AM radio is more strongly band‑limited than FM (or DAB) but there’s typically lots of processing involved too.Photo: Pixabay (Creative Commons CC0)
There’s an AM music station I listen to, and I’m very familiar with the songs on the playlist. For the first few minutes of listening after switching from an FM station, it sounds very different. I don’t think I mean the mono versus stereo thing. I don’t know how to describe the AM sound compared to FM accurately beyond that, the words that come to mind are: “congested, like there’s a veil over it, hard to hear vocals, like there’s something missing...”. Then, after a few minutes of listening, I get accustomed to the AM music station, and it all sounds fine.
Is there something in AM technology that makes this sound unavoidable? I’ve listened to other AM stations to compare, but as I’m not familiar with the songs, it’s hard to know what (if anything) is missing.
SOS Forum post
SOS Technical Editor Hugh Robjohns replies: AM stations broadcast signals with very limited bandwidth: there’s a very steep low‑pass filter at around 4.5kHz. Often, the bass is dialled back too, in the receiver if not the transmitter. So AM radio inherently delivers a very mid‑forward sound. Then, it’s usually compressed and limited heavily too. This is partly because AM is prone to interference and fading effects, but it’s also to maximise the transmitter coverage area. In many cases that (multiband) compression is also set up to give the station a recognisable ‘sound character’.