[old_tv_nut]

FM modulation is described by Bessel functions - college level theory for sure.

What this means is that video frequencies produce a spectrum covering the sync tip to white frequency range, PLUS some sidebands beyond that range.

Low video frequencies produce only a little extra spread beyond the deviation, while higher video frequencies produce additional sidebands that are at least as far away from the background gray frequency as the higher video frequency, above and below the low-frequency deviation they are riding on.

So, for example: say sync tips are 5.4 MHz and white is at 7 MHz. If there is a high frequency pattern of 2.5 MHz video riding on a gray background that produces an FM frequency of 6.2 MHz, sidebands will be generated that extend at least from 3.7 MHz (6.2-2.5) to 8.7 MHz (6.2+2.5) (well beyond the deviation of 5.4 to 7).

Because of the limited upper frequency of the heads and tape, the high-side 8.7 MHz sideband will be attenuated, and the signal becomes a sort of single sideband (or vestigial sideband) narrow-deviation FM when read from the tape.

The lower sideband of 3.7 MHz MUST be present in order to demodulate the entire video including the gray background video component at that point AND the 2.5 MHz video component. If the system bandwidth was strictly limited to the deviation, no high frequency video patterns could be recorded.

No matter what deviation you choose, you must have these sidebands to reproduce higher video frequencies.