Audio/Video, Data, and Control Systems
What this domain tests
Audio/Video, Data, and Control Systems tests whether you can match signal type to cable, impedance, and topology. Expect balanced versus unbalanced audio, 70 V distributed systems, RS-485, 0-10 V control, HDMI and HDBaseT, and PoE data links.
Core rules
- Balanced audio uses two signal conductors and a shield, with common-mode rejection. Unbalanced audio is more susceptible to hum and noise on long runs.
- A 70 V distributed audio line uses transformer taps. Lower tap power means higher impedance and less load on the amplifier. Total tap wattage must not exceed the amplifier rating.
- RS-485 is a differential bus. Use twisted pair, observe polarity, and terminate at the ends of long buses. Do not star-wire a high-speed bus.
- 0-10 V control is susceptible to voltage drop and induced noise. Keep control wiring separated from power and verify the controller's source or sink requirement.
- HDMI has limited passive cable length. HDBaseT extends over twisted pair but requires the correct cable, power, and endpoint compatibility.
- PoE data links need the correct cable category and length; poor termination causes intermittent faults.
Worked example
A 70.7 V amplifier feeds a speaker with a 25 W transformer tap. The tap impedance is Z = V² ÷ P = 70.7² ÷ 25 = 5,000 ÷ 25 = 200 Ω. The current is 70.7 ÷ 200 = 0.354 A. If the installer instead uses a 5 W tap, the impedance is 1,000 Ω and the current is about 0.071 A. The lower tap draws less power but produces less output. The reasoning is to sum tap wattages and compare with the amplifier rating, not to add speaker impedances as if they were direct-connected voice coils.
Traps
- Connecting a 70 V line to a low-impedance speaker input.
- Star-wiring an RS-485 bus.
- Running unbalanced audio next to dimmers or power feeders.
- Assuming any HDMI cable will work at 4K over a long run.
- Forgetting that 0-10 V control wires can pick up noise.