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. It also covers home theater components, microphones, loudspeakers, recording and playback, alarm interfacing, signal conflicts and fault isolation, and the output levels to expect.
On the exam from this page
- Class AB conducts for more than 180° but less than 360°
- the standard amplitude for audio/video output feeding a TV, modulator, or similar input is about 1 volt peak-to-peak
- liquid crystals rotate light about 90° through polarizers, and a key disadvantage is sensitivity to temperature
- a condenser mic needs power, normally 48 V phantom power on XLR pins 2 and 3
- two equal speakers in parallel halve the impedance (two 8 Ω speakers make 4 Ω)
- if the fault moves with the swap, it is upstream of that point
Amplifier classes
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).
- Class A — 360°
- Class B — 180°
- Class C — <180°
more conduction = more linear · less conduction = more efficient
On the exam
Exam staple: Class AB conducts for more than 180° but less than 360° — the answer to a classic question. It's the standard audio output stage because it avoids Class B's crossover distortion without Class A's waste.Standard A/V signal levels
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. That is consumer gear (−10 dBV ≈ 0.9 V p-p); professional +4 dBu line level is higher, about 3.5 V p-p. |
| 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. |
On the exam
Exam point: the standard amplitude for audio/video output feeding a TV, modulator, or similar input is about 1 volt peak-to-peak. Composite video is 1 V p-p into 75 Ω.Display technology
| Display | How it works & notes |
|---|---|
| LCD | Liquid crystals twist polarized light by about 90° to pass or block light from a backlight (or, in a reflective LCD, room light). 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. |
On the exam
LCD specifics the exam tests: liquid crystals rotate light about 90° through polarizers, and a key disadvantage is sensitivity to temperature. An LCD is a low-current display that makes no light of its own: a transmissive LCD (TVs, monitors) needs a backlight, while a reflective one (watches, calculators, meters) uses room light.Common A/V faults
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
The pitch of a hum is a clue. A 120 Hz hum most often comes from bad power-supply filter capacitors — full-wave rectifier ripple leaking through to the audio. A 60 Hz hum points elsewhere: a ground loop, induced pickup or poor shielding, or a supply that has dropped to half-wave (an open rectifier diode). If hum appears only on one input, suspect a bad shielded cable to that source.
Good sound and distorted sound. Competency 17.6 asks you to tell good sound from distorted sound and then to give the electronic and the acoustical reasons for each. Good sound is a faithful copy of the source: nothing added that was not in it (distortion adds new frequencies, harmonics measured as THD), no hum or hiss, and nothing that rattles or buzzes. The electronic causes are in the two cards above and in the distortion table of the Amplifiers guide (section 9): clipping from overdrive, bias errors, crossover distortion, a failing output stage and supply ripple. The acoustical causes lie in the speaker, its cabinet and the room:
| Where | What you hear | Acoustical cause |
|---|---|---|
| Voice coil | A buzz or rasp, worst on loud low notes | The coil rubbing in the magnetic gap, or its former bottoming on the back plate when the cone moves too far (Klippel). Section 7 shows where these parts sit. |
| Cone, surround and leads | Buzzing, rattling or slapping | Buzzing parts, loose particles, lead wires beating against the cone, or the surround reaching its hard limit (Klippel) |
| Cabinet | A rushing air noise | Air forced through leaks in the enclosure (Klippel) |
| Room | Boomy or thin bass that changes from seat to seat | Room modes, the standing waves of the room at low frequencies: Toole measured a 16 dB difference at 36 Hz between the center and side seats of one room. Moving the subwoofer or the listener, two subwoofers on opposite sides of the front of the room and absorption help; equalize only with careful measurement, and never try to fill a dip (Toole, Harman). |
Speaker defects such as a rubbing coil make short, impulsive sounds with little energy spread over a wide band, so an ordinary THD reading is not a sensitive test for them (Klippel). Listen as well as measure, and if the sound changes when you move the speaker or your seat, suspect the room before the electronics.
