Audio/Video, Home Automation, and Control Systems
Distributed audio
Commercial audio often uses a 70 V or 100 V constant-voltage distribution system. Each speaker has a transformer with power taps (for example 2, 4, 8, 16 W). The amplifier sees the sum of the tap wattages, not the speaker’s 8 Ω impedance. Add the taps, then choose an amplifier with 20–25 percent headroom so the system is not run at full output continuously. A 100 W amplifier should not be loaded with 120 W of taps.
Signal transport
HDMI carries video, audio, and control but is distance-limited; passive copper HDMI may be unreliable beyond about 5–10 m depending on resolution and cable. HDBaseT extends HDMI, Ethernet, control, and power over twisted-pair to about 100 m. EDID tells the source what the display supports; HDCP is copy protection and can fail when splitters or extenders are added. Control uses RS-232, RS-485, IP, IR, or wireless protocols such as Zigbee, Z-Wave, or KNX; each has different topology and interference issues.
Network and control
Put AV and automation devices on a documented VLAN or separate network when the manufacturer requires it. PoE can power cameras, access points, touch panels, and small controllers, but check the PoE class and the switch budget. Wireless automation needs a clear channel plan and adequate repeater placement; “mesh” does not eliminate distance limits.
Worked example
A 70 V system has 12 ceiling speakers set to 5 W taps. Total load = 12 × 5 = 60 W. With 25 percent headroom, the amplifier should be rated at least 60 × 1.25 = 75 W. A 100 W amplifier is the next standard size and is acceptable; a 60 W amplifier would run at full rated output with no margin for transformer losses and program peaks.
Traps
- Adding 8 Ω speaker impedances when the system uses 70 V taps; add wattages instead.
- Selecting an amplifier equal to the tap total with no headroom.
- Assuming HDMI will run 30 m over passive copper.
- Forgetting EDID/HDCP when adding a matrix switcher.
- Mixing IR and RF control without line of sight or a clear RF path.
- Loading a PoE switch beyond its budget because each device is “only” Class 4.