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Although my favorite digital (finger-powered) mode is CW, I dabble in others from time to time, bagging DX stations that don’t CW, assessing propagation or trying something new.


Quick links

  1. Digimode hardware (sound cards)
  2. MMVARI multimode software
  3. JT65 and other JT modes
  4. Hinson tips for FT8 Updated Jan 13 - on a separate page and now available as a PDF download
  5. WSPR and WSPRnet plus SDRs
  6. QRSS - dead slow computerized CW
  7. Operating tips for digital DXers - general advice, not specific to any mode


Digimode hardware (sound cards)

Box shot 449 GIFA few years ago, this section would have covered Creed teletypes and terminal units full of 88mH chokes. These days, we’re mostly using PC sound cards to code and decode all manner of digital mode signals with DSP.

For years a SoundBlaster Audigy PCI card worked fine for me, better than the motherboard sound system anyway (the motherboard still runs my PC audio , so I don’t accidentally transmit the bleeps, CDs and movie soundtracks). After installing Windows 8.1, the Audigy card played up and eventually refused to run, so I replaced it with a USB sound system, an ASUS Xonar U7 7.1 USB soundcard and headphone amplifier. I chose the U7 because it:

  • Uses a “high definition” Cirrus Logic CS4398 DAC (Digital-to-Analog Converter) audio chip capable of 192kHz 24-bit audio sampling;
  • Has a claimed SNR of 114dB and low THD which I guess helps dig out the weak among the strong ones (although I suspect the receiver noise floor and band noise predominate);
  • Has a stereo microphone/line input, alllowing me to record and decode signals from both receivers in my K3 simultaneously (e.g. a DX station and his pileup on split, or two different 10m beacons);
  • Is an external USB device, dead simple to install and easier to reboot than PCI cards or motherboard sounds if (when!) I need to do that;
  • Is packaged in a nice little slimline box, USB powered, with standard 3.5mm stereo audio connectors on the back and a volume knob on top;
  • Is a current, supported product, certified for Windows 8.1, from a well respected supplier;
  • Costs about US$80, better value than those fancy USB sound boxes aimed at the ham market and better specified than the sub-$10 USB laptop sound dongles flooding the market.

Sometimes the right channel stops recording, and occasionally the WSJT-X waterfall fills with a spurious ladder-pattern of bars, a bit like OTHR or some weird new wideband digital mode. I suspect it’s RF getting in. Re-starting WSJT-X, power-cycling the Xonar by unplugging and reconnecting the USB cable, or a full PC reboot cures it for a while. I’ll try fitting HF-rated ferrite suppressors to the cables to see how it goes ... no cure as yet ...

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mmmmmmmmmMMVARI

I use MMVARI through Logger32 for RTTY and PSK. Running it within Logger32 means I can log QSOs as I make them, and ‘new ones’ are highlit for me on the decode panes. Here’s a typical screenshot of the MMVARI window within Logger32:

MMVARI screenshot using RTTY

    A: This area is the decoded text from the signal selected on the waterfall (see D).

    B: The text in red is my transmission.

    C: This is the transmit text area where I can type-ahead/buffer and edit text before transmitting it.

    D: ... is the waterfall. Audio from the rig passes to the sound card and is analyzed here by frequency spectrum over time. MMVARI/Logger32 makes an attempt to display the on-air frequencies, combining  VFO frequency information from the rig with the audio frequency. The two vertical red bars at the top show the sound card transmitting my message, centred where the little triangle points (I clicked there to select UN1L’s frequency). Below that is a mostly blue specled area of received noise with 5 pairs of bars representing 5 RTTY signals within the rig’s 2.7kHz audio spectrum. Below that are three rows of 12 macro buttons, 36 in total. The bottom line is a status bar with other operating information (e.g. ‘Net On’ means I will transmit at the same frequency that I receive, while ‘AFC On’ means the decoder will adjust itself across a small frequency range to peak the received signal).

    E: The MultiRX button opens another decode window ...

MMVARI screenshot MultiRX channels

    A: Below are 24 rows of decoded text from 24 simultaneous decoders spread evenly across about 2.5kHz of audio spectrum. [The number of decoders or channels is user-configurable]

    B: The decoded text flows right to left. Logger32 highlights CQ calls with green backgrounds, and callsigns with yellow which is the colour that I chose for new countries on this mode and band (same for DXcluster spots). Reports are highlit in grey. I can click any call to pop it in the log entry window, look up the country, check for previous QSOs etc. Logger32 also substitutes the chosen callsign in place of $call$ in any macros.

MMVARI can do a lot more than that, such as PSK and other modes, but that’s enough for now. Go explore it yourself!

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JT65, JT9, FT8 ...