Home entertainment systems
Competency 17.1 asks you to explain the major components of the most common home entertainment products. A home theater is a chain: sources make the signals, an A/V receiver selects, decodes and amplifies them, and the display and speakers turn them into picture and sound.
| Component | What it does |
|---|---|
| Sources | Blu-ray and DVD players, cable or satellite boxes, recorders and game consoles. Each puts out audio and video at the levels in section 2, or as digital audio and video over HDMI. |
| A/V receiver | The hub. It connects and switches the audio and video sources (its preamplifier section selects the source), includes a radio tuner, decodes surround-sound formats, holds at least five channels of amplification for the speakers, and provides the user interface: remote, front-panel display and on-screen menus (Briere and Hurley, Home Theater For Dummies). |
| Display | A TV or projector. The video sources feed the receiver, which feeds the display, so the picture follows the selected source (section 3 covers the display types). |
| Speakers | A 5.1 system has five main channels, each with its own amplifier, plus the “.1” low-frequency effects channel, usually sent to a subwoofer with its own amplifier (Wikipedia, AV receiver). Section 7 covers how a speaker is built. |
The preamplifier and line amplifier inside this chain are explained in the Amplifiers guide (section 7); the levels at each connection are in section 11.
On the exam
The receiver is the hub: an A/V receiver switches the sources, decodes surround sound and amplifies at least five speaker channels; the sources feed it, and it feeds the display and the speakers.Microphones
Competency 17.2 asks you to describe how microphones are used. Every microphone is a transducer that turns sound into an electrical signal, and the kind of transducer decides whether it needs power (Shure).
| Type | How it works | Power |
|---|---|---|
| Dynamic (moving coil) | Sound moves a diaphragm with a voice coil attached; the coil moves in a small magnet's field, which generates the signal in the wire (Shure). | None. It makes its own signal. |
| Condenser | A thin diaphragm sits in front of an electrically charged back plate. Vibration changes the spacing between them, which creates the signal (Shure). | Needed: a battery in the microphone or phantom power from the mixer or recorder (Shure). |
| Ribbon | A ribbon element, which is naturally figure-of-eight: it picks up front and rear (Shure). | None for a passive ribbon; ribbons with built-in electronics use phantom power. An unbalanced ribbon can be damaged by phantom power (DPA). |
Phantom power is the standard way to power a professional condenser microphone through its balanced XLR cable: pins 2 and 3 both carry +48 V ± 4 V DC and pin 1 is 0 V, so a balanced dynamic microphone on the same input is not affected by it (DPA, IEC 61938). The standard allows 7 mA of rated current and 10 mA at most. To check it, meter pin 1 to pin 2 and pin 1 to pin 3: each should read 48 V DC with no microphone connected, and pin 2 to pin 3 should read 0 V (DPA).
Pickup pattern. A microphone's polar pattern is how its sensitivity changes with direction (Shure):
| Pattern | Pickup and use |
|---|---|
| Omnidirectional | Equally sensitive from all angles: an open, natural sound, but no rejection of room noise or monitor feedback. |
| Cardioid | The most common directional pattern. Least sensitive directly behind (180° off axis), so a stage monitor placed there gives high gain before feedback; it picks up about two-thirds less ambient noise than an omni. |
| Supercardioid and hypercardioid | Narrower pickup (about 115° and 105°), least sensitive at 125° and 110°, with some pickup from directly behind. |
| Bidirectional (figure-of-eight) | Picks up front and rear equally, about 90° each way, and rejects the sides. |
Directional microphones also show the proximity effect: close to the source they boost the bass, so a singer working close sounds warmer; backing off or rolling off the bass corrects it (Shure). A microphone's output is only millivolts (−60 to −40 dBu), so it always feeds a preamplifier first (Interfacing, section 5).
