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freedv - Digital Voice for HF
FreeDV is a GUI application that allows any SSB radio to be used for low bit rate digital
Speech is compressed down to 1400 bit/s then modulated onto a 1100 Hz wide QPSK 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.
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.
New Upgrade as of March 2013
The new version 0.96 provides a 1600 bit-per-second mode that communicates at much lower
signal levels than previously. Communications should be readable down to 2 dB SNR, and
long distance contacts are reported using 1 to 2 watts power. A compatibility mode for
communication with the older 0.91 version is included.
Amateur Radio is transitioning from analog to digital, much as it transitioned from AM to
SSB in the 1950s and 1960s. How would you feel if one or two companies owned the patents
for SSB, then forced you to use their technology, made it illegal to experiment with or
even understand the technology, and insisted you stay locked to it for the next 100
years?? That is exactly what was happening with digital voice. But now, hams are in
control of their technology again.
FreeDV is unique as it uses 100 percent 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.
Watch this video of a FreeDV QSO.
Here is what you need:
A SSB receiver or transceiver
A computer with one (receive only) or two sound cards.
Cables to connect your computer to your SSB radio.
Test your Transmitter Frequency Response
When you play this 10 second 1 kHz to 2 kHz sweep .wav file(external link) 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 "processed". The audio processing in your radio
does not help any digital mode.
Configuring Your Radio
Turn off as much processing as possible. In general noise blankers, DSP band limit
filtering, and narrow bandpass filters are likely to hurt rather than help. Compression,
DSP noise and carrier elimination, and voice processing are definitely wrong for Digital
modes. FreeDV's FDM modem does its own DSP, and in general this is true for other digital
programs as well. The only things that we would expect to hurt the signal are intrusion of
the opposite sideband, images of out-of-passband signals, and intermodulation distortion.
You can see the effect of different settings in the S/N display of FreeDV.
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 the FDM 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. Ask manufacturers and reviewers to start rating linearity and headroom for digital
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