18.3 Digital Transmitter 515 first probes the USB port to locate the requested board number, then it uses the appropriate version-specific source or sink. The source_c is a function defined in the usrp module. It returns a class object that represents the data source. The suffix _c means the data type of the signal is complex, 2 because the signal coming into the USRP is complex (actually a real/imaginary pair). From the next line, we start seeing the real signals coming in and being pro- cessed. Note that, in Python, a comment starts after the symbol #. All statements after # in each line are ignored by Python interpreter. Besides, we save the details of signal processing for the next part. The last line in this part finally completes our signal flow graph. Listing 18.3 Connect flow graph function 1 self.connect (self.usrp, self.channel_filter, self.receiver_block, self.sink) This method is designed for the flow graph to bind all the blocks together. The signal flow graph is done at this point. When we use MATLAB to do simulation, it is believed that, to write the code cleanly and efficiently, we need to memorize a number of MATLAB built-