It’s well known that the most important speech information is between 300 and 3000 Hz. Energy outside that range makes the speech sounds nicer, but doesn’t help intelligibility much. Analog modes such as SSB exploit this by band limiting the speech so that the transmitter power is used just to punch through information in the narrow bandwidth that matters.
With digital speech the RF bandwidth is not directly linked to the bandwidth of the decoded speech. For example FreeDV 700D uses around 1100 Hz of RF bandwidth, but the decoded speech has energy covering most of the 0 to 4000 Hz range.
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