Detecting Zero-Crossings

Hi,

I'm developing a system to read LTC timecode (cf wiki, Good Article), which is an audio signal (I'd guess between .32V and .77V RMS). TL;DR the links, it's a binary pattern in Bi-phase mark coding. I want to sense the period between zero crossings to decode the signal. Will reading a Pin in "both" mode:

setWatch( BMCman.tick, A0, {repeat:true, edge:'both'} ) ;

have the desired result, or should I use a Zero-Crossing detection circuit to condition the signal to a rising edge before sensing it?

Thanks.