If you want to monitor digital modes that use FSK or PSK (e.g. POCSAG, FLEX, ERMES and AIS), you get the best results if you use the unfiltered audio from your receiver. You can find that signal at the discriminator of your receiver. A discriminator is the 'heart' of an FM receiver. After the discriminator, audio filters and amplifiers follow that may heavily distort a digital signal. If you want to decode and monitor digital (FSK and PSK) systems seriously, a discriminator tap is an absolute prerequisite.