Television Principles

🎯Course Objectives

What you should be able to do after studying this unit.

1. Explain the physiological principle of persistence of vision and its role in producing continuous motion from discrete frames.
2. Define and quantify flicker and derive the conditions under which it disappears.
3. Analyze line merging and interlaced scanning as flicker-reduction techniques.
4. Evaluate picture quality in terms of brightness, contrast, resolution, and signal-to-noise ratio.
5. Apply the Kell factor to relate vertical resolution to the number of active scanning lines.
6. Describe the scanning process (progressive and interlaced), sync pulses, and blanking intervals.
7. Relate resolution (vertical, horizontal) to scanning standards and derive video bandwidth.
8. Compare the major TV systems: NTSC, PAL, and SECAM.

1Persistence of Vision

The human eye as an integrator — the foundation of all television.

When the human eye views a bright object and the object is suddenly removed, the sensation of vision does not vanish instantaneously. Instead, the image persists on the retina for a short time after the stimulus is withdrawn. This phenomenon is called persistence of vision.

The Physiological Facts

• The sensation produced by a flash of light persists on the retina for approximately 1/16 s (≈ 62.5 ms).
• The visual sensation also takes a finite time to build up (about 1/16 s) after the stimulus is applied.
• Persistence is a property of the photo-sensitive rods and cones in the retina and the neural processing time of the visual cortex.
• This time lag is greatest for blue and least for red light — one reason flicker is more noticeable on blue content.

Consequence for Television

If a picture is flashed on a screen, removed, and replaced with a second picture within 1/16 s, the eye cannot detect the dark interval between them — the two pictures fuse into one. Therefore, if 16 or more different pictures (frames) per second are shown in rapid succession, each slightly different from the previous, the eye perceives smooth, continuous motion instead of a series of still images.

2Flicker

When the frame rate is too low, the picture flashes and irritates the eye.

If pictures are projected at a rate below about 16 per second, the eye can perceive the dark interval between successive frames as a rapid fluctuation of brightness. This annoying effect is called flicker.

Critical Flicker Frequency (CFF)

The minimum rate at which flashes must recur so that the sensation appears continuous is the critical flicker frequency (also called critical fusion frequency or flicker fusion threshold).

• CFF increases with brightness: a bright cinema screen flickers more readily than a dim one.
• CFF is higher for peripheral vision than for central (foveal) vision.
• Exceeding the CFF alone is not enough for smooth motion — you also need enough frames per second for natural movement; for that reason film runs at 24 fps while cinema projectors use shutter tricks to flash each frame 2–3 times, raising the flicker rate to 48–72 Hz without adding motion frames.

Solutions to Flicker in TV

1. Higher frame rate — costly in bandwidth (more pictures to transmit per second).
2. Interlaced scanning (line merging) — flashes each frame in two halves (fields) at twice the frame rate. This is the solution adopted in analogue TV.
3. Long-persistence phosphors on CRT screens (store light between refreshes).

3Line Merging (Interlaced Scanning)

Doubling the flicker rate without doubling the transmission bandwidth.

Suppose a picture is made up of 525 scanning lines. If all 525 lines are scanned from top to bottom in one pass (one frame) taking 1/30 s, the flicker frequency is only 30 Hz — flicker would be objectionable. Doubling the frame rate to 60/s would double the required channel bandwidth, which is uneconomical.

The Interlacing Trick

The complete picture (frame) is transmitted in two parts called fields:

• Odd field (first field): scans lines 1, 3, 5, 7, … (every odd line).
• Even field (second field): scans lines 2, 4, 6, 8, … (every even line), interleaved between the odd lines.

The odd lines persist on the retina thanks to persistence of vision while the even lines are traced. The odd and even lines merge visually into a single complete picture — hence the term line merging. Meanwhile, the screen is refreshed at the field rate, which is twice the frame rate, eliminating flicker without any extra bandwidth.

Advantages & Disadvantages

4Picture Quality

What makes a television picture "good"?

Picture quality is a subjective judgment, but it is determined by several objective and perceptual factors:

Subjective Quality Measurement

Perceived quality is formally rated using the Mean Opinion Score (MOS) on a 5-point scale (5 = excellent, 4 = good, 3 = fair, 2 = poor, 1 = bad), or objective metrics such as PSNR (Peak Signal-to-Noise Ratio) for digital video:

5Kell Factor

Why effective resolution is less than the number of scanning lines.

