AVE Preliminary cheat sheet

Decibel reference values: dBSPL re 20 µPa, dBu re 0.775 V, dBV re 1 V, dBW re 1 W; dBm re 1 mW into 600 Ω.

Power ratio: dB = 10·log10(P2/P1). Voltage ratio (same impedance): dB = 20·log10(V2/V1).

Doubling power adds 3 dB; doubling voltage or distance-related pressure adds 6 dB; tenfold power adds 10 dB.

Wavelength: λ = c/f. Sound in air at 20 °C: c ≈ 343 m/s, so λ ≈ 343/f. Light in vacuum: c = 299,792,458 m/s.

Inverse-square law: SPL drops 6 dB per doubling of distance in a free field (point source); line sources drop about 3 dB per doubling.

Frequency bands: audio 20 Hz–20 kHz, ultrasonic above 20 kHz; PAL/NTSC video baseband 0–6 MHz; FM broadcast 88–108 MHz.

Combining incoherent sources: L_total = 10·log10(Σ10^(Li/10)). Two equal levels sum to +3 dB; a source 10 dB lower adds 0.4 dB.

Nyquist criterion: sample rate must exceed twice the highest frequency of interest; 48 kHz audio supports content to 24 kHz before aliasing.

Balanced interconnect rejects common-mode noise; a properly balanced input has CMRR typically 60–90 dB at 1 kHz, falling at high frequency.

Phantom power: 48 V DC via 6.8 kΩ matched resistors on pins 2 and 3 of an XLR; dynamic mics and most line devices must not receive it.

Mic level ≈ 1–10 mV, line level ≈ 0.775 V (−10 dBV consumer, +4 dBu pro), speaker level tens of volts; gain staging must match these.

Amplifier output power: P = V²/R. A 100 V constant-voltage line driving 8 Ω gives 1250 W; the same line into 100 Ω gives 100 W.

Constant-voltage (70.7 V / 100 V) distributed systems use transformer taps; total tap wattage must not exceed amplifier rating.

Loudspeaker impedance varies with frequency; minimum impedance, not nominal, determines amplifier current demand and stability.

Filters: 1st-order = 6 dB/octave, 2nd = 12, 3rd = 18, 4th = 24. Linkwitz-Riley even-order filters sum to flat magnitude with all-pass phase.

DSP latency budget: AEC and FIR blocks add milliseconds; keep total under about 10 ms for live speech reinforcement to avoid echo perception.

Resolution and aspect: 1280×720 (16:9), 1920×1080, 3840×2160 (UHD), 4096×2160 (DCI 4K). SD 720×480 (NTSC) and 720×576 (PAL) are 4:3.

Frame rates: 23.976/24, 25, 29.97/30, 50, 59.94/60, 120 Hz; 29.97 and 59.94 derive from the 3:2 pulldown legacy of NTSC color.

Pixel clock ≈ H_total × V_total × frame rate; 1080p60 with CEA-861 blanking is 2200 × 1125 × 60 ≈ 148.5 MHz.

Color: Rec.709 for HDTV, Rec.2020 for UHD, DCI-P3 for cinema; chroma subsampling 4:4:4, 4:2:2, 4:2:0 reduces bandwidth by 1/3 or 1/2.

HDMI 2.0 carries 18 Gbps and 4K60 4:2:0; HDMI 2.1 raises this to 48 Gbps for 4K120 and 8K. DisplayPort 1.4 is 32.4 Gbps.

Luminance and contrast: contrast ratio is white/black luminance; HDR10 targets 1000 nits peak, Dolby Vision up to 4000 nits with dynamic metadata.

Projection: throw ratio = throw distance / image width. Screen gain >1 concentrates light on-axis and narrows viewing angle.

Display panel types: LCD (backlit, high brightness), OLED (per-pixel emission, true black, burn-in risk), LED wall (modular, pixel pitch in mm).

Control protocols: RS-232 (point-to-point, typically 9600–115200 baud), RS-485 (multidrop, up to 1200 m), IR (carrier 36–40 kHz), IP (TCP/UDP), relay and GPIO contact closure.

RS-232 framing must match on both ends: baud, data bits, parity, stop bits, and flow control; mismatch produces garbage or no response.

CEC over HDMI allows limited device control but is unreliable across brands; discrete RS-232 or IP commands are preferred for deterministic control.

EDID describes display capabilities (resolutions, timings, audio formats); a missing or wrong EDID causes blank screens or forced low resolution.

HDCP 2.2/2.3 protects UHD content; any non-compliant splitter, extender, or switcher in the chain yields a black screen with an error.

Control system logic: macros, conditionals, and feedback polling; state feedback (power sense, projector lamp status) prevents desynchronised UI.

