Designing a 2x interpolation filter for audio
@Keith: Here's the situation as I see it. Your original signal's spectrum is shown in panel (a) below. The solid lines are spectral energy and the dotted lines are spectral replications. After the zero-stuffing (upsampling by two), the stuffed sequence's spectrum is shown in panel (b). Notice the spectral image centered at 44.1 kHz that needs to be removed (attenuated). So after zero-stuffing you need to implement a digital filter whose passband cutoff freq is 0.45*22.05 kHz and whose stopband begins at 0.55*22.05 kHz. (I show that filter as the dashed line in panel (b).) But keep in mind, the filter’s Nyquist rate is 44.1 kHz (88.2/2 kHz) and not 22.05 kHz. The final spectrum you want is shown in panel (c).