Hardware's purpose is to let the radio signal enter the computer.
It's composed by two parts:
The best radio receiver for this applications is the scanner. There is many models at many prices.
A scanner is, generally, a large band radio receiver, able to detect a signal and stop on it. (For example, car-radios have a scanner for frequencies between 88 and 108 MHz FMW).
They permit to choose a modulation type. Modulations could be FM (Frequency Modulation) or AM (Amplitude Modulation).
POCSAG messages are sent on FM by FSK (Frequency Shift Keying) with NRZ (No Return to Zero) code.
It means that the frequency changes function of the bits to transmit.
On the scanner output, we find a signal like this:
Fig. 1 - Audio Output Scope
As you can see, bits of the message are not clearly differentiated. All we see is a suite of curves, corresponding to the frequencies used to modulate the FM signal.
But, if we take a look at the discriminator output, we find something like that:
Fig 2. - Discriminator Output Scope
Here, the signal is not really clean (we find residues of the FM signal), but the bits are clearly differentiated. What we have here, is a signal wich the voltage correspond to the output frequency. That's the signal we must output from the scanner to put it on the interface. Here is a scope of the two signals at the same time.
Fig. 3 - Scanner Outputs Scope
Mosts scanners don't have a discriminator output. You have to modify your scanner to get one. Every scanner is different. There's no general rules to modify one. Try to find for your scanner on AltaVista, or search about HAMRADIO.
I've made an HowTo about the ALINCO DJ-X1.
Once the scanner has is discrimator output, you must make the PC interface.