DQPSK demodulation.

Hi, Thanks for the add, I have been hinted this place, and im looking forward to digging deeper into the realm of TETRA / DQPSK. What im trying to do is write a C# program for demodulation of TETRA. and have come so far as to be able to view the IQ vector run around on the unity circle, but it appears a bit distorted. I think its due to a crude low pass filter i used on the I/Q signal causing this. From reading books and sites i find that I should probably used a raised cosine filter to keep the signal form beeing distorted (ISI). am i correct in my assumption? and do you have the open source code i can inspect, regardless if programming language. I know there is tons of programs out there that can decode and even produce audio from TETRA DQPSK signals, and I have one installed, but i want to have an edge to this, i need to understand and learn first hand. It may take time, but thats how it is. I have what is required regarding programming skills and radio knowledge - I am radio Amateur, and 20 years ago i did have Digital communication classes at Engineering college - But thats 20 years ago, and the equations in the books are kind of far away in my head.. but the theory sticks. Please help me get up to speed in decoding DQPSK, im not expecting to do this quickly, but will be in steps when i have time, and will lead to questions. But at some point i could be on your frontier digging into TETRA as a whole. Best Regards Kim Mortensen