AM, FM, SSB, and Digital Modes: What's Actually Different About Them

A radio wave by itself, an unmodulated carrier, carries no information, it's just a steady signal at a fixed frequency. Modulation is the process of varying some property of that carrier, its amplitude, its frequency, or more complex properties for digital modes, in a way that encodes information (voice, data, images) onto it. Every mode you'll encounter is really just a different answer to the question 'which property of the carrier do we vary, and how?'

AM (amplitude modulation) varies the carrier's amplitude (strength) in step with the audio signal. It's simple and was the original broadcast standard, but it's inefficient: a meaningful fraction of the transmitted power sits in the carrier itself, which carries no information, only the sidebands around it actually carry the audio. This inefficiency is exactly why SSB exists.

SSB (single sideband) is, in a real sense, AM with the wasted parts removed: it suppresses the carrier and one of the two redundant sidebands, transmitting only the sideband that actually carries the information. The result uses significantly less bandwidth and puts all the transmitted power into the part of the signal that matters, which is why SSB is the dominant voice mode on HF, where both bandwidth and power efficiency matter more than they do on VHF/UHF.

FM (frequency modulation) takes a different approach entirely: instead of varying amplitude, it holds amplitude constant and varies the carrier's frequency in step with the audio. This makes FM inherently resistant to the kind of amplitude-based noise (electrical interference, atmospheric static) that plagues AM and, to a lesser extent, SSB, which is a large part of why FM became the standard for VHF/UHF voice and repeater operation, where clean, consistent audio matters more than squeezing into minimal bandwidth. The tradeoff is that FM uses more bandwidth than SSB for the same audio, an acceptable cost on the relatively spacious VHF/UHF bands.

Digital modes extend the same basic idea, varying some property of a carrier to encode information, but encode data (text, images, telemetry) rather than analog audio directly, often using both frequency and phase shifts in combination, and frequently layering error correction on top so a message can be reconstructed even if part of the transmission is corrupted by noise. This is why digital modes can often complete a contact under conditions (weak signal, high noise) where a voice contact using the same power and antenna would be unreadable, a well-designed digital mode is extracting a small amount of highly redundant, tightly encoded information out of a noisy signal, where a voice signal has none of that structure to lean on.