FM frequency spacing is 200 kHz. The number called bandwidth is determined by how it is calculated.
Such as:
en.wikipedia.org
The digital radio broadcasting system used in the USA complies with the FCC "spurious emissions" rule 73.317, see below.
Digital power of -10 dB below carrier effective radiated power "fits" underneath the "mask" that is produced when (b), (c) and (d) are plotted on a chart.
That is how USA digital radio essentially was approved. From a literal regulatory view, the digital radio signal is a spurious emission that just barely complies with FCC rules.
The current NPRM proposes less interference protection to adjacent channel stations, potentially permitting more stations to increase digital power to -10 dB below carrier. How much reduction of service area and increase of interference will stakeholders agree on?
Stay tuned.
This is where it gets interesting...
73.317 was developed a long time ago, testing analog FM interference between adjacent channel stations with programming and receivers of that era. That is, the FM sidebands produced by audio of that era heard on typical receivers, with a test audience that was accustomed to audio of the era.
It is logical that digital radio sidebands are higher density than today's FM analog sidebands, which are higher density than in the era when 73.317 was developed.
What matters is how objectional interference defined at a given received signal strength of the desired station, at what percentage of locations and percentage of time. Would you be OK if your cellphone worked at 50 percent of locations 50 percent of the time?
Determining this in the real-world seems correct. However incoming signal strength of the desired and undesired stations must be measured at each location. How long will listening be done at each location, given that the rule correctly anticipates changes in propagation over time?
Thus, to make a decision that is equitable to the radio listening public, the process becomes a serious research project that requires careful methodology. It might be lab testing of interference tolerance, followed by carefully moderated real-world listening tests, taking into account the changing nature of propagation.
Here's the FCC rule:
§ 73.317 FM transmission system requirements.
(a) FM broadcast stations employing transmitters authorized after January 1, 1960, must maintain the bandwidth occupied by their emissions in accordance with the specification detailed below. FM broadcast stations employing transmitters installed or type accepted before January 1, 1960, must achieve the highest degree of compliance with these specifications practicable with their existing equipment. In either case, should harmful interference to other authorized stations occur, the licensee shall correct the problem promptly or cease operation.
(b) Any emission appearing on a frequency removed from the carrier by between 120 kHz and 240 kHz inclusive must be attenuated at least 25 dB below the level of the unmodulated carrier. Compliance with this requirement will be deemed to show the occupied bandwidth to be 240 kHz or less.
(c) Any emission appearing on a frequency removed from the carrier by more than 240 kHz and up to and including 600 kHz must be attenuated at least 35 dB below the level of the unmodulated carrier.
(d) Any emission appearing on a frequency removed from the carrier by more than 600 kHz must be attenuated at least 43 + 10 Log10 (Power, in watts) dB below the level of the unmodulated carrier, or 80 dB, whichever is the lesser attenuation.
(e) Preemphasis shall not be greater than the impedance-frequency characteristics of a series inductance resistance network having a time constant of 75 microseconds. (See upper curve of Figure 2 of
§ 73.333.)
