POCSAG paging uses the direct FSK method of keying.  This direct method
of keying requires a "data ready" transceiver. For both 9600 baud packet
and POCSAG transmissions, a bi-polar digital signal must be used to
drive the frequency determining element of the FM rig, usually the
varactor. For 9600 baud packet and POCSAG reception, discriminator audio
must be used.  Audio processing, in either case, distorts the signal
enough that it cannot be copied consistently.

There are differences between 9600 baud packet and POCSAG signalling in
addition to baud rate.  9600 signals are scrambled during transmission
while POCSAG signals are not. Scrambling keeps a signal balanced; that
is, the number of ones and zeros are kept in balance over a fixed number
of bits.  With POCSAG, it is possible to have a long string of ones or
zeros, and the result is a signal with low audio frequency content.
Hence, POCSAG transmitters must have a low audio responce, around 20 to
50 hertz.  That means that data ready radios that have a good low audio
responce as well as wide audio responce do well with POCSAG. For this
reason, some rigs advertised as 9600 "data ready" work for paging and
some do not.
--
     Looking at the format of the POCSAG signal
The transmission format of the code is a preamble of 576 bits -
alternating ones and zeros followed by one or more batches of codewords.
Each batch consists of a 32 bit syncronization codeword (SC) followed by
eight frames, each consisting of two 32 bit codewords.  Codewords are
defined as synchronous, idle, address, and data.  The format of the data
codewords differs slightly from the rest, but all codewords contain ten
check bits for error detection and correction.

According to the POCSAG code, each pager is assigned one slot in eight,
i.e. one of the 8 frames in a batch for an address codeword.  Hence, in
addition to the 18 bits assigned for addressing withing an address
codeword itself, 3 more bits are (in effect) added to the address,
establishing unique addressing for over 2 million pagers on a given
frequency.  This assignment, of course, allows a pager to be turned off
during the time in which the non-assigned frames are received (without a
message) which extends battery life.  All messages for a particular
pager follow the address codeword for that pager.
--
A capcode of 8:
16, 24, 32 ... 1234568, 123576 and so on.
By picking a capcode divisible by 8, we'll insure that the address
codeword is the first codeword transmitted following the preamble and
synchronization codeword.
--