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The VARA Modem is a relatively new high-performance Windows "soft-modem" a.k.a. "soft TNC" soundcard app for transmitting data over radio. It was developed by EA5HVK.
VARA comes in two versions. One is optimized for operation in the noisy interfence-plagued HF spectrum over SSB transceivers. The other takes advantage of the greater audio bandwidth of FM transceivers on VHF and UHF to achieve far higher data rates.
The VARA HF version is widely used with the Winlink
radio email system, as an alternative to the very expensive (USD $500 or more)
hardware-based SCS Pactor 4 modems. VARA's IP-based interface allows it to be used
as the transmit/receive modem for a variety of applications, including the
Winlink RMS radio email mail system, or the VarAC
keyboard-to-keyboard live chat program.
VARA also has a packet-TNC-like "KISS" interface.
allowing it to used as a replacement "TNC" for any application that would
normally use a classic 300- , 1200-, or 9600-baud AX.25 packet TNC. This
includes APRS, BBS systems, specialty EMCOMMS programs, etc. VARA offers vastly
superior noise immunity and transmission speeds compared to classic AX.25
packet.
VARA-HF can exceed the data transmission speed of a 1200-baud VHF packet TNC in a 500-Hz bandwidth (i.e. CW filter) on an HF/SSB transceiver. Using the full voice bandwidth of an SSB transceiver (2300 Hz), VARA can exceed 7000 bits per second.
VARA-HF Speeds in SSB and CW bandwidths:
VARA-FM can exceed 12,000 BPS when used with a traditional mike-&-speaker hookup like 1200-baud packet. When used with a wide-band "9600-baud packet"-type audio input on the 6-pin MiniDIN "data port" on many radios, VARA can exceed 25,000 BPS on a strong-signal path on VHF.
VARA "FM Wide" speeds need to use the rear-panel "data port" -- often a 6-pin mini-DIN connector -- with the radio set for "9600 baud" packet. "FM Narrow" speeds can use mic/speaker connections on an FM radio.
VARA-FM Speeds in Wide and Narrow
Bandwidths:
Further, VARA uses very sophisticated forward error correction. It can reliably receive data at very low signal-to-noise ratios,and over HF radio paths plagued with selective fading, multi-path and just plain weak signals buried in the noise. It can provide error-free copy (slowly) on signals that literally can not be heard in the radio speaker.
The highest speed modes of both the HF and VHF-FM flavors of VARA are unlocked by purchasing a license for USD $69 from the author's web site. Even the lower speeds available with the free version are far faster than classic AX.25 packet on both HF and VHF-FM.
VARA uses a very complex audio waveform that uses 52 simultaneous QAM audio subcarriers. This means it is far less tolerant of incorrectly-set audio levels and distortion in radio audio channels, either on the transmit or receive end. This image is simplified - it only shows 11 of the 52 audio sub-carriers actually used by VARA.
From the VARA website:
The VARA modems can be down-loaded (and
purchased/registered) from the author's web
site at:
https://rosmodem.wordpress.com/
The VarAC Chat program can be down-loaded from the
author's website at
https://www.varac-hamradio.com/
Note the free versions of the VARA modem are speed-limited. To get the maximum speeds VARA is capable of, you must register/purchase the program for about USD $70. For use with the slow-speed VarAC chat, the free versions will work perfectly.
