Finally having all the pieces in place I need, I want to use the new
GMSK code to decode an EDACS trunked radio control channel.
Given my rudimentary knowledge on the topic (and gnuradio in general)
which (if any) of the examples present would be my best starting point
given that I’m hoping to do this, even initially, right off the air
with the TV RX?
Some details at
http://users.netropolis.net/maverick/scanners/edacs.htm for anyone
Most relevant snippet for the uninitiated:
Modulation: It looks like GMSK (Gaussian Minimum Shift Keying) which is basically the same thing as two level FSK keying except that the data stream is passed through a low pass filter before modulating the carrier. This reduces the high frequency components allowing the control channel to fit within a 12.5KHz channel. Accordingly, a simple data-slicer circuit can be used to receive the control channel information. Baud Rate 9600 Baud Frame Sync Frame synchronization is achieved by sending the following 48 bit sequence: 000101010101010101010111000100100101010101010101 or 0x155557125555 Hexadecimal Data Frames After transmission of the frame synchronization sequence TWO data frames will be transmitted and then the whole cycle repeats ad infinitum. Each data frame is 120 bits long; this means 288 bits are transmitted in each cycle (2x120 bits for the data frames and 48 bits for frame sync).