Introduction
Echolink is a VOIP (Voice Over IP) application that allows amateurs to connect to one another using the internet. It is similar to IRLP but allows users to connect via either radio or computer. Application versions are available for PC and Mac computers.
Echolink is not usually described as a digital mode – although VOIP is the digital transmission of voice – as most operators will use an FM radio to connect to the Echolink network.
VOIP connections of any sort – including Echolink – require a DSL connection to the internet. A minimum connection speed of 256KBs is usually recommended for effective, clear VOIP applications.
Using Echolink on a computer
As Echolink is using internet protocols for transmission of voice, various ports must be open in your firewall for it to operate properly, the Echolink website summarising the requirements thus:
- Allow UDP destination ports 5198-5199 between Internet and PC in both directions
- Allow TCP (source port any, destination port 5200) from PC to Internet
Because the firewalls only allow port directs to particular IP addresses – and hence computers – Echolink can in most cases only be used on one computer at a time in a particular household. Some ISP’s may allow multiple unique external IP addresses, in which case Echolink can be used on multiple computers.
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The Echolink application presents a window such as the one below to the user.
The user selects a contact from the list and communication is carried out using the computer microphone.
Using Echolink via a radio
Step by step process
- Look here to find out which nodes are available and where they are located. This list is updated regularly, and gives information about which nodes are connected to each other. If you use Google Earth, this kmz file will show you the connections between Echolink nodes.
- Locate a node close to you and one that you would like to contact. Repeater listings such as this one for Australia, will provide useful information in your search.
- Use a DTMF microphone and dial in the node number that you wish to contact. A message will tell you if you either are successful or unsuccessful.
- There will be a delay between sending and receiving when using Echolink or IRLP. The best protocol is to wait a couple of seconds between hearing the other person and starting to talk yourself.
- When you have finished talking, use the DTMF buttons and dial in 73. This will disconnect the nodes.
EchoLink DTMF Functions
EchoLink can be configured to accept commands through the local radio receiver using DTMF tones (TouchTones). These commands are used to enable or disable the link, or to connect or disconnect a station on the Internet.
Each command consists of a sequence of digits (or the special keys *, #, and A through D). Although a set of default sequences is assigned to each function, any sequence can be customized using the DTMF tab of the Sysop Settings page.
The table below lists each of the DTMF commands.
//Note:// If you have upgraded from an earlier version of EchoLink, you may need to choose “Reset to Defaults” to make all of the following commands available.
Command | Description | Default |
Connect | Connects to a station on the Internet, based on its node number. | num |
Connect by Call | Connects to a station on the Internet, based on its callsign. | C+call+ |
Random Node | Selects an available node (of any type) at random, and tries to connect to it. | 00 |
Random Link | Selects an available link or repeater (-L or -R) at random, and tries to connect to it. | 01 |
Random Conf | Selects a conference server at random, and tries to connect to it. | 02 |
Random User | Selects an available single-user station at random, and tries to connect to it. | 03 |
RandomFavNode | Selects an available node (of any type) at random from the Favorites List, and tries to connect to it. | 001 |
RandomFavLink | Selects an available link or repeater (-L or -R) at random from the Favorites List, and tries to connect to it. | 011 |
RandomFavConf | Selects a conference server at random from the Favorites List, and tries to connect to it. | 021 |
RandomFavUser | Selects an available single-user station at random, and tries to connect to it. | 031 |
Disconnect | Disconnects the station that is currently connected. If more than one station is connected, disconnects only the most-recently-connected station. | |
Disconnect All | Disconnects all stations. | ## |
Reconnect | Re-connects to the station that most recently disconnected. | 09 |
Status | Announces the callsign of each station currently connected. | 08 |
Link Down | Disables EchoLink (no connections can be established). | (none) |
Link Up | Enables EchoLink. | (none) |
Play Info | Plays a brief ID message. | * |
Query by Call | Looks up a station by its callsign, and reads back its node number and status. | 07+call+ |
Query by Node | Looks up a station by its node number, and reads back its callsign and status. | 06+num |
Profile Select | Switches to a different stored set of configuration settings (0 through 9). | B#+num |
Listen-Only On | Inhibits transmission from RF to the Internet. | 0511 |
Listen-Only Off | Restores normal transmission from RF to the Internet. | 0510 |
Connect
The default for the Connect command is to simply enter the 4- 5-, or 6-digit node number to which you wish to connect. To prevent interference with other DTMF functions, however, you may wish to configure a special prefix, such as A or 99.
Link Up and Link Down
No defaults are provided for these functions. To enable these functions, enter a DTMF sequence for each one, using the DTMF tab of the Sysop Settings page.
