Audio & Video
Audio and video systems are where most consumer electronics work happens — amplifying signals cleanly, moving them at standard levels, and displaying them. The Associate CET exam covers amplifier classes, standard A/V signal levels, display technology, and the common faults that show up in this gear.
An amplifier's class describes how much of the input waveform the active device conducts for — its conduction angle. It's the central trade-off between linearity (clean sound) and efficiency (less wasted heat).
A/V gear interconnects at agreed signal levels — knowing them tells you instantly whether a stage is working:
| Signal | Standard level |
|---|---|
| Line-level audio | Roughly 1 V peak-to-peak — the level out of DVD/Blu-ray players, satellite boxes, and preamps feeding a TV or modulator. |
| 0 dBm reference | 0 dBm = 1 milliwatt (often into 600 Ω in audio). A power reference, not a ratio. |
| Composite video | 1 V peak-to-peak into 75 Ω — the classic baseband video level (sync tip to peak white). |
| Speaker level | Much higher — volts to tens of volts, driving low-impedance (4–8 Ω) speakers with real power. |
| Display | How it works & notes |
|---|---|
| LCD | Liquid crystals twist polarized light by about 90°iAn LCD pixel sits between crossed polarizers. With no field, the liquid crystal rotates light 90° so it passes the second polarizer (bright); a voltage untwists it and blocks the light (dark). LCDs don't emit light — they need a backlight. to pass or block a backlight. Low power, but performance is temperature-sensitive — sluggish in the cold. Doesn't emit its own light. |
| LED | An array of light-emitting diodes (or an LED-backlit LCD). Bright, efficient, long-lived. "LED TVs" are usually LCDs with LED backlights. |
| Plasma | Tiny cells of ionized gas glow to form the image. Excellent contrast on large panels, but heavier and more power-hungry — now largely obsolete. |
| OLED | Organic LEDs that emit light per-pixel — true blacks (pixels switch fully off) and no backlight needed. |
| CRT | The legacy cathode-ray tube — an electron beam scanned across a phosphor screen. Bulky and high-voltage. |
Distortion
If audio is distorted, the most common cause is overdriving the input stage — too much signal level clips the waveform. Also check bias errors and a failing output stage.
Hum
A 60 Hz (or 120 Hz) hum in an amplifier most often comes from bad power-supply filter capacitors — ripple leaking through to the audio. If hum appears only on one input, suspect a bad shielded cable to that source.