FSK and Timing Recovery Questions

[Alex Roberts]

Hello Mailing List, 

Happy New Year!

I've been playing around with this (http://aaronscher.com/GNU_Radio_Companion_Collection/Audio_Modem_loop_back_test_FSK.html) FSK audio modem and have a few issues/questions. 

1) Is the PFB block the way to go for FSK timing recovery? I have issues where if I start the receiver before the transmitter, the system doesn't work. And if I start the transmitter before the receiver, it will eventually lose sync and never recover. I added a message strobe to send dummy data once a second (like a keep alive/heartbeat) and this helps it from losing sync in between time of when I send meaningful data.

1a) Reading docs on both PFB and M&M blocks, do I need a moving avg. filter after quad demod to round out the square pulses to allow PFB and M&M to better find peaks or is the moving avg filter implemented within the block?

2) Is the pre-amble being muxed doing anything more than just padding my data to help the PFB lock in before actual data arrives?

3) Would the Correlation Estimator help since it can send "time_est" tags downstream to the PFB and if so, where would be the ideal place to put it?

Cheers,

Alex