Profile Select
Profiles are numbered from 0 to one less than the number of profiles shown under File→Profiles. Profile 0 is always MAIN.
Station Shortcuts
Custom DTMF commands can be created to connect to specific stations. These commands are called Station Shortcuts and are not shown in the table above. To manage your Station Shortcuts, click the Station Shortcuts button on the DTMF tab of Sysop Settings.
Entering Node Numbers
To enter a node number (for the Connect or Query by Node commands), enter the 4-, 5-, or 6-digit node number. If the specified node is not among the stations currently logged on, EchoLink will say “NOT FOUND”.
Entering Callsigns
To enter a callsign (for the Connect by Call or Query by Call commands), press two digits for each letter and number in the callsign. The first digit is the key on which the letter appears (using 1 for Q and Z), and the second digit is 1, 2, or 3, to indicate which letter is being entered. To enter a digit, press the digit followed by 0. When finished, end with the pound key (#).
For example, the letter “K” is entered as “52”, the letter “Q” is entered as “11”, and the digit “7” is entered as “70”.
1 Q-11 Z-12 | 2 A-21 B-22 C-23 | 3 D-31 E-32 F-33 | A |
4 G-41 H-42 I-43 | 5 J-51 K-52 L-53 | 6 M-61 N-62 O-63 | B |
7 P-71 R-72 S-73 | 8 T-81 U-82 V-83 | 9 W-91 X-92 Y-93 | C |
* | 0 | # | D |
Callsigns need not be entered in full. If a partial callsign is entered, EchoLink will find the first match among the stations currently logged on. If no match is found among the stations currently logged on, EchoLink will say “NOT FOUND”.
Examples
(These examples assume that the default DTMF codes are configured.)
EchoLink responds with:
“CONNECTING TO CONFERENCE E-C-H-O-T-E-S-T”
followed by
“CONNECTED”
because 9999 is the node number of conference server ”*ECHOTEST*”.
Enter: 0 7 5 2 1 0 7 2 3 3 3 1 #
EchoLink responds with:
“K-1-R-F-D 1-3-6-4-4 BUSY”
because 13644 is the node number of station K1RFD, and K1RFD is currently busy.
Enter: 0 1
EchoLink responds with:
“CONNECTING TO K-1-O-F REPEATER”
followed by
“CONNECTED”
because K1OF-R was selected at random.
Getting Help
The Echolink community provides a number of forums for users to get assistance:
- Echolink Help files These are available in PDF.
- Yahoo group for Echolink users
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Presentation
- Built using Beamer version 3.10 and compiled using Latex version 3.1415926.
Programming Practices
Case Study 1: Quadrature Demodulator
Source code for gr::analog::quadrature_demod_fc.
Use the example fm_demod_example.py (updated for GNU Radio 3.7)
This reads in the file philly_93.3MHz_500ksps.32fc. This file was captured using a USRP N210 with a WBX board in Philadelphia on FM channel 93.3 MHz, a local rock station. The command to capture this using the latest GNU Radio with UHD is:
uhd_rx_cfile -a 'addr=192.168.11.2' -g 25 -f 93.3M --samp-rate=500k -N 5000000 philly_93.3Mhz.32fc
The file is captured at 500 kHz sampling rate, single-precision complex float (the 32fc is an indicator of that).
The GNU Radio Python script provided here will read in the samples from the file, demodulate, filter, and resample the signal and output it to an audio sink so that we can listen to it.
Using the GNU Radio tool gr_plot_psd_c, which reads in complex binary samples and plots the PSD, we can see what the original signal looks like (gr_plot_psd_c -R 500k philly_93.3Mhz.32fc):
When viewing the file, this image has skipped ahead a few frames (using the bottom right arrow) to get away from initial transients present when starting up the hardware collection.
Notice that this is a very strong, clear signal. The PSD shows a fairly narrow FM modulated signal in the middle of the spectrum. When we listen to it, we'll see that it is because there is only the DJ talking at this point in time. Notice also the two flat signals at plus/minus 175 kHz. Those are the high definition digital stations that overlay the original analog signal and could be decoded separately.
Now, when we run the fm_demod_example.py, which has all of the variables hard-coded inside, we can hear the DJ talking about the last song played, a Green Day song apparently. This is only a 10 second capture.
Exercises
1. Add a visualization tool to see the audio output signal (from self.lpf). You can either dump the signal to a file and look at it with the gr_plot_* scripts, your own Python or Matlab code, or you can use a real-time GUI like the qtgui.sink_f block.
