[amsat-bb] Re: BeeSat at 4800 GMSK

Nathaniel S. Parsons nsp25 at cornell.edu
Sat Apr 10 22:31:30 PDT 2010

Hi Mike,

Thanks for the reply while we still have time to fix things and stay on

Do you think it would be feasible to implement Mobitex in software on a µC,
given a transceiver in transparent mode (will modulate and send any bits it
receives on serial, and demodulate/receive any RF it receives)? We already
have one dedicated to comms, and all other algorithms take place on a
separate processor. I ask because at this point, it would be easier to
change the software than the hardware (software has no mass or volume).

I'd say Block formatting, CRC and bit & Frame Sync would be easy enough
(everything's a multiple of a byte) because they're completely described,
but the interleaving and scrambling aren't which makes things much harder.

-Nate KC2SVI

On Fri, Apr 9, 2010 at 4:03 AM, Mike Rupprecht <mail at mike-rupprecht.de>wrote:

> Hi Nate,
> Sorry for the long delay....
> MixW doesn't support GMSK. You mix that with MFSK.
> BeeSat has two CMX909B modems (CML) onboard. The data is sent in the
> format, not AX.25 !
> See here for more information about Mobitex:
> http://en.wikipedia.org/wiki/Mobitex
> The CMX909B is a half-duplex Gaussian Minimum Shift Keyed (GMSK) BT = 0.3
> modem data pump with on-chip packet data handling. GMSK modulation
> optimises
> the data throughput for a given bandwidth RF channel and the on-chip packet
> data handling relieves the host µC of regular processing tasks, such as
> maintaining Bit and Frame Synchronisation, Block Formatting, CRC and FEC
> Error Processing, Data Interleaving and Scrambling.
> 73, Mike
> -----Ursprüngliche Nachricht-----
> Von: amsat-bb-bounces at amsat.org [mailto:amsat-bb-bounces at amsat.org] Im
> Auftrag von Tom Azlin N4ZPT
> Gesendet: Freitag, 9. April 2010 00:28
> Betreff: [amsat-bb] Re: BeeSat at 4800 GMSK
> Hi Nate,
> I was not aware MixW had MSK in it but still identical to GMSK. Perhaps
> you are thinking of MFSK? In the MixW program there is MSFK16, a
> multitone frequency shift signal, is supported, but not Minimum Shift
> Keying or Gaussian Minimum Shift Keying signals. And also FSK, aka RTTY.
> There is work going on to make a sound card 4800 bps 0.5 time-bandwidth
> product GMSK soundcard modems for use in GMSK DV adapters to use with
> D-STAR radios. Perhaps that could be adapted for your purpose?
> http://www.qsl.net/kb9mwr/projects/voip/dstar/G4KLX.html seems to
> describe some of the work.
> http://groups.yahoo.com/group/dstar_development/ and
> ttp://groups.yahoo.com/group/pcrepeatercontroller/ are where I see the
> discussion underway.
> Separate discussion is using an external sound card interface like the
> Rigblaster, to connect a computer and radio.
> 73, Tom n4zpt
> Nathaniel S. Parsons wrote:
> > Can I assume MSK=GMSK? I thought they were different, but they are the
> same,
> > and since MixW has MSK, and I could use that with a Rigblaster, etc,
> right?
> >
> > -Nate
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