scan lines N = 8 (small, so the raster is easy to see), choose Progressive mode, and press Start Scan.
- Observe the spot moving left-to-right (active trace), the rapid return during horizontal retrace, and the return to the top during vertical retrace.
- Count the number of lines in one frame; confirm that in progressive mode all 8 lines are traced consecutively in one sweep. Record the time for one frame (frame period).
- Switch to Interlaced mode and restart. Observe that the odd field (lines 1, 3, 5, 7) is traced first, followed by the even field (lines 2, 4, 6, 8).
- Using the timing readouts, verify that the field rate = 2 × frame rate, i.e. each field takes half a frame.
- Increase N to 16 and 32 and repeat. Note how the raster appears more continuous; relate this to real TV (625 lines, interlaced, 50 fields/s → 25 frames/s).
- Sketch one frame of each raster type in your report and annotate the active trace, both retrace intervals, and the field structure.
▶ Simulation 1: Raster Scanner
Observations (Record in Your Report)
| N (lines) | Mode | Lines per field | Frame period (arb. units) | Field period (arb. units) | Observed raster appearance |
|---|---|---|---|---|---|
| 8 | Progressive | — | |||
| 8 | Interlaced | ||||
| 16 | Interlaced | ||||
| 32 | Interlaced |
Experiment 2: Picture Resolution (Vertical & Horizontal)
Aim: To investigate how the number of scanning lines and the video bandwidth limit picture resolution, and to verify the role of the Kell factor.
Apparatus (virtual): Test-pattern generator (fine wedge + stripes), variable-line raster sampler, resolution readout.
Theory Recap
The test pattern contains vertical bars whose spacing decreases toward the right (like a TV wedge). If the raster has N lines, only picture features larger than the line spacing can be reproduced vertically. Horizontally, the number of brightness alternations per line is limited by the bandwidth slider: as the bandwidth is reduced, fine detail smears into grey. The vertical resolution counter applies the Kell factor: Nv = K·Nvisible.
Procedure
- Set scan lines = 64 and bandwidth = 100% (full detail). Run the sampler and observe that all bars, including the finest, are reproduced.
- Reduce the scan lines to 32, then 16. Note the finest bar group that still appears as separate black/white stripes — this is the effective vertical resolution. Compare it with K·N.
- Restore N = 64. Reduce bandwidth in steps (100 → 50 → 25 → 12%). Observe the horizontal smearing of fine bars while coarse bars remain sharp.
- At each bandwidth setting record: (a) finest vertical-bar group resolved, (b) the computed horizontal TV lines shown by the simulator, (c) whether vertical or horizontal resolution limits first.
- Explain in your report why reducing N affects vertical detail while reducing bandwidth affects horizontal detail, and why Nv < N (Kell factor: an element lying between two scan lines is illuminated by both and averaged).
▶ Simulation 2: Resolution Test Pattern Sampler
Top: ideal test pattern. Bottom: what the TV raster actually reproduces, sampled at N lines and low-pass filtered by the bandwidth setting.
Observations
| N (lines) | Bandwidth (%) | Nv = K·N | Nh (sim.) | Finest bar group resolved? | Limiting factor |
|---|---|---|---|---|---|
| 64 | 100 | ||||
| 32 | 100 | ||||
| 16 | 100 | ||||
| 64 | 50 | ||||
| 64 | 25 | ||||
| 64 | 12 |
Experiment 3: Video Bandwidth of a Television Signal
Aim: To derive the video bandwidth from the system parameters (lines, frame rate, aspect ratio) and to verify it against the standard 625-line and 525-line TV systems.
Apparatus (virtual): TV standards calculator, waveform display with selectable bandwidth limiting, spectrum sketch panel.
Theory Recap
Rearranging the theory formula, a TV system transmitting N lines per frame at fframe frames/s with aspect ratio A and Kell factor K requires video bandwidth B = 0.5·K·α·N2·fframe·A. The waveform panel shows the worst-case video signal (alternating black/white picture elements at the resolution limit) and the effect of truncating its spectrum at the computed B. The spectrum sketch shows the picture-signal band extending from DC to B, with the sound carrier and vestigial sideband of a real 8 MHz TV channel marked for reference.
Procedure
- Load the CCIR 625-line preset. Record N = 625, fframe = 25 Hz, A = 4/3, K = 0.7, α = 0.84. Compute fh = N·fframe (line frequency) and fv (field rate = 2·fframe).
- Read the computed video bandwidth and compare with the standard value of 5 MHz. Comment on the small difference (guard margins, rounded active-line assumptions).
- Repeat with the NTSC 525-line preset (30 frames/s) and verify B ≈ 4.2 MHz.
- Vary one parameter at a time (N, fframe, A) from the 625-line baseline and record B each time. Confirm from your table that B is proportional to N2 — doubling the lines quadruples the bandwidth. Explain why HDTV (1080+ lines) needed digital compression (MPEG) to fit terrestrial channels.
