[BrendaAnnD]

Mikethedruid,

You need to understand that AM is much more complicated than just varying the strength of a carrier wave. There's lots of math involved, much of which even I have trouble with. The short story is, all information in AM is carried in the sidebands, and there is a direct mathematical correlation between the audio bandwidth (frequency response) and the width of those sidebands. If you only want 4 KHz of audio bandwidth, you only need 8 KHz modulation bandwidth (say, you have a carrier on 1000 KHz, you would have sidebands out to 996 and 1004 KHz.) For more audio bandwidth, you need correspondingly more modulation bandwidth. For a 10 KHz audio bandwidth, you would create sidebands out to 990 and 1010 KHz. You can't get something for nothing.

FM is complex in a different way. To get a 15 KHz audio bandwidth on FM, you COULD use 15 KHz deviation either side of the carrier (30 KHz total), but this gives limited signal to noise ratio, so for broadcast purposes, a deviation of 75 KHz either side of the carrier (150 KHz total) was chosen. In this case, the audio bandwidth remains the same, but other information is placed onto the carrier at certain frequencies and deviation levels (like the 19 KHz pilot subcarrier and the 38 KHz L-R subcarrier) and allows for a "louder" modulation with better signal to noise ratio.

Digital (IBOC, or as I call it, I-BLOCK in the US) is another whole kettle of fish. It use fixed sidebands with neither AM nor FM modulation, but Pulse Width Modulation and a codec that allows for information other than the L+R modulation of a standard AM signal. These include a L-R and metadata information.