2. Add a visualization tool to view the output of the self.demod block. How does this compare to the signal in question 1?
The following values are hard-coded into this example program: Cyberbyte antivirus and internet security premium 3 0 59.
- input sample rate (fs)
- FM bandwidth or sample rate (fm_bw)
- FM deviation (deviation)
- Audio rate (audio_rate)
- Audio passband width (audio_pass)
- Audio transition width (audio_tw)
- Audio stopband attenuation (audio_atten)
3. What happens when you change the input sampling rate?
4. What happens when you change the FM deviation (make it much larger and much smaller)? What is this doing and why does it seem to make sense to reduce it?
5. Increase the audio passband bandwidth. What is happening to the signal? Why does the quality get worse?
6. Adjust any other parts or values in the system to see what happens.
Case Study 2: Costas Loop
Can be found in source code at: gr-digital/examples/examples_costas.py
Source code for costas_loop_cc.
Source code for gr::blocks::control_loop.
Why it's of interest
- A sync block with a loop.
- Inherits from gr::blocks::control_loop; implements:
- advance_loop <- from current error estimate.
- sets and gets for all control values (including: damping factor, loop bandwidth, alpha and beta gains, current frequency and phase estimates).
- Can be used with BPSK, QPSK, 8PSK.
- Two loops if second output of frequency estimate is used.
- Done for performance reasons: reduce branches in inner loop.
Running
- Requires scipy and matplotlib Python modules
- Can run without arguments for default settings: ./examples_costas.py
- Use ./examples_costas.py --help for list of command-line options
- Includes setting things like:
- Number of samples in the simulation
- Samples per symbol
- Roll-off factor for the RRC filter
- Loop bandwidth
- Number of RRC filter taps
- AWGN noise voltage
- Initial frequency offset
- Initial timing offset
- Initial phase offset
Output
Produces a multi-subplot figure showing the frequency response, constellation, and input and output signals for comparison. The frequency response graph should resemble a standard control loop converging to a static frequency offset.
Exercises
1. Adjust the noise in the simulation. You can do this in steps from 0 to 1. What happens to the system behavior as the noise increases?
2. Adjust the loop bandwidth. How does this affect the convergence of the loop?
3. Set the initial frequency offset to 0.01. What happens?
4. What happens when you set the inital frequency offset to 0.1?
5. Set the initial phase to 1. What happens?
6. What happens when you set the initial phase to 0? Why?
COST-TERRA Summer School, Dublin, Ireland
This tutorial is based off of GNU Radio 3.7. Please make sure you have a version of GNU Radio using the 3.7 API.
Make sure that at least the following components are to be build. The output of cmake will tell you this:
- python-support
- testing-support
- volk
- gruel
- gnuradio-core
- gnuradio-companion
- gr-fft
- gr-filter
- gr-audio
- gr-digital
- gr-qtgui
- gr-utils
- gr-wavelet
- gr-wxgui
Study Material
All of the following study material are .grc files that are meant to be opened and run inside of the GNU Radio Companion.
Filtering Example: creates a low pass filter to filter noise with adjustable bandwidth.
Sources and Sinks: demonstrating the use of multiple sources and sinks in a flowgraph.
Representation of Bits: example that tries to demonstrate the idea of bit representation in multiple dimensions.
Command Tab Plus 1 93 Fm Radio Online
MPSK Receiver Development
The following files are various stages of developing an M-ary Phase Shift Keying (MPSK) digital modulation receiver. They go from simple to complex and robust against channel and system impairments.
(Note: due to some quirk in the web hosting service, you may not be able to untar these files directly using the standard 'tar xzf mpsk_script.tar.gz. Instead, first use gunzip and then untar them.)
Stage 1 Terraria multiplayer save world. : using GNU Radio to create a MPSK-modulated signal and view the results in multiple domains.
Stage 2: Add a channel model to simulate noise, frequency and timing offsets, and multipath simulation.
Stage 3: Recover symbol timing (find the optimal sampling point for every symbol).
Multipath Simulation: A simple simulation for exploring multipath as modeled by a FIR filter.
Stage 4: Using the Constant Modulus Algorithm (CMA) equalizer for blind channel estimation and correction.
Stage 5: Correcting phase and fine frequency offsets using a digital implementation of a Costas Loop.
Command Tab Plus 1 93 Fm Radio Station
By the end of stage 5, we should have a receiver that will work with real transmitted MPSK signals under standard operating conditions. We did not look at correcting large frequency offsets, which can be done using a specific frequency locked loop or by estimating the coarse offset at the receiver and letting the Costas Loop pick up the slack.