On to the Software

GNU Radio provides a library of signal processing blocks and the glue to tie it all together. The programmer builds a radio by creating a graph (as in graph theory) where the vertices are signal processing blocks and the edges represent the data flow between them. The signal processing blocks are implemented in C++. Conceptually, blocks process infinite streams of data flowing from their input ports to their output ports. Blocks' attributes include the number of input and output ports they have as well as the type of data that flows through each. The most frequently used types are short, float and complex.
Some blocks have only output ports or input ports. These serve as data sources and sinks in the graph. There are sources that read from a file or ADC, and sinks that write to a file, digital-to-analog converter (DAC) or graphical display. About 100 blocks come with GNU Radio. Writing new blocks is not difficult.
Graphs are constructed and run in Python. Example 1 is the "Hello World" of GNU Radio. It generates two sine waves and outputs them to the sound card, one on the left channel, one on the right.

Example 1. Dial Tone Output

#!/usr/bin/env python

from gnuradio import gr
from gnuradio import audio

def build_graph ():
    sampling_freq = 48000
    ampl = 0.1

    fg = gr.flow_graph ()
    src0 = gr.sig_source_f (sampling_freq, gr.GR_SIN_WAVE, 350, ampl)
    src1 = gr.sig_source_f (sampling_freq, gr.GR_SIN_WAVE, 440, ampl)
    dst = audio.sink (sampling_freq)
    fg.connect ((src0, 0), (dst, 0))
    fg.connect ((src1, 0), (dst, 1))

    return fg

if __name__ == '__main__':
    fg = build_graph ()
    fg.start ()
    raw_input ('Press Enter to quit: ')
    fg.stop ()
We start by creating a flow graph to hold the blocks and connections between them. The two sine waves are generated by the gr.sig_source_f calls. The f suffix indicates that the source produces floats. One sine wave is at 350 Hz, and the other is at 440 Hz. Together, they sound like the US dial tone.
audio.sink is a sink that writes its input to the sound card. It takes one or more streams of floats in the range -1 to +1 as its input. We connect the three blocks together using the connect method of the flow graph.
connect takes two parameters, the source endpoint and the destination endpoint, and creates a connection from the source to the destination. An endpoint has two components: a signal processing block and a port number. The port number specifies which input or output port of the specified block is to be connected. In the most general form, an endpoint is represented as a python tuple like this: (block, port_number). When port_number is zero, the block may be used alone.
These two expressions are equivalent:
fg.connect ((src1, 0), (dst, 1))
fg.connect (src1, (dst, 1))
Once the graph is built, we start it. Calling start forks one or more threads to run the computation described by the graph and returns control immediately to the caller. In this case, we simply wait for any keystroke.