Television Principles Quiz

Explanation: The photo-sensitive rods and cones of the retina, together with neural processing in the visual cortex, retain the image for about 1/16 s (≈62.5 ms). This is the physiological basis of television: flashing 16 or more frames per second makes the images fuse into continuous motion.

Explanation: Because of the finite scanning spot (aperture effect), scan-line position uncertainty, and imperfect interlace line pairing, the eye resolves only about 70% of the scanning lines. Effective vertical resolution = K × N_active with K ≈ 0.7.

Explanation: NTSC (National Television System Committee) uses 525 lines, 30 fps (60 Hz fields), 4.2 MHz video bandwidth, and encodes I and Q colour components in quadrature on a 3.58 MHz subcarrier. PAL and SECAM use 625 lines at 25 fps with a 4.43 MHz subcarrier.

Explanation: Horizontal and vertical synchronising (sync) pulses are inserted in the blanking intervals (with equalising pulses around the vertical interval). They trigger the receiver's deflection oscillators so the reconstructed raster exactly matches the camera's scanning; without them the picture tears or rolls. The colour burst (option c) is a separate reference for colour demodulation.

Explanation: Because each image persists for ~1/16 s, a new frame presented within that interval overlaps the fading previous one. The eye therefore integrates the sequence of still frames into one smoothly moving image — the fundamental illusion exploited by film and television. It does not improve brightness sensitivity or resolution.

Explanation: The critical flicker frequency (CFF, ≈16 Hz) is the minimum flash rate at which the eye perceives continuous brightness. If the screen is refreshed below this rate, the dark intervals between flashes are perceived as brightness fluctuation — flicker. Note that flicker depends on the refresh rate, which interlacing raises to the field rate without changing the frame rate.

Explanation: Thanks to persistence of vision, the first (odd-line) field has not faded from the retina when the second (even-line) field is scanned into the gaps between the odd lines. The two sets of lines therefore appear simultaneously as one complete frame — this is "line merging".

Explanation: Random noise superimposed on the video waveform appears as luminous "snow" because noise amplitude modulates the beam current across the whole raster simultaneously. Acceptable viewing requires a weighted video SNR of about 40–45 dB or better. Losing the colour burst (option d) would affect colour only, not produce snow.

Explanation: Apply R_v = K × N_active = 0.7 × 576 = 403.2 ≈ 403 TV lines (≈ 200 resolvable black-white line pairs vertically). The 625 total and 576 active figures must not be confused — only active lines contribute to the picture.

Explanation: Every line (including those lost in vertical blanking) must be traced once per frame: f_h = N × f_frame = 625 × 25 = 15 625 Hz. This is the standard line frequency of the 625/50 system and is why it is robust against 50 Hz mains hum — hum bars remain stationary on the screen.

Explanation: B = 0.7 × 525² × 30 × (4/3) / 2 = 0.7 × 275 625 × 30 × 0.667 ≈ 3.86 MHz — standardised at 4.2 MHz in NTSC (the extra margin accommodates the colour subcarrier sidebands). Compare with the 625-line system, which gives ≈ 4.5–5 MHz.

Explanation: Y = 0.299(0.8) + 0.587(0.4) + 0.114(0.2) = 0.239 + 0.235 + 0.023 = 0.497 ≈ 0.46–0.50 V depending on rounding. Green dominates the luminance signal (0.587 weight) — the eye is most sensitive to green — which is why noise in the G channel is most visible.

Explanation: From B ≈ K·N²·f_frame·AR/2, bandwidth is proportional to the square of the number of lines (doubling lines doubles both vertical resolution AND line frequency). Doubling N therefore multiplies the bandwidth by four (≈4.5 MHz → ≈18 MHz). This square-law relationship is why increasing picture resolution was historically so expensive in spectrum — a key argument for digital compression.

Explanation: A strip of horizontal lines occupies, say, only odd-numbered lines of the raster. It is therefore written only during the odd field and goes unrefreshed for a whole field period — effectively flashing at 25 Hz instead of 50 Hz, below the CFF for fine detail. This interline flicker ("twitter") and the similar venetian-blind effect on camera pans are inherent interlace defects — one reason modern digital TV prefers progressive scanning.

Explanation: Flicker is a brightness-refresh phenomenon (CFF ≈ 16 Hz+), whereas motion smoothness is a temporal-sampling phenomenon (frame rate). Interlacing decouples the two: each frame's two fields refresh the screen 20 ms apart, giving 50 flashes/s (above CFF) while motion is still sampled at only 25 fps. No extra bandwidth is used because a field carries only half the lines.

Explanation: Television picture quality in a noise-limited system is governed by the received SNR (weighted video SNR should exceed ~40–45 dB). In fringe areas, nighttime tropospheric ducting increases co-channel interference, and multipath changes can produce frequency-selective nulls across the wide TV channel, degrading the carrier-to-noise ratio — hence snow. This links directly to ECE 523E propagation topics: coverage prediction and interference are propagation-limited, not transmitter-power-limited, phenomena.

Explanation (evaluation): NTSC encodes both colour components in quadrature; phase distortion rotates the chrominance vector and visibly shifts hue, requiring a manual tint control — poor for phase-distorted paths. SECAM's FM is phase-immune, but sequential R-Y/B-Y line transmission needs a glass/semiconductor delay-line memory and complicates mixing standards. PAL alternates the V-component phase by 180° each line so that averaging adjacent lines cancels phase errors automatically, while still using one quadrature subcarrier and moderate bandwidth (5 MHz). Given the stated constraint, PAL is the most defensible engineering choice. (Option d is wrong: PAL still needs a one-line memory for the averaging, and does not use the least bandwidth.)

Explanation (evaluation): Bandwidth B ≈ K·N²·f·AR/2 is proportional to frame rate. Proposal A at 50 fps needs roughly twice the bandwidth of B at 25 fps (≈9 MHz vs ≈4.5 MHz). Interlacing gives B the same 625-line spatial resolution and a 50 Hz refresh (no flicker) within that half bandwidth — the engineering trade-off accepted worldwide for analogue TV. The honest cost is interline twitter and reduced vertical resolution on moving detail (interlace artefacts), which is why proposal A's progressive approach is preferred where bandwidth is affordable (computer displays, modern digital HDTV). Kell factor is unchanged by interlacing, ruling out (d).

Explanation (evaluation): Quality is ultimately perceptual, so subjective measurement (MOS ≥ 4.5 "excellent" on the 5-point ITU-R scale, averaged over a statistically meaningful panel) is the authoritative criterion. Objective metrics like PSNR corroborate it: PSNR > 40 dB corresponds to imperceptible or near-imperceptible distortion, whereas ~28 dB shows clearly visible artefacts. Bitrate and chip generation alone are unreliable predictors because content difficulty (motion, texture) varies — the same bitrate can yield very different quality. The convergent subjective + objective evidence in (b) is therefore the strongest justification.

Explanation (evaluation): An 8 MHz-wide channel is frequency-selective in a multipath environment: echoes arriving with different delays produce different amplitude/phase responses at the vision carrier, colour subcarrier and sound carrier, visible as ghosts and colour errors. A broadband antenna (e.g., log-periodic or bow-tie array) must present flat gain/VSWR across the whole 8 MHz. Height increases line-of-sight field strength (C/N — remember picture quality is SNR-limited), and directivity discriminates against off-axis reflections. AGC only corrects overall level, not frequency-selective distortion, ruling out (d); loops and narrowband antennas cannot cover the channel flatly, ruling out (a) and (c).