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Comment by SamReidHughes | Hacker News Reader

The article doesn’t say it explicitly, but it’s because the FM band is bounded by round numbers, 88 MHz and 108 MHz, as opposed to 87.9 and 107.9.

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Er, that's in the first sentence. What could be said a little more clearly is that the frequency 90.1 is actually the channel 90.0-90.2 since the convention is to tune to the center frequency. A convention of tuning to the lowest or highest frequency of the channel would be equally valid - its just not how its done.

If you tune to the lowest frequency in the channel, wouldn't that be bad because it's on the border with the channel just below it? Tuning to the center of the channel seems like the only sensible thing to do, it's not just a convention. Or am I way off here?

All I mean is that in the modern digital world, there is no difference between a channel being defined as 90.0 + 200kHz, 90.2 - 200kHz, or 90.1 +- 100kHz. "Tuning" isn't even really a thing in modern SDR receivers that can receive a huge band all at once (it is, but more as a UI convention than an analog necessity).

I assume the convention applies to both receivers and transmitters.

This is probably just splitting hairs, but I don't think it is equally valid. Compared to the channel width, the center frequency is known / required to be set much more precisely. You could wiggle around by the bandwidth by 100s of Hertz and maybe lose some fidelity, or get some interference from neighboring channels, but you can't say the same for the center frequency.

It used to be relatively common for FM radio stations to run at over 100% modulation because it would allow them to be "louder" than the competition. Obviously this is illegal because it results in "spilling over" the assigned frequency band. But it absolutely was a thing.

Only by convention though. The radio systems I use have a configurable channel width and center frequency - there are only so many legal options. Center frequency is just a convention though - a 100kHz channel being defined as being bounded by (freq + 50kHz, freq - 50kHz) is no more logically valid than (freq, freq + 100kHz) or (freq, freq - 100kHz).

What I'm saying is that the filter that shapes the bandwidth (on either Rx or Tx) is not a brick-wall filter. The channel width isn't exactly as stated, because there's a gradual roll-off.

If I misspecified the bandwidth, I can still tune into and broadcast at the right frequency. It's not just a convention, it follows how the transmitted and received signals are actually processed. I do not think it's more logical because setting the center frequency and setting the bandwidth of the baseband signal are two independent operations. Conceptually, they are orthogonal concepts. So it's logical to specify them in an orthogonal basis. Choosing another basis, as you're saying, works. But the current "convention" is more than just a convention. It's the natural choice.

If I take my FM radio and take it somewhere that uses different channel spacing, I can still tune it to the advertised frequency. That would not be the case with what your proposing.

What could be said a little more clearly is that the frequency 90.1 is actually the channel 90.0-90.2

Not strictly. The main channel is 90.1. But many FM stations have sideband channels for various audio and data services, like reading books and newspapers to the blind.

After reading the whole article, I was still wondering why they didn't just put the center of the frequencies on the even numbers. Then I realized that's not the order in which they decided what to do.

Some of my favorite radio stations broadcast(ed) below 88MHz. 87.7 and 87.9 are favorites of college and low budget FM broadcast.

Most likely a TV station. Known as "Franken-FM stations".

https://en.wikipedia.org/wiki/Channel_6_radio_stations_in_th...

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