Professor Joe Taylor (K1JT) from Princeton University has developed the JT narrowband digital modes for extreme weak signal work. Using DSP/sound card and software processing, audio signals well below the level discernable by ear can be decoded successfully. By integrating the audio over several seconds, the signal can be distinguished from random noise. Forward Error Correcting encoding means that (with rare exceptions) messages are either correctly decoded and displayed, or not displayed at all.

WSJTX misdecode

I doubt someone transmitted the message “+7+/4ZS7JTJ0L” on 15m FT8!

The downside is a low bit-rate meaning tediously long overs. The upside includes working the world with QRP or QRPp, and efficient use of the shared spectrum.

FT8 is a faster, narrow-band digimode that is proving extremely popular on HF. Each FT8 over lasts 15 seconds, taking at least a minute to exchange and confirm receipt of callsigns, locators and reports (niceties such as 73s take another 30 seconds). The downside of the extra speed (compared to, say, JT65) is less sensitivity and very little thinking time about what message to send next in response to the message received and decoded. The sensitivity is still remarkable, hence a few watts are generally adequate if a band is open. Automating the normal sequence of standard messages reduces the thinking time issue, at the cost of turning us into passive observers as our computers make QSOs for us.

Under the hood, there’s some complex mathematics going on. The WSJT-X help file tells us:

“Forward error correction (FEC) in FT8 uses a low-density parity check (LDPC) code with 75 information bits, a 12-bit cyclic redundancy check (CRC), and 87 parity bits making a 174-bit codeword. It is thus called an LDPC (174,87) code. Synchronization uses 7x7 Costas arrays at the beginning, middle, and end of each transmission. Modulation is 8-tone frequency-shift keying (8-FSK) at 12000/1920 = 6.25 baud. Each transmitted symbol carries three bits, so the total number of channel symbols is 174/3 + 21 = 79. The total occupied bandwidth is 8 x 6.25 = 50 Hz.”

Most of that flies way over my head ... but there are enough clues in there to start exploring the protocol, if not designing the Fast Fourier Transforms (whatever they might be) to decode it. Meanwhile, I’m happy to use it, in the same way that I drive a car without being a metallurgist.

I believe there are several programs capable of running the JT modes on HF. I’m using WSJT-X written by Joe K1JT himself plus a team of elves. It sends info on logged QSOs to other programs via UDP: Logger32 can now be configured to receive and use the QSO info.

JTAlert by VK3AMA is another way to supercharge the alerting on new ones within WSJT-X.

The Hinson Tips on HF DXing with FT8 are now on a separate page.

Click to download this

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WSPR and WSPRnet

WSPRnet (“W eak S ignal P ropagation R eporting net work”) is an HF propagation monitoring system using WSPR, a weak-signal data mode similar to JT65 that uses digital audio processing on PC soundcards to encode and decode very narrowband (just 6 Hz!) QRP data transmissions. Each transmission takes 2 long minutes to transmit the callsign, locator and power. QRP and QRPp signals are routinely received over thousands of kilometers when the bands are open. In the best amateur tradition, the software was written and released free-of-charge by K1JT to help others experiment. The really neat thing about it is that the program automatically uploads details of transmitters and receivers to a website where the information is listed and mapped in near-real -time, so you can see at a glance where in the world your QRP signals are currently being heard on whatever band you are using.

Here’s a screenshot (with some notes for new WSPR users since the help file is rather basic):

WSPR screenshot

WSPR measures transmitter power in dBm rather than milliwatts or watts, so here’s a conversion chart to make things easier:

Power to dB conversion chart

 

 

 

The chart is a simple Excel spreadsheet. By all means download it if you want to print it for the shack wall or calculate different equivalents.

 

 

 

 

 

 

 

 

 

 

A similar project is running on PSK31 mode - see PropNET.org.

My pal Holger ZL2IO is currently experimenting with Skimmer using the Red Pitaya SDR sold as a learning/development system and bench test instrument for about US$260. It can monitor 6 HF amateur bands simultaneously. The high impedance input needs a wideband pre-amp on a typical 50 ohm feedline, and it is vulnerable to local transmitters so it’s best placed at a remote receiving site - like maybe our previous contest club site along the coast North of Napier. We’ll see.

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QRSS

QRSS VD by AJ4VD is easy tonuse: literally within a few minutes of downloading and running the program, I received the KC0TKS QRSS 10m beacon on 28.221.510 sending “TKS” with QRSS30 (30 seconds per dot). Here’s an image from QRSS VD (click it for a much bigger version):

KC0TKS QRSS30 10m beacon

This grainy image may not look like much to you but I find it amazing, given that the beacon is transmitting just 25 mW and is about 12,000 km from ZL (that’s 480,000 km per watt, roughly ten times further than I have ever achieved by ear, even on an outstandingly good day)!