Trap
Phantom power is not harmless to every microphone: a dynamic microphone on a balanced input ignores it, but an unbalanced microphone or cable, such as a sensitive ribbon microphone wired unbalanced, can be damaged. Never connect unbalanced microphones or cables to a phantom-powered input (DPA).On the exam
Microphones to know: a dynamic mic generates its own signal with a coil moving in a magnet's field and needs no power; a condenser mic needs power, normally 48 V phantom power on XLR pins 2 and 3; a cardioid mic rejects sound best from directly behind (180°).Loudspeakers: construction and precautions
Competency 17.3 asks you to explain speaker construction and precautions. A moving-coil loudspeaker works like a dynamic microphone in reverse: audio current in the voice coil makes a magnetic field that pushes against a permanent magnet, which moves the cone back and forth to make sound waves (Eminence).
| Part | What it does |
|---|---|
| Voice coil | The electromagnet: copper or copper-clad aluminum wire wound on a former chosen for its heat handling, and glued to the cone. It sits in the magnetic gap. |
| Magnet and plates | A ferrite, neodymium or alnico magnet sandwiched between the top plate and a back plate with a center core; the narrow space between magnet and core is the gap. |
| Cone and dust cap | The cone, usually paper, moves the air; the dust cap keeps dirt out of the gap. |
| Spider and surround | The suspension: together they bring the cone back to its resting position. |
| Basket | The frame, cast aluminum or stamped steel, that holds it all and mounts in the enclosure. |
Precautions. The coil's wire has resistance, which with its other electrical properties makes the speaker's impedance, and the amplifier relies on it to keep from over-working. The speakers on an amplifier output must not total less than the amplifier's rated minimum impedance, and they must be rated for the power the amplifier delivers at that impedance. Two equal speakers in parallel (+ to + and − to −) halve the impedance (AVSL). AVSL's example: an amplifier rated 2 × 100 W at its 4 Ω minimum gives only about 70 W into one 8 Ω speaker, so each speaker needs at least 70 W handling; a pair of 8 Ω speakers in parallel makes 4 Ω, takes the full 100 W and shares it, 50 W each. Distributed 70 V systems are different again: each speaker needs its own line transformer (section 11).
Worked example
Why a low-impedance load overloads the amplifier: two 8 Ω speakers in parallel make 8 × 8 / (8 + 8) = 4 Ω; in series they make 8 + 8 = 16 Ω. An amplifier putting 100 W into 8 Ω has V = √(100 × 8) ≈ 28.28 V RMS on its output. The same voltage across 4 Ω asks for P = V² / R = 28.28² / 4 ≈ 200 W, twice the power and twice the current, which is why going below the rated minimum overloads the output stage.On the exam
Speaker precautions: never load an amplifier below its rated minimum impedance; two equal speakers in parallel halve the impedance (two 8 Ω speakers make 4 Ω), so each added speaker in parallel asks the amplifier for more current.Recording and playback
Competency 17.4 asks you to explain basic recording and playback technology. Three methods cover nearly all of it: magnetic tape, optical discs and digital audio.
Magnetic tape. The heads are the transducers. Each is a C-shaped core of iron or ferrite wound with a coil, and recording happens at the head gap. The record head turns the signal into a magnetic field on the tape; the reproduce (playback) head turns the field back into a signal and has more turns, because the signal on tape is weak. Along the tape path the erase head comes first, then record, then reproduce, so the tape is erased before recording and can be monitored just after. The erase head saturates the tape with a high-frequency signal, or in some audio recorders with DC. A high-frequency AC bias added to the signal gives a stronger output, a better signal-to-noise ratio and less hiss; too much bias erases the high frequencies and too little gives distortion. The head gap and the tape speed set the highest frequency a recorder can handle, and poor tape-to-head contact loses the highs first (NEETS Module 23).
Optical discs. A laser diode reads microscopic pits in the disc. CDs use 780 nm infrared, DVDs 650 nm red and Blu-ray 405 nm blue-violet with a 0.85 numerical-aperture lens: the shorter wavelength and higher aperture focus a smaller spot, so more data fits in the same area (Kumar and Krishnaiah, IJCEA).
