Digital Radio Post 3 - FLDIGI and PSK-31

Session 3 - Continuing a Summer of Digital Communications Fun
June 24, 2009

This is Curt Black, WR5J with the Educational Radio Net
– Please stand by for a DIGITAL QST

Send File ALLMODE.wav

CHECKINS


Tonight I want to practice more with FLDIGI and work our way through more on-screen controls as we continue to acoustically couple our radios to our computers.

So, before the net please set up your laptop near your radio with a microphone plugged into the computer so that the sound in your shack shows up visually on the waterfall display on the FLDIGI software. Set it up so if you clap your hands you see an indication of that sound moving down the screen.

If this is your first session - go back to the previous blog entries (just lower down on this page) and read about downloading and installing the free software in the Narrow Band Emergency Messaging System (NBEMS) suite. It would be great if you could get your sound card calibrated with the CheckSR.exe software before the net to make sure your copy is as good as possible. If you can't you will still have good copy with MT63, but some of the other modes may not be as forgiving.

PSK-31 - Phase Shift Keying
(at 31 baud and with two tones 31 Hz apart).

As the most popular digital mode on HF, I thought I would be remiss if we didn't start with PSK-31. I want to emphasize that we will be moving on to lots of other modes, so if your eyes are rolling back in your head because this is too pedestrian, have faith, we will be getting to the really cool stuff like WSPR, WSJT and SSTV real soon now. But this week - an intro to the PSK Modes.

I have put some great PDF files of PowerPoint presentations up on the wa-digital group site at

http://groups.yahoo.com/group/wa-digital/files/

Please join this Yahoo group - it seems the best way for me to get some of these great visual resources into your hands quickly.

The two files by Clint Hurd, KK7UQ are from his Anchorage Convention talk last summer in Alaska. Clint lives in Port Orchard and has made interfaces and tools for helping people get the most out of digital modes for many years. Start with the file Alaska Basic Slides.pdf and check out all his information on digital modes. For the adventurous check out the second file, Alaska Advanced Slides.pdf for great tips on adjusting the waterfall for maximum sensitivity and resistance to strong signals in the audio passband. Much more great information is available directly from Clint at his website:

http://kk7uq.com/html/hamfest.htm

Here are some hints for working with PSK31 or any of the PSK flavors.

Hints to New PSK Users

1. Make sure you are putting out a pure signal. Don't overdrive the rig (make sure ALC is showing absolutely no sign of life) You cannot achieve better communications by over driving the audio on any of the digital modes that require linearity. These include all of the modes that have an amplitude modulation component. Some like PSK and THROB have both an amplitude and a frequency or phase modulation component. An overdriven signal may produce more decoder errors than a properly driven signal of lesser power. This is particularly true for all variants of PSK

2. The best way to know what your signal is like is to ask on the bands for a report from others – the software of the person receiving your signal will report out what your intermodulation distortion 3rd order intercept is in decibels (IMD) – you want an IMD value below minus 24dB. You don't need a slide rule to figure this value out - the software calculates it anytime there is an idling PSK-31 station being decoded. It also gives you the signal to noise ratio - you need to pay attention to that before you believe the value you are seeing for the IMD window. If you have a nice -34dB IMD you are transmitting one fine signal. At minus 18 people will probably start giving you unsolicited reports since your signal will be about twice as wide as necessary. At –11dB you will have much of the band after you. For more good news - since we are using FM for these examples, the overdriving part is greatly reduced. But it is extremely important for your HF future as an A-1 Operator.


3. Don't type in all caps. This slows down your transmission, makes it more prone to errors, and is just annoying.


4. Remember that your HF rig is not set up to operate at a 100% duty cycle – voice operations tax your finals much less than most data modes. Lower your power to a level of 50% of what your rig can produce so you don’t burn out your finals. This will help the bands as well since many of our digital modes are detectable at very low power levels. Particularly for the weak signal modes like WSJT – one loud signal in the passband of your radio can make everyone else, especially DX, sadly unreadable. Chances are you will produce good copy at the stations listening to you with just a few watts. That is particularly true when we get to JT65A on 14.076 USB...

5. There is way more to digital operation than PSK31. Tune a little above the PSK activity and call with Hell or MFSK16 or Olivia 16/500. You can use the sked page to arrange experiments with new or more esoteric modes. Try these pages for scheduling some activity –


http://www.obriensweb.com/sked/


http://www.hamspots.net/

These websites aren’t needed for PSK31 – it is nearly always available on the frequencies listed below. But the less common modes are more challenging to find folks for a QSO. Below is a table of common PSK31 frequencies – you will frequently see the wider signals of PSK63 just a little higher.

PSK31 FREQUENCIES
PSK31 HF Frequencies PSK31 VHF Frequencies
Band Frequency Band Frequency
160 meters 1.838 MHz 6 meters 50.290 MHz
80 meters 3.580 MHz 2 meters 144.144 MHz
40 meters 7.035 MHz 1.25 meters 222.07 MHz
30 meters 10.140 MHz 70 centi-meters 432.2 MHz
20 meters 14.070 MHz 33 centi-meters 909 MHz
17 meters 18.100 MHz
15 meters 21.080 MHz
10 meters 28.120 MHz
:

We also have permission to practice on the West Seattle repeater at 441.800 output (up 5 MHz for input)with a tone of 141.3. Just ID with voice before and after your digital transmissions and maybe yield to any voice traffic that wants the repeater... It has been pretty quiet lately. Hopefully that will change as we bring the West Seattle Amature Radio Club back to life...


The yahoo group is another great way to set up a schedule with your fellow Educational Radio Net folk. Just send your message to wa-digital@yahoogroups.com

As we send PSK-31 back and forth tonight - you will probably notice that you can lose copy if the volume gets very loud. The phase shifts that the software is looking for can be really messed up by echos in your shack- particularly from a wall about 16 feet away. The software is looking for phase shifts every 32 milliseconds - sound travels about 1000 feet per second - so that wall 16 feet away is just right to produce an echo at a really bad time - actually all the sound bouncing around your shack is the bad part of acoustic coupling. For PSK-31 just try turning the sound down and getting the mic closer to the speaker on the radio to avoid echos.

I guess, this brings us to the desirability of having an interface between your radio and the computer.

There are some great advantages to acoustic coupling - for one it couldn't be simpler. Additionally, there is no problem with RF getting into the computer when you are transmitting - at least not from the interface cables. But some of the modes we will want to use are going to be increasingly choosy about phase and the timing of sound arriving for processing.

There are lots of choices of interfaces from junk-box parts to the $99 Tigertronics SignaLink to the $360 US Interface Navigator - each has pros and cons.

There is lots of satisfaction from doing things out of the junk box. You can build a very workable interface with jacks and wires. You can build a better interface with isolation transformers. You can build a great interface with optoisolators and at some point, the time begins to make some of the commercial products look pretty good. The more expensive units provide lots of control over the radio, actual FSK keying (as opposed to audio-frequency shift keying, AFSK)and great user interfaces with just the controls you need where you might want them.

We will continue to do an acoustically coupled transmission each week for practice and will probably try a round-robin through the repeater in another week or so. However, eventually I suspect you will want to have an interface to get signals into and out of your computer in the best possible form. So, please consider if your time and financial resources will allow it - it can open up an amazing world.

Please take a look at Clint Hurd's presentations from the Alaska Hamfest last August on the Yahoo Group wa-digital if you haven't already.

See you on the waterfall,

vy 73 de WR5J

Curt Black
black@nwfirst.com