The "hidden bit" (which I didn't see mentioned in the site above) gets you 1
bit, plus I include the sign bit as a mantissa bit. This is justified because
it 24-bit signed fixed point, the sign bit is included as part of the mantissa.
The >= comes because you get 25 bits of resolution regardless of the level of
the signal, as opposed to fixed point where the lower level the signal, the
fewer bits you use.