Digital audio. Recording starts with an analog-to-digital converter (Interfacing, section 1) and playback ends with a digital-to-analog converter. CD audio is two channels of 16-bit samples taken 44100 times a second (the Red Book standard); its Nyquist limit is 22.05 kHz, and the specified 20 kHz upper limit leaves room for the anti-aliasing filter (Wikipedia, Compact Disc Digital Audio).
Worked example
The CD's bit rate: 44100 samples a second × 16 bits × 2 channels = 1411200 bits a second, about 1411.2 kbit/s. Half the sample rate, 44100 / 2 = 22050 Hz, is the highest frequency the samples can represent.On the exam
Recording basics: a tape recorder's heads run erase, record, then reproduce in the direction of tape travel; digital recording starts with an analog-to-digital converter; a CD stores 16-bit samples taken 44100 times a second on each of two channels.Interfacing with alarm and security systems
Competency 17.5 asks you to explain how alarm and security systems may be interfaced with entertainment and information products. The usual link is a relay. In public buildings a sound system often must be muted during a fire alarm: the fire alarm installer gives the sound system installer a set of dry relay contacts, normally open or normally closed, that start the muting sequence the project specification defines. One method interrupts the audio between the mixer or processor output and the power amplifier inputs with a line-level switch that avoids pops and ticks; the same switch can select a line-level feed from the fire alarm system instead (RDL). The same relay method lets a home alarm panel mute the A/V system so the alarm is heard.
Video from security cameras often ends up on the same displays. Surveillance systems can have large ground-voltage differences between cameras and monitors, and the resulting ground loops show as 60 Hz hum bars in the picture; a 75 Ω video isolator in the cable prevents them (Jensen ISO-MAX VB-1BB). Isolation between systems is also what an opto-isolator gives (Optical, section 12).
Safety
A fire alarm is a life-safety system: get the approval of the Authority Having Jurisdiction (the fire marshal) before installing anything that interfaces with a required fire alarm system. Requirements vary between jurisdictions (RDL).On the exam
Alarm interfacing: the alarm panel gives the sound system a set of dry relay contacts, and the sound system mutes, or switches to the alarm's audio feed, when they change state.Signal conflicts and isolating a fault
Competency 17.7 asks you to explain how signals may conflict and the symptoms that conflict may produce, and 17.8 how to isolate faults between separate pieces of equipment. The Interfacing guide (section 8) lists the general symptoms; these are the ones you meet in A/V systems.
| Symptom | Likely conflict |
|---|---|
| Hum or buzz in the sound | A ground loop between units (Interfacing, section 7); a 120 Hz hum on every input points instead to supply filter capacitors (section 4). |
| Dark bars rolling through the picture | 60 Hz hum bars from a ground loop on a video cable (Jensen). |
| Loud and distorted, or almost silent | A level mismatch: line level into a microphone input, or a microphone into a line input (Shure). |
| Reflections on a video cable | An impedance mismatch in what should be a 75 Ω path (section 2; Cabling). |
| A 70 V system goes dead when a speaker is added | An ordinary low-impedance speaker connected without its line transformer (ProSoundWeb). |
Isolating the fault. With several units in a chain, work by elimination, as in the half-split method (Work Procedures, section 3). Disconnect interconnects one at a time: if a hum stops when one cable comes off, the loop runs through that path. Substitute a known-good source, cable or unit and listen again. Swap left and right at one connection: a fault that moves to the other side is upstream of the swap, and one that stays put is downstream of it.