The QRSS beacon was totally inaudible by ear, of course, but integrated over about 10 minutes the Morse characters are clearly readable. We can even make out a tiny chirp as the transmitter drifts about 1 Hz HF during each transmission and a short blip after the S character (which was actually his full callsign sent in normal CW at 13 WPM, acting as a punctuation mark before the sequence repeats).

At the time this was recorded, 10m was in good shape: I could hear numerous conventional QRP beacons and stations in the USA and central America, even Junior, a mobile Jamaican in the Kingston traffic. It will be interesting to monitor KC0TKS and other QRSS beacons for signs of life when the band appears to be dead but is, in fact, merely resting.

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Operating tips for digiDXers

In the same vein as my tips for those chasing or being DX on CW and phone modes, here is my advice for digital mode DXing.

 

Pileup-beating advice for digiDXers

Pileup management advice
for digiDX stations

Lock your RX frequency on the DX and turn off all automatic tuning (e.g. AFC and NET in MMTTY).  Manually select your TX frequency, lock it or pop it in the memory and for sure don’t touch that VFO if the DX calls you! Keep to a sensible range but look for a quiet spot away from the DX stn’s TX frequency (up to 1 kHz away on PSK, probably more on RTTY) and stay put for a while. If the DX seems to drift off frequency, use the RIT on your rig to keep them in tune rather than moving your VFO or adjusting the receive frequency in software. Don’t forget to listen on your chosen TX frequency and watch the waterfall to make sure it’s still clear.

Stick to the preferred band segments for each mode, especially if this is a condition of your licence. Avoid the beacon frequencies and, of course, listen first to find a clear spot.

Always split. Never operate simplex. Avoid listening too close to your own TX frequency. Turn off all automatic tuning and lock your TX frequency to avoid wandering across the band. Use suitable filters to pick out individual callers. Remember your responsibility to tune within a limited range to avoid spreading the pileup out too far. If stations are clearly not hearing you well, double check that your TX frequency remains clear and don’t forget to send “UP” or “QSX 14085-6” or similar.

Setup your digital mode software and macros appropriately . When calling DX in a pileup, give your own callsign two or three times and listen. It is not normally necessary to include the DX callsign in your calls.

Get your macros ready e.g. in MMVARI: “$transmit$ $mycall$ $mycall$ $receive$” and “$transmit$ $call$ TU $sentrst$ $mycall$ $receive$” (note: using $sentrst$ lets you send genuine reports from your log)

Make sure you can actually copy the DX properly and have his call correct (DXcluster is peppered with busted calls).

The worst thing for callers is not knowing who got called due to QRM.  Repeat a caller’s callsign at least twice and give your own callsign frequently, especially if there are other DXpeditions active at the same time.

Get your macros ready e.g. in MMVARI: “$transmit$ CQ DE $mycall$ $mycall$ UP 1 $receive$” and “$transmit$ $call$ $sentrst$ $call$ $receive$” and “$transmit$ $call$ TU $mycall$ UP $log$ $receive$” (note: the leading and trailing spaces are important for readability)

Be consistent and get into a rhythm for more efficient QSOs and to reduce out-of-turn calling.

Use multi-frequency decoding software if possible e.g. $multirx$ in MMVARI. Monitor the pileup to identify who is working the DX and so where he is listening. Look for holes in the pileup in which to transmit. Stay well clear of the DX station’s frequency and respect the band limits.

Use multi-frequency decoding software if possible e.g. $multirx$ in MMVARI. Monitor the pileup to identify numerous callers simultaneously. Avoid working callers too close to your TX frequency or out of band, and try not to let your pileup spread too far (no more than a few kHz please).

Do not overdrive your transmitter.  Apart from perhaps overheating and damaging it, your signal will probably become unreadable and create QRM for others.  This is especially important if you are using AFSK with tones generated by a PC audio card. Use your rig’s transmit monitor function for a simple but crude quality check.  If you have a separate receiver or a monitor scope, listen to/monitor your own data transmissions to check the levels.  If not, find a local ham who is willing to help you conduct some tests.  If you have trouble contacting reasonably strong stations normally, and especially if you receive reports indicating poor quality signals, check those settings again.

Look out for well modulated signals and make certain your own signal is clean. Do not overdrive your transmitter and be careful not to knock the microphone or PC audio level controls once set. It pays to keep a written note of the correct settings. You should really have figured those out before you left home but small adjustments may be needed in the field: ask a local to check the quality and width of your data signal when you first set up, and act on any adverse signal reports, such as ghost signals either side of your transmission.

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