Listeners to the Arlington County Net and subscribers to the w4wvp-members mailing list have heard and seen a lot recently about the MT63 multi-tone digital modulation technique. Why?
Some Arlington Amateur Radio Club (ARC) members took an interest in digital communications at the July 2011 meeting, when Ken, WB4AKK, and I (assisting, with no speaking lines) demonstrated use of FLDIGI software for several modes of digital communication. Interest in the MT63 mode, in particular, ramped up when Kim, KD9XB, announced on the June 5, 2012, Arlington County Net that he had checked into the Fairfax County ARES Net and found them using MT63 to transmit data, such as rosters of check-ins, over the repeater. They use it because the MT63 modulation scheme, using a 2000 Hz audio bandwidth (MT63-2000 or -2k), efficiently uses most of the audio spectrum available on ham VHF/UHF FM voice radios and repeaters to achieve good throughput of text and (with the FLWRAP application) binary data, such as images or word-processor documents. It has some drawbacks, such as long latency (delay from time text is entered until it is decoded), which make it unattractive for keyboard-to-keyboard chats on HF (for that, try PSK31), but it is very good if one has, say, dozens of FEMA, Red Cross, Hospital, or other specially formatted messages to communicate using common ham gear (HTs, mobile VHF/UHF FM transceivers, and laptops running Windows, Mac OS-X, or various GNU/Linux distributions.
Other digital modes are also used on Fairfax County ARES Net, and you may want to try them, simplex or on the W4WVP/R. In any case, please give voice traffic priority on W4WVP/R. For the post-Arlington County Net digital sessions, let’s first strive to communicate among ourselves using MT63-2000.
As noted, FLDIGI software may be used for MT63 on PCs running Windows, Mac OS-X, or GNU/Linux. It is free and open-source. But other software can also do MT63, such as the Digital Master 780 (DM780) program in the Ham Radio Deluxe software suite.
There is also an MT63-2000 app for devices running AndroidOS. [Thanks to KI4DHW and KD9XB for this info. N4ERD has looked for an app for iPhone but found none, so far.]
When setting up software for MT63 on the Arlington County Net, choose the 2000 Hz bandwidth option and the [2012-08-18 update by KD9XB:] short interleaving option. The extended character set option is probably not needed, but we may learn more about this as we experiment.
You may interface your PC to your transceiver acoustically (the air-gap method) or using an interface cable and associated circuitry. The air-gap method is easiest; I recommend you try it first. You may decide you want to invest the effort to construct an interface cable to make operating more convenient.
Some interfacing information is provided in the FLDIGI Manual. One size does not fit all; the interface must be compatible with your transceiver and the connectors of your PC.
For users of the Tigertronics SignaLink USB Interface, Tigertronics provides interfacing information and hardware for many transceivers.
KA8OPJ shared this info on interfacing his Yaesu FT-817ND:
Various radios take different cabling, and sometimes different sets for particular modes. Also, the PTT driver (unless you actually push the button) may take external circuitry. Audio coupling has its advantages!!
- My old Baycom packet controller (the same model NW3V used to have) took only two 2-conductor mono plugs to an ICOM IC2AT HT: one to the microphone and one to the earphone. Because it used a serial port, I hadn’t used it in several years: There was no serial port on my recent computers.
- My FLDIGI PSK31 to Yaesu FT817ND “DATA” connector (mini-DIN, like the plug from a $3 PS/2 mouse) took a $15 USB-to-serial converter plus a small switching network (two resistors, one diode, and one 2N2222 transistor). However, it does NOT have the microphone and earphone connectors needed for MT63, because they are NOT present on the DATA connector. This is similar to the SignaLink adaptor. My total investment for PSK31 was $15 plus an hour of assembly time, plus a trip to MicroCenter for USB-serial. WB4AKK gave me the min-DIN connector.
- My FLDIGI MT63 to FT817ND connections use a more complex network, similar to the one in the FLDIGI instructions. The microphone, earphone, and PTT lines are all transformer-isolated. The PTT uses the tone from the right stereo channel of the computer rather than the manual switches some of the folks use. Instead of a bipolar transistor like 2N2222, I used a HEXFET. The radio’s earphone connector is mono. The radio’s microphone and PTT connector use the RJ45 microphone connector, but there’s no microphone or earphone line on the DATA connector. (KK4JQR please note.) Parts took a trip to MicroCenter; assembly >3 hours.
- My Yaesu VX3R has a 3-conductor stereo earphone plug on the side: This can be used for listening, by connecting directly to the computer’s microphone plug. The 4-conductor microphone+stereo earphone plug on the top uses the FT817ND network, plus it needs additional circuitry (one resistor + one capacitor) to combine microphone AC (left stereo channel from the computer) and PTT DC (right stereo channel through the transistor).
Again, audio coupling for MT63 took <5 minutes to install FLDIGI, and no additional hardware ….
For users of Yaesu FT-250R HTs, the CT-44 accessory may be useful. The FT-250R user manual provides interfacing information.