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From: steveu on 18 Dec 2009 11:11
>
>
>Flex - it is a Motorola propriatary paging standard. It uses 4 level
>FSK.
>
>It supports 4 mixes of rate and number of levels. Essentially the
>rates are 1600 or 3200 bauds and each baud can be either 1 or 2 bits.
>The header is always 1600 - 2 level and then may change to a higher
>data rate based upon the header's content. The 4 level data is grey
>encoded and FSK modulated on an RF carrier. The spec calls for a 10th
>order Bessel filter (IIRC cutoff = 3.8kHz) to be used to slew limit
>the modulation. The RF lives in a 25kHz wide channel, although the
>actuall spectral occupancy is much less. But the RF may be viewed as a
>4 FSK.
>
>There's your example.

Ermes was 4 level FSK, too. The paging people seemed to like 4 level FSK.

Steve

From: steveu on 18 Dec 2009 23:20
>
>
>Clay wrote:
>
>> On Dec 18, 12:06 pm, Vladimir Vassilevsky <nos...(a)nowhere.com> wrote:
>
>
>>>Besides, there is no good way to achieve simulcast coverage with
4-FSK.
>>
>>
>> Basically data packets are built with a time to transmit
>> tag in them at the paging terminal and the packets are distributed to
>> the transmitters. The smart transmitter emits the scheduled paging
>> data at the right time.
>
>Timing isn't a very big problem. The problem is interferrence between
>the transmitters. With 2-FSK, you can shift the carrier frequencies of
>transmitters by +/- 0.5 x baud rate. Then the mutual interference will
>be averaged out per duration of one bit, and everything works great.
>With 4-PSK, you can't do that. Which makes simulcast networking pretty
>much impossible.
>
>> At one paging equipment manufacturer I worked on both the encoders and
>> then the protocol monitors and pager intercept devices. I used
>> Moto56309 DSPs for those projects. The company was already using that
>> processor and its predessor (56001 and 56002) in its product line, so
>> were very comfortable with manufacturing. I certainly enjoyed the
>> available horsepower and 24 bit depth! The specialized equipment was
>> not very price sensitive, so $30 or more for a DSP was not an issue.
>
>I developed paging and PMR stuff. We had to do everything by i8051 and
>M68HC11. Analog circuitry and tons of assembly code. When ADSP-21xx
>became available, that was great relief.

Its interesting that so many people here have worked on paging equipment,
when it was always such a very niche business, with very few DSP people
involved.

Yours, another who developed paging systems,
Steve

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