Speech is compressed down to 1600 bit/s then modulated onto a 1.25 kHz wide 16QPSK signal which is sent to the Mic input of a SSB radio. On receive, the signal is received by the SSB radio, then demodulated and decoded by FreeDV. Communications should be readable down to 2 dB S/N, and long-distance contacts are reported using 1-2 watts power.
FreeDV was built by an international team of Radio Amateurs working together on coding, design, user interface and testing. FreeDV is open source software, released under the GNU Public License version 2.1. The FDMDV modem and Codec 2 Speech codec used in FreeDV are also open source.
FreeDV is unique as it uses 100% Open Source Software, including the audio codec. No secrets, nothing proprietary! FreeDV represents a path for 21st century Amateur Radio where Hams are free to experiment and innovate, rather than a future locked into a single manufacturers closed technology.
Here is what you need:
- A SSB receiver or transceiver
- FreeDV software, download links are below.
- A Windows or Linux PC with one (receive only) or two sound cards.
- Cables to connect your PC to your SSB radio.
Test your Transmitter Frequency Response
When you play this 10 second 1 kHz to 2 kHz sweep .wav file through your transmitter, the power level should remain constant. If not, look for filtering and processing to turn off.
Connecting Your Radio
If you are lucky enough to have a "9600" input and output on your radio, this is the best connection for every digital mode, even 1200 packet, and your audio box should be configured for 9600 or "no pre-emphasis/de-emphasis" if it has that setting. If the radio's configuration menu has a 1200/9600 setting, leave it permanently on 9600.
The "9600" and "1200" settings are misnamed. "9600" should really be called "direct connection", and "1200" should be called "pre-emphasis". The pre-emphasis that comes with the 1200 setting doesn't help any digital mode. The 9600 connection is the most direct and unprocessed path to the modulator and demodulator of your radio.
Those who don't have a special connection for digital modes can use the normal audio inputs and outputs of your radio. The same cables and hardware that you use for other digital modes that are based on PC programs will work with FreeDV, but you will need a second sound interface for the microphone and speaker connections to the FreeDV program. A USB headset of the sort used by gamers is all you need for the second sound interface.
Configuring Your Radio
Turn off as much processing as possible. In general noise blankers, DSP band limit filtering and narrow bandpass filters are more likely to hurt than help, while compression, DSP noise or carrier elimination, and voice processing are definitely wrong for Digital modes. FreeDV's HF modem does its own DSP, and in general this is true for other digital programs as well.
You can see the received effect of different settings in the S/N (signal to noise ratio) display of FreeDV. A higher S/N is better.
Drive your transmitter and amplifier so that it emits 10% to 20% of its rated power continuously. There is a 12 dB peak-to-average power ratio in our HF modem, and peak clipping in your amplifier will reduce the received S/N. Modern transmitters and amplifiers are only as linear, and only have as much headroom, as is necessary for voice SSB, thus we suggest you maintain amplifier headroom by operating well below your full power output. FreeDV is more efficient than SSB voice, and will achieve similar range to an SSB signal driven at higher levels, and better audio quality. We encourage you to ask manufacturers and reviewers to start rating transmitter and amplifier linearity and headroom for digital modes, not just SSB voice.
For more info, visit http://freedv.org/tiki-index.php