RF glossary for production sound

Definitions are given in the sense they carry on a set, not the sense they carry in a regulatory filing. Where the two differ meaningfully, both are noted.

Spectrum and allocation

Band

A named range of frequencies. In wireless microphone usage it usually means the broad category — VHF, UHF, 900 MHz — rather than a specific tuning range.

Block

A manufacturer's named tuning range for a transmitter or receiver, such as Lectrosonics B1 or Shure G53. Blocks are marketing designations, not regulatory ones, and different manufacturers' blocks overlap arbitrarily. See the block catalog.

TV channel

A 6 MHz slice of spectrum in the US. UHF channel N spans 470 + (N − 14) × 6 MHz, so channel 14 is 470–476 MHz and channel 36 is 602–608 MHz. Wireless microphones live in the gaps between occupied channels.

White space

A TV channel that is unoccupied at a given location. Wireless microphones operate in white space; so do unlicensed white space devices, which is why the white space database exists.

Duplex gap

The 652–663 MHz guard region between 600 MHz downlink and uplink. Only part of it is available to microphones, and the two halves have different rules: 653–657 MHz licensed only, 657–663 MHz unlicensed only.

Guard band

Spectrum deliberately left unused between two services to stop them interfering. The 600 MHz guard band at 614–616 MHz is open to unlicensed microphones at 20 mW EIRP.

Repack

The post-2017 reorganization of TV stations into a narrower band after the Broadcast Incentive Auction. Explained in the 600 MHz repack.

Secondary status

The legal footing wireless microphones operate on in the TV bands. Primary users — broadcasters — have priority. If you interfere with them, you stop; if they interfere with you, that is your problem to solve.

Licensing

Part 15

The FCC rules covering unlicensed devices. §15.236 is the wireless-microphone-specific section: 50 mW EIRP in the TV bands, 20 mW in the 600 MHz slivers, no interference protection.

Part 74

The FCC rules covering broadcast auxiliary services. Subpart H covers low power auxiliary stations — licensed wireless microphones — with more spectrum and more power. See do you need a license.

LPAS

Low Power Auxiliary Station. The FCC's term for a licensed wireless microphone, IFB, cue or intercom link.

ULS

The FCC's Universal Licensing System, where Part 74 applications are filed on Form 601 and where existing licenses can be searched.

WMAS

Wireless Multichannel Audio System. A wideband device class authorized in 2024 that fits more audio channels into a wider RF channel. See WMAS explained.

Signal and propagation

ERP

Effective Radiated Power. Transmitter power multiplied by antenna gain, relative to a dipole — the number FCC records use to describe how loud a broadcast station is. A full-power UHF station can run hundreds of kilowatts ERP against your 50 milliwatts.

EIRP

Effective Isotropic Radiated Power. The same idea as ERP but referenced to a theoretical isotropic radiator. Part 15 microphone limits are written in EIRP; Part 74 UHF limits are written in conducted power at the antenna terminal, which is why the two numbers are not directly comparable.

HAAT

Height Above Average Terrain. How high a transmitting antenna sits relative to the surrounding landscape, averaged over a radius. A high-HAAT station reaches much further than its power alone suggests.

Multipath

The same signal arriving at the receiver by more than one path — direct plus reflections off walls, vehicles, water. Where paths arrive out of phase they cancel, producing a null that can be a few inches wide.

Null

A physical location where multipath cancellation drops received signal to near nothing. Moving the antenna, not raising the gain, is the fix.

Diversity

Using two spatially separated antennas (and often two receiver sections) so that a null at one is unlikely to coincide with a null at the other. True diversity switches or combines between complete receiver chains.

Line of sight

An unobstructed path between transmitter and receiver. UHF tolerates obstruction poorly; a human body between a lav transmitter and the receive antenna can cost more than doubling the distance.

Interference and coordination

Coordination

Choosing a set of carrier frequencies that are individually clear and mutually compatible. Both halves matter — see coordinating a multi-channel bag.

Intermodulation (intermod, IM)

New frequencies generated when two or more signals mix in a non-linear stage. Third-order products of the form 2A − B and A + B − C land close to the originals and are the practical enemy. See intermodulation explained.

Co-channel

Interference from a signal on the same frequency as yours. The obvious case, and the one a scan will show you.

Adjacent channel

Interference from a signal on a neighbouring frequency, leaking in because no filter is perfect.

Blocking / desensitization

A strong nearby signal — often a broadcast transmitter — overloading a receiver's front end so it goes deaf even on a completely different frequency. Explained in why wireless drops out near a TV tower.

Squelch

The threshold below which a receiver mutes rather than passing noise. Set too high it drops usable audio; set too low it passes a burst of hash on every hit.

Scan

A receiver's sweep of a tuning range, showing measured RF energy per frequency. Only valid for the position, time and transmitter state it was taken in — see scanning and walk-testing.

Walk test

Physically walking the transmitter through every position the talent will occupy, watching the receiver for drops. The only test that catches nulls and body-blocking.

Emitters you will meet in an advisory

DTV (full power)

A full-power digital television station. The heaviest contributor to congestion in any market — hundreds of kilowatts from a tall tower, occupying an entire 6 MHz channel continuously.

Class A

A low power television station with protected status. Lower power than full-power DTV but still a real occupant of its channel.

LPTV

Low Power Television. Secondary status, modest power, often overlooked — and often exactly what ruins a channel that looked clear on paper.

BAS

Broadcast Auxiliary Service. Studio-to-transmitter links, electronic news gathering trucks, remote pickup. Intermittent, which makes it worse: it is not there when you scan and very much there when the news truck parks.

Part 74 licensee

Another licensed wireless microphone operator in the same area. Visible in FCC records, invisible on a scan until they power up.