Re: GMSK modem


On Thu, May 26, 2016 at 11:40 PM, Ed Ingber WA6AXX <ed.ingber@...> wrote:

Is the GMSK modem implemented in software in the Raspberry Pi?

Yes -- G4KLX wrote a GMSK modem for soundcards and that is what is being used by the UDRC (albeit with a higher quality sound chip than typically available).

How does its performance compare to the CMX589 GMSK modem chip that's used in the DV MEGA, for example?

This is a question without a simple answer.  Apocryphally, in my installation and testing, it appears that the UDRC implementation works better, but I don't have laboratory numbers for you.

The reason that there is no simple answer is that there are adjustments in the signal chain that can be made for both the UDRC and the CMX589 and for a given set of settings, the results could vary widely.

If someone wanted to build up a test setup, they would record a baseline GMSK signal at 1.0 volts peak to peak from the UDRC and the CMX589 chip.  The signal would a well defined bit pattern.  Then they could take the UDRC and CMX589 and readjust them for optimum 0 BER reception.  Then start attenuating the source signal until the BER rose above 0, then reoptimize each.  Repeat until you find the lowest source signal that each can decode at 0 BER, the one with the lowest signal wins :) 




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