AVE Preliminary flashcards

34 cards. Read the term and the definition here. The deck at the top of this page is optional.

dBSPL reference

Sound pressure level is referenced to 20 micropascals, roughly the threshold of hearing at 1 kHz. A 6 dB increase doubles the pressure; 10 dB is perceived as about twice as loud.

Inverse-square law

In a free field, sound pressure from a point source falls 6 dB each time distance doubles. Line arrays approximate a 3 dB per doubling loss near the array because of cylindrical spreading.

Wavelength formula

Wavelength equals propagation speed divided by frequency. In air at 20 degrees Celsius, 100 Hz has a wavelength of about 3.43 metres; 1 kHz about 34 centimetres.

Nyquist criterion

A sampled signal must be sampled at more than twice its highest frequency component. Otherwise higher frequencies alias down into the audio band as spurious tones that cannot be removed later.

Decibel power vs voltage

For power ratios use 10 times the base-10 logarithm. For voltage or pressure ratios at equal impedance use 20 times the logarithm, because power varies with the square of voltage.

Phantom power

48 volts DC supplied equally through 6.8 kilohm resistors on pins 2 and 3 of an XLR. It powers condenser microphones and active DI boxes but can damage unbalanced or ribbon devices.

Common-mode rejection ratio

CMRR measures how well a balanced input rejects noise appearing equally on both conductors. Good inputs achieve 60 to 90 dB at 1 kHz, degrading at higher frequencies where balance is harder to maintain.

Gain staging

Setting each stage so signal passes near but below clipping, from microphone preamp through DSP to amplifier. One stage set too hot raises noise and distortion for the whole chain.

Constant-voltage distributed line

Loudspeaker systems using a high-voltage line, typically 70.7 or 100 volts, with transformer taps on each loudspeaker. Total tap wattage must not exceed the amplifier's rated output.

Linkwitz-Riley crossover

Even-order filters, usually fourth order at 24 dB per octave, whose outputs sum to flat magnitude. They are standard in loudspeaker processors because they preserve a coherent polar response through the crossover region.

Amplifier power formula

Output power equals voltage squared divided by load impedance. A 100 volt line delivers 100 watts into 100 ohms and 1250 watts into 8 ohms, so tap selection must match the amplifier rating.

Pixel clock calculation

Pixel clock equals total horizontal pixels times total vertical lines times frame rate, including blanking. 1080p60 with standard blanking gives about 148.5 megahertz.

Throw ratio

Throw ratio equals projection distance divided by image width. A 1.5 to 1 lens throwing 6 metres produces a 4 metre wide image; shorter ratios allow closer mounting.

HDCP 2.2

High-bandwidth Digital Content Protection for UHD content. Every device in the signal chain must support it; one non-compliant splitter or extender causes a black screen or authentication error.

EDID

Extended Display Identification Data, read over the display interface, lists supported resolutions, timings, and audio formats. A missing or incorrect EDID forces fallback to a low common resolution.

Chroma subsampling

Reducing colour detail relative to luminance. 4:2:2 keeps half the horizontal chroma and 4:2:0 keeps a quarter, cutting raw video bandwidth by one third or one half with little visible loss.

Rec.709 and Rec.2020

Colour primaries and transfer standards for HDTV and UHD respectively. Rec.2020 covers a much wider gamut, so content mastered in 709 shown in 2020 without conversion looks desaturated.

RS-232 framing

Serial parameters that must match at both ends: baud rate, data bits, parity, stop bits, and flow control. A mismatch produces garbled characters or no response at all.

CEC limitations

Consumer Electronics Control carries simple commands over HDMI but behaves inconsistently across brands. Deterministic room control uses discrete RS-232 or IP command sets instead.

Relay and GPIO control

Contact closure outputs and general-purpose inputs let a control system trigger screens, curtains, or sense device power state. They provide simple, protocol-independent integration with third-party equipment.

Control macro

A stored sequence of commands, such as power on, select input, and lower screen, executed from one button press. Macros need state feedback to stay synchronised with real device status.

PoE power classes

802.3af supplies 15.4 watts at the source, 802.3at 30 watts, and 802.3bt 60 or 90 watts. Cable heating and budget per switch port limit how many powered devices can share a switch.

IGMP snooping

A switch feature that forwards multicast traffic only to ports that requested the group. Without it, audio and video streams flood every port and congest the network.

Copper channel length limit

Structured cabling allows 90 metres of permanent horizontal link plus 10 metres of patch cords, a 100 metre channel. Category 6 supports 10 gigabit to 55 metres, Category 6A to 100 metres.

QoS DSCP marking

Differentiated Services code points tag packets for priority handling. Real-time audio uses EF, class 46; video commonly uses AF41, class 34; QoS must be enabled on every link.

HDBaseT extension

A standard that carries HDMI video, audio, Ethernet, control, and power over a single Category cable up to 100 metres. Longer or noisy runs typically switch to fiber.

Sabine reverberation formula

Reverberation time equals 0.161 times room volume divided by total absorption in sabins, using metres. Speech rooms target roughly 0.4 to 0.6 seconds at mid frequencies.

Viewing distance rule

Nearest viewer no closer than twice the image height; farthest no more than about six times height for standard definition or eight times for high definition, keeping text legible.

Ground loop

Multiple earth paths at different potentials create current on the signal shield, producing hum at mains frequency and harmonics. Cure by single-point grounding, not by lifting safety earth.

Commissioning sequence

Verify power and grounding, then cable continuity, then device communication, then audio and video alignment, and finally a documented user acceptance test signed by the client.

Hum versus buzz diagnosis

Steady 50 or 60 hertz hum points to a ground loop. Buzzing rich in harmonics usually comes from dimmers, fluorescent ballasts, or a failing power supply on the same circuit.

Troubleshooting no-video fault

Check in order: EDID handshake, HDCP compliance, cable length versus resolution, and source output format. Each produces a distinct symptom pattern that narrows the cause quickly.

Hearing exposure limit

Sustained exposure above 85 A-weighted decibels requires hearing protection. Permissible duration halves for every 3 decibel increase, so 88 dBA allows half the time of 85 dBA.

Secondary safety bond

An independent cable or fitting that catches overhead equipment if the primary mount fails. Required by most codes and manufacturer instructions for suspended loudspeakers and projectors.