-snip----snip----and semi major snip--

  But I agree, writing a repeater controller does seem to be some sort of bizzar
e
sort of right of passage......

> >> What's the protocol look like for POCSAG?
> >
> >        Pretty straight forward. The basic codeword is 32 bits. 20
> >bits of data,
> >11 bits BCH ECC and one parity bit. You also have to generate sync
> >words and
> >idle words.
>
> Sync words and idle words are actually just two of the possible 20-bit
> data words.  The ECC and parity codes are the same.
>

        Yep.

> The main problem I see with transmitting POCSAG (which would be a very
> useful feature I agree) is that the time slot when the data is
> transmitted is part of the address.  In order to page more than one pager
> per transmission, you need to organize the paging data into blocks of 16
> codewords (about 48 bytes before ECC).  It would be pushing the RAM space
> in the 84.  A special-purpose transmitter that doesn't pack multiple
> pages into one transmission would be much simpler, since all the
> non-applicable words would just be "Idle" words.
>


        I know,
        That's kind of the point, I don't want to handle  paging needs for the
entire east coast. I don't care about conserving airtime, I just want to signal
a
single beeper. We can just send a nice simple batch stuffed with idle
codewords.

---- more casual snipping--------

>
> Yes, it is hard to do approximately DC modulation on simple synthesized
> transmitters because the PLL will fight it.  I converted a synthesized
> scanner into a POCSAG test transmitter (by placing the pager near the
> scanner, it will receive the LO).  In order to make it work, I modified
> the loop filter to have a very long time constant.  It took about 30
> seconds to lock onto frequency.  Once locked, any modulation added after
> the loop filter would directly FM the oscillator and the PLL couldn't do
> anything about it other than control the long term average frequency.  Of
> course it was on the air all the time.
>
> A more practical, but complicated, technique could FSK the reference
> oscillator as well.  Maybe it's possible to reprogram the PLL for +- 5
> KHz on the fly, but there'd likely be a lot of problem transients that
> would go out of the channel bandwidth.  (A pag ing transmitter with a
> dirty signal?  Never heard of that!)


        You're absolutely right. The standard trick is to  just put a varacter
diode on the reference oscillator and modulate that as well.

        As to dirty pager transmitters...
        Our repeater (here in NYC) is plagued with intermod and noise from
the paging transmitters that share our site. They push 200+ watts (ten feet from
our receive antennae) and receive all their data on land lines or microwave link
s.
        We just learned to live with it.


And on a parting note:
        I'm off to Dayton, OH for the annual hamfest. (hey! last year I got a
picmaster for $600) so I won't be able to respond for a few days. Everyone who
wanted the pocsag encoding source code will have to wait till next week.  I'll
post it, but I still have to generate schematics, and add a few more comments.

        Anyone who would like to look at  pocsag decoding on a pic, I suggest
you point your browsers at the following url: http://members.tripod.com/~dejank/



later............










carl

--------------------------------------------------------
Henry Carl Ott   N2RVQ    carlott@interport.net
http://www.interport.net/~carlott/
--------------------------------------------------------
Diplomacy is the art of saying "nice doggy"
until you can find a rock.