Broadcast audio processing

This is not a manual. It is an attempt to write down the things I had to learn the hard way about processing audio for broadcast, in the order that they actually make sense.

Broadcast processing has a reputation for being a black art practised by a small number of people who won't tell you what they do. Some of that is real — a lot of the craft is tuning by ear over months — but the underlying engineering is not mysterious at all. Every stage in a broadcast chain exists because of a specific, describable problem: a listener in a noisy car, a transmitter with a hard modulation limit, a playlist that spans fifty years of mastering practice, a receiver that costs four dollars. Once you know the problem, the stage stops looking arbitrary.

I write these chapters as someone who runs a radio station and builds the processing for it, which means I am on both sides of the argument about how much of this you should do. The chapters try to be honest about that. Where TRANSMIT Tower appears, it appears as one concrete example of a general idea, not as the subject. You should be able to read the whole thing, never touch the plugin, and still process a stream better afterwards.

The chapters are meant to be read in order, but each one stands alone.

• 01 — Why stations process — The listening environment as an engineering problem, and why "just play the records" fails on air.
• 02 — Levelling and AGC — Loudness, level and peak are three different things; what a slow gain servo does that a compressor cannot.
• 03 — Multiband — Why you split the spectrum at all, what a crossover costs you, and what band linkage really controls.
• 04 — Pre-emphasis — The FM tone curve: why it exists, the 75 µs / 50 µs regional split, and why it is the reason FM has a sound.
• 05 — Clipping vs limiting — How loudness is actually won, what distortion buys you, and why a clipper ahead of a limiter beats a limiter alone.
• 06 — Stereo in broadcast — Mid/side, correlation, mono compatibility, and why width is paid for in loudness.
• 07 — Low frequency — Mono bass, level-dependent low shelving, bass clipping, and why low peaks are the most expensive peaks you own.
• 08 — The FM signal path — End to end from the studio output to the antenna, and exactly where a plugin's responsibility stops.
• 09 — From FM to web — What carries over to streaming, what doesn't, and the honest arguments on both sides.

— Omer Senesh