- In the waveform display, observe that limiting the bandwidth to B removes the higher harmonics of the finest picture elements — the square edges round off — while picture elements larger than the resolution limit pass unaffected.
- Sketch the channel occupancy (DC–5 MHz picture, vestigial lower sideband, FM sound carrier at +6 MHz) and relate the 8 MHz channel to VHF/UHF frequency planning in Kenya (antenna topics from ECE 523E).
▶ Simulation 3: Bandwidth Calculator & Waveform
Waveform: worst-case video signal at the resolution limit. The shaded region shows the spectrum passed (0 to B). Grey trace = unlimited signal.
Observations
| System / variation | N | fframe (Hz) | A | K | B computed (MHz) | Standard B (MHz) |
|---|---|---|---|---|---|---|
| CCIR 625 (baseline) | 625 | 25 | 4:3 | 0.70 | 5.0 | |
| NTSC 525 | 525 | 30 | 4:3 | 0.70 | 4.2 | |
| 625-line, N doubled (1250) | 1250 | 25 | 4:3 | 0.70 | — | |
| 625-line, fframe doubled (50) | 625 | 50 | 4:3 | 0.70 | — | |
| 625-line, 16:9 | 625 | 25 | 16:9 | 0.70 | — | |
| Your own variation |
Guidelines for Writing the Laboratory Report
Reports must be typed, submitted as PDF, and follow the MMUST School of Engineering format. Suggested length: 6–10 pages (excluding appendices). Use the structure below.
| 1. Title Page | Course code & title (ECE 523E — Antenna & Radiowave Propagation), experiment title (Television Principles), your name, registration number, group number, date of experiment, date of submission, lecturer's name. |
| 2. Abstract | A single paragraph (100–150 words) summarising objectives, key methods, principal results (e.g. computed B for the 625-line system), and conclusions. |
| 3. Objectives | List the objectives as given in this manual, rephrased where appropriate. |
| 4. Theory | Explain scanning (progressive & interlaced), vertical/horizontal resolution, the Kell factor, and the derivation of B = 0.5KαN²fframeA. Include your own clearly-labelled diagrams of the raster and the TV channel spectrum. Cite lecture notes or a textbook. |
| 5. Procedure | Describe what you actually did, in your own words and in the past tense. Reference each simulator and setting (e.g. "Interlaced mode, N = 16 lines, maximum scan speed"). Include screenshots of each simulation at meaningful settings. |
| 6. Results & Analysis | Present the completed observation tables from Experiments 1–3. Show all sample calculations (bandwidth computation, line frequency, Kell-adjusted resolution). Plot any trends (e.g. B vs N, confirming the N² law). Compare simulated values with theoretical/standard values and compute the percentage error, stating its likely cause. |
| 7. Discussion | Answer the discussion questions: Why is interlacing used? Why does Nv < N? What physical justification does the Kell factor have? How does video bandwidth map to the 8 MHz VHF/UHF channel, and what does this imply for TV receiving-antenna bandwidth (link to ECE 523E antenna topics)? |
| 8. Conclusion | Concise statements of what was verified, in the same order as the objectives. No new material here. |
| 9. References | Numbered list in IEEE style, e.g. [1] S. O. Kasap, Optoelectronics and Photonics; [2] A. B. Carlson & P. Crilly, Communication Systems; lecture notes for ECE 523E. |
| 10. Appendix | Raw screenshots, additional parameter sweeps, and any code or worksheets used. |
Marking Guide (Indicative)
| Component | Marks |
|---|---|
| Theory understanding & quality of diagrams | 20% |
| Procedure description & evidence of simulation runs | 15% |
| Results completeness, calculations & error analysis | 30% |
| Discussion & ECE 523E linkage (antennas/propagation) | 20% |
| Presentation, structure & referencing | 15% |
References
- Carlson, A. B., & Crilly, P. (2009). Communication Systems: An Introduction to Signals and Noise in Electrical Communication (5th ed.). McGraw-Hill. (TV bandwidth derivation, Chapter on pulse/analogue transmission.)
- Gulati, R. R. (2012). Modern Television Practice: Transmission, Reception and Applications. New Age International. (625-line CCIR scanning standards.)
- Whitaker, J. C., & Benson, K. B. (Eds.). (2003). Standard Handbook of Video and Television Engineering (4th ed.). McGraw-Hill. (Resolution, Kell factor, bandwidth.)
- MMUST Department of Electrical & Communication Engineering (2026). ECE 523E: Antenna & Radiowave Propagation Lecture Notes. Masinde Muliro University of Science & Technology.
- ITU-R Recommendation BT.470-7 (2005). Conventional Television Systems. International Telecommunication Union. (System B/G 625-line parameters used in Kenya.)