[PCR-1000] packet output POCSAG

[Dave]

POCSAG stands for Post Office Code Standarization Advisory
Group.   Post office in this context is the British Post Office
which used to be the supplier of all telecommunications services in
England before privatization.

	POCSAG as defined in the standard, (original POCSAG) is 512 bits
per second direct FSK (not AFSK) of the carrier wave with +- 4.5 khz shift
(less deviation than that is used in some US systems). Some systems run 1200
bits.
Data is NRZ coded with the higher frequency representing 0 (space) and the
lower one representing 1 (mark).

	The basic unit of data in a POCSAG message is the codeword which
is always a 32 bit long entity.  The most significant bit of a codeword
is transmitted first followed immediately by the next most significant
bit and so forth.  The data is NRZ, so that mark and space values (plus
and minus voltages) as sampled on the output of the receiver
discriminator at a 512 hz rate correspond directly to bits in the
codeword starting with the MSB.   (Note that the audio output circuitry
following the discriminator in a typical voice scanner may considerably
distort this square wave pattern of bits, so it is best to take the
signal directly off the discriminator before the audio filtering).