Worked example
The fault that follows the swap: the left speaker is silent. Swap the left and right speaker leads at the amplifier's outputs. If the right speaker now goes silent, the fault moved with the amplifier channel, so it is in the amplifier or before it; swap the inputs to the amplifier next to tell the two apart. If the left speaker stays silent, the fault is in that speaker or its wire.On the exam
Isolating a fault between units: substitute a known-good source, cable or unit, or swap left and right at a connection; if the fault moves with the swap, it is upstream of that point.Output levels to expect
Competency 17.9 asks you to list the signal or voltage levels to expect at the output circuits of audio and video equipment. Speaker outputs follow from the power: V = √(P × R).
| Output | Level to expect |
|---|---|
| Microphone | −60 to −40 dBu, about 0.775 to 7.75 mV (Shure). |
| Consumer line output | −10 dBV ≈ 0.316 V RMS, about 0.9 V peak-to-peak (section 2). |
| Professional line output | +4 dBu ≈ 1.23 V RMS, about 3.5 V peak-to-peak. |
| Composite video | 1 V peak-to-peak into 75 Ω. |
| Speaker output | Volts to tens of volts into 4 to 8 Ω: 1 W into 8 Ω is √8 ≈ 2.83 V RMS, and 100 W into 8 Ω is √800 ≈ 28.28 V RMS. |
| 70 V distributed line | A constant-voltage amplifier output rated 70.7 V, used where runs are hundreds of feet. Each speaker has a transformer whose taps (1, 2, 5, 10 W) set how much power it takes, and the taps must not add up to more than the amplifier can give (ProSoundWeb). |
Worked example
Loading a 70 V line: a transformer set to its 10 W tap presents R = V² / P = 70.7² / 10 ≈ 500 Ω to the line, and a 5 W tap about 1000 Ω. Eight speakers on 5 W taps and four on 10 W taps ask for 8 × 5 + 4 × 10 = 80 W, so a 100 W amplifier carries them. (70.7 V is the RMS value of a sine wave with a 100 V peak: 100 / √2 ≈ 70.71.)On the exam
Output levels to expect: line outputs about 1 V (−10 dBV consumer, +4 dBu professional), composite video 1 V peak-to-peak into 75 Ω, and speaker outputs of volts to tens of volts: 100 W into 8 Ω is about 28 V RMS.On the job: no sound from a home theater
Job practice, not exam content. A typical sequence when a customer reports a home theater that shows a picture but plays no sound. The manufacturer's service literature comes first.
- Check the settings. The receiver selects the source (section 5): confirm the right input is selected, mute is off and the speaker outputs are switched on.
- Every source, or one? If only one source is silent, suspect that source or its cable; if every source is silent, suspect the receiver, the speakers or their wiring.
- Substitute and swap. Try a known-good cable or source, then swap left and right at one connection to see whether the fault follows (section 10).
- Check the load. Count the speakers on each output: speakers in parallel halve the impedance, and the total must not fall below the amplifier's rated minimum (section 7).
- Check any alarm interface. If the system is muted by a fire or security alarm relay (section 9), an alarm or a fault holding that relay will silence it; check the interface before replacing parts, and record what you found (Work Procedures).
Check yourself
Answer in your head first, then reveal.
For how much of the input cycle does a Class AB stage conduct?
Class AB conducts for more than 180° but less than 360°
What is the standard amplitude of audio/video output feeding a TV or modulator?
about 1 volt peak-to-peak
If audio is distorted, what is the most common cause?
If audio is distorted, the most common cause is overdriving the input stage
What does a 120 Hz hum most often come from?
A 120 Hz hum most often comes from bad power-supply filter capacitors
What is a key disadvantage of an LCD?
a key disadvantage is sensitivity to temperature
What power does a condenser microphone need, and how does it reach the microphone?
a condenser mic needs power, normally 48 V phantom power on XLR pins 2 and 3
What happens to the load when two 8 Ω speakers are wired in parallel?
two equal speakers in parallel halve the impedance (two 8 Ω speakers make 4 Ω)
How do you find which unit in a chain of A/V equipment holds a fault?
substitute a known-good source, cable or unit, or swap left and right at a connection; if the fault moves with the swap, it is upstream of that point
Practice this topic: Audio/Video questions are in the Study Hub quiz bank, the dB math is in the Decibel tool, and amplifier basics are in the Transistor & Amplifier tool.