01The source: where sound becomes signal
A microphone does one thing — it converts air pressure into voltage. A presenter speaks, the diaphragm moves, and the capsule produces a tiny fluctuating electrical signal that is an exact analogue of those pressure changes. At this stage the signal is extremely fragile: just a few millivolts, easily swamped by interference, easily degraded by a bad cable.
Dynamic microphones generate that voltage through electromagnetic induction — a coil attached to the diaphragm moves within a magnetic field. Condensers use a different principle, a capacitor whose capacitance changes as the diaphragm vibrates, which is why they need power (phantom power, typically 48 volts, supplied down the same cable) to operate. The practical difference matters to presenters: dynamics are robust and handle high sound pressure levels well, which is why they dominate broadcast studios. Condensers are more sensitive to detail, which is why they appear in music recording, and increasingly in podcast setups where a controlled acoustic space is guaranteed.
The signal leaving a microphone is balanced — carried across two signal conductors and a ground, with the two signal conductors carrying mirror-image versions of the same audio. Any interference picked up along the cable hits both conductors equally, and the desk's input circuitry cancels it when it subtracts one from the other. This is why broadcast studios run XLR cables rather than unbalanced jacks: the rejection of interference is not a nicety but a necessity when cables run alongside mains wiring for tens of metres.
02The desk: shaping, routing and riding the level
- MicrophoneAir pressure becomes a small electrical signal.
- DeskLevels set, sources mixed, the presenter's channel opened and closed.
- ProcessingCompression and limiting even out the dynamics and raise perceived loudness.
- Studio-transmitter linkThe finished programme is carried to the transmitter site.
- TransmitterThe audio modulates a carrier wave and is amplified.
- AerialThe signal is radiated; height and pattern decide who can hear it.
- ReceiverThe set tunes the carrier, demodulates it and drives a speaker.
The signal arrives at the mixing desk, and the desk is where the chain becomes genuinely complex. Each input channel strips away anything below the voice's useful range (the high-pass filter, which cuts rumble and low-frequency handling noise), adjusts the gain to bring that millivolt-level microphone signal up to line level, then gives the operator control over the sound before it is mixed with everything else.
Line level is the standard operating voltage for professional audio equipment — roughly 1.228 volts in broadcast practice, sometimes referred to by its dBu equivalent of +4 dBu. The reason for standardising here is practical: once every source (microphone, playback machine, telephone hybrid, network feed) is operating at the same reference level, the desk can mix them without one overwhelming another. Getting any source to that level is the gain stage's job, and setting gain correctly is the most underrated craft skill in live broadcasting. Set it too low and the operator rides the fader hard and picks up noise; set it too high and transients clip before they even reach the fader.
The fader is the large slider that most people picture when they imagine a mixing desk. It does not set the fundamental level — that is gain — but controls the proportion of the already-amplified signal that passes into the mix bus. The operator pushes the fader up to open a microphone and pulls it down to close it, which is where the studio discipline of cue-and-fade originates. On a live show, the presenter's microphone fader goes up fractionally before they speak and comes down the moment they finish, preventing the open mic from picking up off-mic noise, the producer's talkback or the monitor speaker feed.
At the output of the desk the mixed signal passes through a master bus, where overall programme level is set. Most broadcast desks also incorporate metering at this point — traditionally PPM (Peak Programme Meters), which respond quickly to transients, rather than VU meters, which are slower and closer to perceived loudness but can miss a peak that will cause distortion downstream.
03Processing: loudness, dynamics and protection
Between the desk and the transmitter sits a layer of signal processing that most listeners never see and most new broadcasters underestimate. Its purpose is to protect the transmission chain, ensure consistent loudness, and shape the station's characteristic sound.
The limiter is not a creative tool at this stage; it is a guard.
A compressor reduces the dynamic range of the signal — the difference between its quietest and loudest moments — by automatically reducing gain when the signal exceeds a set threshold. In speech this means a presenter who moves toward the microphone for emphasis doesn't suddenly blow everything else out of the mix; in music it means a quiet verse and a loud chorus sit closer in level than the recording engineer originally intended.
A limiter is a compressor set to a very fast attack at a very high ratio — effectively a ceiling. Nothing above the threshold passes. Broadcast limiters are placed near the end of the chain specifically to prevent the transmission signal from exceeding the maximum permitted deviation: on FM, over-deviation distorts the received signal at the listener's end; on digital systems, over-level can cause clipping artefacts. The limiter is not a creative tool at this stage; it is a guard.
Most broadcast stations also run a multiband processor, which splits the audio into several frequency bands and compresses each independently. This is partly for protection and partly for loudness: a processed station simply sounds fuller and louder on a car radio than an unprocessed one, because the processing fills in the quieter moments. The trade-off is a reduction in dynamic contrast, which is why heavily processed stations can feel fatiguing to listen to over long periods.
04Transmission: the signal leaves the building
The processed audio feeds the transmitter, which combines it with a carrier signal — a radio-frequency wave at the station's assigned frequency — and radiates it from the aerial. The exact method of combination is modulation: on FM, the audio varies the frequency of the carrier; on AM, it varies the amplitude. Each approach carries different trade-offs in coverage, quality and susceptibility to interference, but both are ways of encoding the same original audio fluctuation into a form that can travel through air.
The transmitter aerial is engineered to direct radiated power toward the intended coverage area. Height matters enormously on FM, because the signal travels in broadly straight lines and cannot follow the curve of the earth — a higher aerial means a wider horizon, and therefore a wider coverage area. Power matters too, though it is regulated: a station cannot simply increase power to drown out a neighbour on an adjacent frequency.
05Reception: reversing the chain
At the listener's end, a receiver aerial intercepts a tiny fraction of the transmitted energy — typically a few microvolts of signal. The tuner selects the desired frequency and rejects everything else, the demodulator extracts the audio from the carrier, and the recovered signal passes through amplification stages before reaching the speaker.
A speaker reverses what the microphone did. A voice coil attached to a cone sits in a magnetic field; as current flows through the coil in response to the audio signal, the cone moves, and moving air creates pressure waves — sound. The physical principles are closely related to a dynamic microphone's, run in reverse. A good speaker in a good room can recover the spatial impression of the original recording. A cheap speaker in a reverberant environment will flatten it considerably.
