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Barry Duggan is a graduate of Georgia Tech in Electrical Engineering and is a career computer programmer specializing in real-time control, data communication systems, and email security. He has been an amateur radio operator since 1953, and now devotes most of his time to the GNU Radio project. He is a member of the GNU Radio General Assembly, the GRCon21 Organizing Committee, and is currently the Documentation Lead for GNURadio.
 
The first section of this tutorial explains how an Amplitude Modulated (AM) signal can be created. Rather than using any real hardware for transmission, the signal is sent via a socket to the second section of the tutorial which explains how to demodulate the received signal. The only actual hardware involved is the computer's microphone input and speaker output. In the case of a Raspberry Pi computer, which has no microphone input, an alternative is presented.
 
This tutorial can be performed with either GNU Radio (GR) version 3.7 or 3.8 (and later). The Graphical User Interface gnuradio-companion (GRC) is used to create a flowgraph for each section.
 
== Prerequisites ==
 
* [[Guided_Tutorial_GRC|'''Intro to GR usage: GRC and flowgraphs''']]
* [[Sample_Rate_Tutorial|'''Understanding sample rate''']]
 
== AM transmitter ==
 
Using gnuradio-companion (GRC) and the following Block descriptions, build this flowgraph of the transmitter section:
 
[[File:AM_transmit_fg.png|900px]]
 
=== Block descriptions ===
 
* The Options block identifies the filename for the flowgraph, a title, author, etc.
** id: AM_transmit
** Click on File -> Save As&nbsp;&nbsp;use the file name 'AM_transmit'. the extension '.grc' is added automatically
* The microphone input is defined by an Audio Source block. The parameters are:
** Sample rate: set to 48khz (use the pull-down)
** Device name: for most microphone jacks built into the computer, the Device name can be left blank; for other cases, see [[Audio_Source#Device_Name]]
* For the remainder of the flowgraph, a sample_rate of 768khz is used. This value was chosen to give the 48khz carrier frequency 16 samples per cycle (48000 x 16 = 768000).
** Variable block<br>
    id: samp_rate<br>
    value: 768000
* To boost the 48khz sample rate of the audio input to the 768khz sample rate, a Repeat block with an Interpolation value of 16 is used.
* The QT GUI Range block defines an Audio gain (volume) control
** id: volume
** default value: 1.2
** start: 0
** stop: 10.0
** step: 0.1
** Widget: choose whatever you like
* The value of the volume control is used as a multiplier in the Multiply Const block.
** Constant: volume
* To create an AM signal, the carrier signal is multiplied by the audio signal plus one. The constant 1 creates the carrier when no audio is present.
** Add Const block<br>
    Constant: 1.0
* The carrier signal (in this example is 48khz) is generated by the Signal Source block.
** Sample Rate: samp_rate
** Frequency: 48000
** Amplitude: 0.5
* The QT GUI Time Sink gives a visual representation of the transmitted signal.
** Number of Points: 4096
** Sample Rate: samp_rate
** Number of Inputs: 2
* For a real radio transmitter, the output of the Multiply block would be fed to band-pass filters and then to Radio Frequency (RF) hardware. For this tutorial, we are sending the transmit signal to a ZMQ PUB Sink data socket connected to the receiver section.
** Address: tcp://127.0.0.1:50001
 
=== Note for Raspberry Pi ===
 
Since a Raspberry Pi has no audio input jack, there are two alternatives:
 
# use a USB audio dongle as is shown in the flowgraph.
# replace the Audio Source block with a Signal Source block and a Throttle block.
 
=== Test transmitter section ===
 
== AM receiver ==
 
=== Block descriptions ===
 
=== Testing ===

Revision as of 19:10, 22 July 2021

Barry Duggan is a graduate of Georgia Tech in Electrical Engineering and is a career computer programmer specializing in real-time control, data communication systems, and email security. He has been an amateur radio operator since 1953, and now devotes most of his time to the GNU Radio project. He is a member of the GNU Radio General Assembly, the GRCon21 Organizing Committee, and is currently the Documentation Lead for GNURadio.