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The Mast Log Notes on broadcast and recorded sound

Section

On Air

What happens to sound once it leaves the building. Modulation, the atmosphere, the geometry of coverage, and the one system that is allowed to interrupt everything else.

A broadcast signal is not a recording that has been sent somewhere. It is a continuous wave that has been deliberately deformed, in a way a receiver knows how to undo. Almost every strange thing about radio, from the hiss at the edge of coverage to the fact that a distant station appears at midnight and vanishes at dawn, comes from the details of that deformation and from the medium the wave crosses.

These four pieces work outward. The first stays inside the transmitter and takes an FM carrier apart layer by layer. The second leaves the tower and follows a medium-wave signal into the ionosphere. The third asks the licensing question, which is really a geometry question: given a class, a height and a power, where does the protected service actually stop? The fourth is about the exception, the alert system that can seize the audio path at any second of any hour.

A flat monochrome map showing three nested signal contours drawn around a single transmitter site, the outermost one crossing a county boundary and stopping short of a distant town.
Coverage is drawn as contours of field strength, not as a circle around a tower. Two stations with identical power can have contours of very different size and shape.

Pieces in this section

  • How an emergency alert interrupts a song

    The header burst, the two-tone attention signal, the relay chain that predates the internet, and why the system is deliberately built to survive the failure of everything modern.

    13 minute read4 notes

  • How far a station is allowed to reach

    Height above average terrain, the class table, the protected contour, and why a low-power station on a hill can beat a high-power station on a plain.

    13 minute read4 notes

  • Why the AM band changes at sunset

    Ground wave by day, sky wave by night, and the whole apparatus of power reductions, directional patterns and sign-off times that exists because of it.

    12 minute read4 notes

  • What is actually riding on an FM carrier

    The baseband, layer by layer: mono sum, pilot tone, stereo difference channel, station text, and the digital sidebands that sit outside all of it.

    14 minute read5 notes

The other section

  • In the Room

    Six pieces on the studio side: microphones, wiring, loudness measurement, the hour clock, format design and music licensing.

  • Glossary

    Every term used in these four pieces, defined in one or two sentences, with a link back to the full explanation.