RCDD Audiovisual Systems Infrastructure 6

Question 1

A new 1,500-seat auditorium needs an assistive listening system (ALS) that meets the 2010 ADA Standards for Accessible Design (Table 219.3). The owner does not want a hearing-loop system. What is the minimum number of receivers that must be hearing-aid compatible (usable with neckloops)?

Accepted Answer

12

Answer

For 1,001–2,000 seats, Table 219.3 requires 35 receivers plus 1 for every 50 seats over 1,000. That is 35 + (500 ÷ 50) = 45 receivers. For assembly areas over 200 seats, 25% (1 in 4) of receivers must be hearing-aid compatible: 45 × 0.25 = 11.25, which rounds up to 12. You always round up because a fraction of a receiver can't be provided. The exception that lets a hearing-loop system satisfy this requirement doesn't apply, since the owner ruled out a loop.

Question 2

A lecture hall in Building A must send 4K video to an overflow room in Building B, about 85 m away. The two buildings are fed from separate electrical services, and a measurable ground potential difference exists between them. Which transport method best addresses the infrastructure risk?

Accepted Answer

Fiber-optic AV extension over dielectric (non-conductive) fiber cable

Answer

When buildings sit at different ground potentials, any copper path between them (including a cable shield bonded at both ends) can carry circulating ground current. That current causes hum bars, data errors, damaged equipment, and safety hazards, and bonding both ends makes it worse. Dielectric fiber gives complete galvanic isolation and also handles the distance and bandwidth easily. Isolated-ground receptacles don't remove the potential difference between the two services, and ferrites only reduce high-frequency common-mode noise, not low-frequency ground currents.

Question 3

A designer specifies Power over HDBaseT (PoH) for wall displays, with up to 100 W delivered to each display over 4-pair balanced cabling. The cables run in large bundles above a plenum ceiling in a warm mechanical zone. Beyond the HDBaseT signal reach, what is the most important cabling infrastructure consideration?

Accepted Answer

Temperature rise inside the bundle from DC current on all four pairs, which may require smaller bundles, higher-rated cable, or shorter channel lengths

Answer

High-wattage 4-pair power such as PoH (up to 100 W) and IEEE 802.3bt heats the conductors. In large bundles, especially in hot or enclosed spaces, that heat builds up. The higher conductor temperature raises insertion loss and can exceed the cable's temperature rating. TIA TSB-184-A guidance covers bundle size, cable gauge and category, and length derating to manage this. NEXT isn't affected by DC power. Bend radius rules don't change with power level. PoH uses all four pairs, so splitting power and video onto separate pairs isn't how it works.

Question 4

Using the traditional 4-6-8 display sizing guideline, a training room's farthest viewer sits 12 m from the screen. Content includes detailed spreadsheets that viewers must read, but not inspect closely. What is the minimum image height?

Accepted Answer

2.0 m

Answer

The 4-6-8 rule ties the farthest viewing distance to image height based on how demanding the viewing task is. Inspection viewing allows 4 × image height (4H), detailed viewing 6H, and general or basic viewing 8H. Reading detailed spreadsheets is detailed viewing, so image height = 12 m ÷ 6 = 2.0 m. 1.5 m is the 8H (basic viewing) answer, which is too small for this content. 3.0 m is the 4H (inspection) answer, which is bigger than needed. 2.4 m doesn't match any ratio in the rule.

Question 5

A Dante audio network works on the old switches, but after the access switches are replaced with new enterprise models, devices show intermittent clock-sync loss and audio dropouts at low traffic levels. QoS and VLANs were copied over exactly. What switch feature is the most likely cause?

Accepted Answer

IEEE 802.3az Energy Efficient Ethernet (EEE) enabled on the access ports

Answer

Energy Efficient Ethernet (802.3az) puts links into low-power idle when traffic is light. Waking the link adds variable latency, which disrupts the precise PTP timing that Dante and other AV-over-IP clocking rely on. That explains failures that appear at low traffic and only after new switches, which often ship with EEE on by default. Audinate and most AV-over-IP vendors require EEE to be disabled. IGMP snooping is recommended. Strict priority for CS7 is the correct setup for PTP. RSTP on uplinks doesn't cause steady-state timing drift.

RCDD - Registered Communications Distribution Designer Practice Test

RCDD - Registered Communications Distribution Designer Practice Test

RCDD Audiovisual Systems Infrastructure 6