With N active scanning lines, one might expect the eye to resolve N distinct black-and-white line pairs vertically. In practice, only about 70% of the lines can be resolved. The ratio of effective vertical resolution to the number of scanning lines is the Kell factor (K).

Reasons for the Reduction

• Line sampling: the scanning spot has finite size (aperture effect); small details smaller than the spot average out to grey.
• Scan-line position uncertainty: a horizontal black-white transition rarely aligns exactly with a scanning line.
• Interlace line pairing: alternate fields may not interleave perfectly (line pairing/jitter of odd and even lines).
• Display spot overlap on the CRT and eye integration further smooths the discrete samples.

Worked Example

A 625-line system has 576 active (visible) lines. Its effective vertical resolution is:

6Scanning

How an optical image is converted into an electrical signal line by line.

A television camera converts the optical image into an electrical signal by examining it in an orderly sequence. An electron beam (CRT camera tube) or read-out register (CCD/CMOS sensor) sweeps across the image from left to right along a line, then quickly jumps back to the left and steps down one line, repeating until the whole frame is covered. This is scanning. At the receiver, the picture tube scans the same pattern in exact synchronism, reconstructing the image.

Scanning Parameters (any raster system)

• Scanning spot / aperture — the small element of the picture examined at any instant; must move fast enough to cover the frame in the frame period.
• Horizontal scanning frequency: f_h = N × f_frame lines per second (N = total lines incl. blanking).
• Horizontal retrace: the fast fly-back at end of each line — hidden by a blanking pulse that cuts off the beam.
• Vertical retrace: the return from bottom to top of the frame — also blanked out, occupying several line periods.
• Synchronising pulses: transmitted within the blanking intervals (horizontal sync, vertical sync, equalising pulses) to lock the receiver's scanning exactly to the camera.

Progressive vs Interlaced Scanning

7Resolution

Vertical and horizontal resolving power of the TV image.

Vertical Resolution (R_v)

Determined by the number of active scanning lines and the Kell factor:

Horizontal Resolution (R_h)

Determined by how fast the video signal can change — i.e., by the channel bandwidth B. The maximum signal change rate limits how many alternating black/white picture elements (pixels) per line can be distinguished:

Ideal Square-Pixel Condition

For geometrically faithful pictures, the horizontal and vertical resolutions should be equal (TV aspect ratio 4:3 historically):

Worked Example (625-line, 50 Hz, 4:3 system)

• N_total = 625, frame rate 25 fps, field rate 50 Hz.
• Line frequency: f_h = 625 × 25 = 15 625 Hz.
• Active lines: 576  ⇒  R_v = 0.7 × 576 ≈ 403 TVL.
• Required horizontal resolution (4:3): R_h = 403 × 4/3 ≈ 537 picture elements/line.

8Bandwidth

Deriving the video channel bandwidth from the scanning parameters.

The highest video frequency occurs when the signal alternates as rapidly as possible — a black-white-black-white (checkerboard) pattern. Each complete black+white cycle requires the signal to go from minimum to maximum and back. Hence the highest frequency is approximately:

The standard approximate design formula used for monochrome TV:

Worked Example: 625-line, 25 fps, 4:3

Worked Example: 525-line, 30 fps, 4:3

Why Bandwidth Matters in ECE 523E

• Bandwidth determines the channel capacity and hence the number of TV channels that fit in a given transmission band (VHF/UHF spectrum planning).
• It dictates the antenna bandwidth requirements for TV transmitting and receiving antennas — a core antenna-design parameter.
• Vestigial-sideband (VSB) modulation is used in analogue TV precisely to halve the RF bandwidth while keeping low-frequency video components intact.

9TV Systems (NTSC, PAL, SECAM)

The three analogue colour television standards.

All three systems encode luminance (Y) — for compatibility with monochrome receivers — plus two colour-difference signals transmitted on a colour subcarrier within the same video bandwidth. They differ mainly in how the colour information is modulated and how phase/colour errors are handled.

Luminance & Colour-Difference Signals

✔Self-Test Questions

Attempt each question, then reveal the answer.

📚Summary of Key Formulas