Networked AV: static IPs or DHCP reservations for codecs, DSPs, and displays simplify troubleshooting; document every address in an as-built.

Touch panel and UI design: minimum touch target about 9 mm, clear power-on/off states, and a help or reset path for every room.

Cat 6 supports 10GBASE-T to 55 m, Cat 6A to 100 m; both are rated to 500 MHz. Cat 5e is limited to 100 MHz and 1 Gbps at 100 m.

Maximum copper horizontal run is 90 m plus 10 m of patch cords (100 m channel); fiber extends to 550 m (multimode OM3 at 10G) or kilometres (single-mode).

PoE types: 802.3af 15.4 W at PSE (12.95 W at PD), 802.3at 30 W (25.5 W), 802.3bt 60 W or 90 W (51 W or 71 W at PD).

Subnetting: /24 gives 254 usable hosts; /30 gives 2 and is used for point-to-point links; keep AV VLANs separate from corporate data.

Multicast for AV-over-IP (Dante, AES67, SDVoE) requires IGMP snooping and a querier; without it, multicast floods all ports.

QoS: mark real-time audio/video with DSCP EF (46) for audio and AF41 (34) for video; enable on all trunk and access links.

Dante/AES67 latency presets typically 0.15–5 ms; clock master must be single and stable, with PTP (IEEE 1588) for AES67.

HDBaseT extends HDMI up to 100 m over Cat 6A with PoH; fiber (SDI, HDMI over fiber) is used for longer or electrically noisy runs.

Room acoustics: RT60 target for speech is about 0.4–0.6 s; Sabine formula RT60 = 0.161·V/A (metric, V in m³, A in sabins).

Coverage design: seat-to-seat SPL variation should stay within ±3 dB; aim loudspeakers so the first rows are off-axis, not directly on-axis.

Viewing geometry: nearest seat no closer than 2× image height, farthest no more than 6× for SD or 8× for HD; sightlines under 35° vertical.

Cable installation: separate power and signal pathways by at least 300 mm, respect bend radius (typically 4× cable diameter for copper), and never run in conduit with mains.

Commissioning sequence: verify power and grounding, then signal continuity, then device communication, then audio/video alignment, then user acceptance test.

As-built documentation includes cable schedules, IP address lists, rack elevations, signal flow drawings, and control code backups.

Grounding: single-point ground for the AV rack; ground loops cause hum at mains frequency (50/60 Hz) and harmonics.

Heat load: total rack watts must be removed; design intake air below 27 °C and keep front-to-back airflow consistent with rack fans.

Hum at 50/60 Hz indicates a ground loop; buzz with harmonics suggests dimmer or power-supply noise; isolate by lifting signal ground only, never safety earth.

No video with valid source: check EDID handshake, HDCP compliance, cable length versus resolution, and source output format in that order.

Audio distortion: check gain structure first (clip at mic preamp, DSP input, or amplifier), then impedance mismatch, then driver fault.

Network faults: verify link LEDs, then ping, then ARP, then VLAN and IGMP membership; use a cable certifier for pair and length faults.

Signal measurement tools: SPL meter (A/C weighting, slow/fast), RTA or FFT analyser, impedance meter, cable certifier, and optical power meter for fiber.

Preventive maintenance schedule: clean projector filters and lenses, verify lamp/laser hours, test backup batteries, check firmware versions, and re-run control macros quarterly.

Intermittent faults often trace to connectors: re-terminate RJ45 to T568B, inspect BNC centre pins, and torque XLR screws; log every intervention.

Baseline documentation of normal readings (SPL, latency, IP responses) turns future faults into fast comparisons.

Rigging and mounting must follow local code and manufacturer load ratings; overhead loudspeakers require a secondary safety bond.

Electrical safety: lockout/tagout before working in racks; verify absence of voltage; never defeat earth ground on any device.

Hearing conservation: sustained exposure above 85 dBA requires protection; NIOSH halves permissible time for each 3 dB increase above 85 dBA.

Laser projectors are classified 1–4 under IEC 60825-1; never stare into the beam or open the housing; service only by trained personnel.

Standards to cite: ANSI/AVIXA V201.01 (audiovisual systems design), ANSI/TIA-568 (cabling), NEC/NFPA 70 (electrical), IEC 60950/62368 (equipment safety).

Ladder and lift work: three points of contact, spotter at the base, and never exceed the rated load; fall protection above the local threshold (often 1.8 m).

Documentation and handover: deliver as-builts, O&M manuals, warranty terms, training records, and a signed acceptance certificate.

Professional conduct: scope changes go through written change orders; never claim manufacturer certification or affiliation you do not hold.