Here is a shot of my computer screen while receiving the ARRL (American Radio Relay League) Propogation Forecast Bulletin which is broadcast from the ARRL station W1AW in Newington CT. Recently the ARRL has started transmitting the bulletins using the digital mode PSK-31. W1AW is a state of the art amateur station with an excellent antenna farm. Geographically W1AW is only a couple states away from my location so the signal is always loud and clear. These informational transmissions are also sent using other modes such as CW and RTTY. For many years hams have sharpened their CW copy skills by listening to the ARRL bulletins broadcast in Morse Code.
Think of PSK-31 as instant messaging by wireless without an internet connection, phone lines, 3G networks and associated baggage that we pay money for every month. PSK-31 was developed by Peter Martinez, G3PLX and introduced into the amateur community in December of 1998 as a way for hams to communicate in a live chat keyboard to keyboard fashion. Note the deep blue area at the bottom of the screen. This is called the waterfall. It is basically a graphical representation of a 3 KHz slice of radio spectrum. When a digital signal appears in this piece of bandwidth it is shown as a yellow stripe that starts at the top of the waterfall and slowly decends. When the transmission ends so does the stripe, the tail end dropping down and finally out of sight off the waterfall. This visual display lets the operator tune in on the narrow signal and allows the software to begin decoding the digital signal into readable text that appears in the larger receive window above. The incoming text prints in real time one character at a time as the sending operator types.
The integration of computers into ham radio is an amazing and ever changing facet of this great hobby. Most of this is very technical and flies way over my head but I am learning and having fun at the same time. One purpose of this blog is an explanation of my hobbies and activities for friends and family. For that reason I try to keep things from turning into a dry technical report. Writing about the digital modes is a real challenge. I barely understand how much of this stuff works myself, let alone try to explain it in layman's terms.
When I am ready to transmit I begin typing into the orange window and the information from my keyboard is encoded into audio tones by the software and a dedicated sound card. Shown below is the Signalink USB interface. This piece of equipment is the link between my laptop and the transmitter. The Signalink has a high quality soundcard that is dedicated to ham radio duty so the laptop's inferior soundcard can be left to do it's thing for the computer. I'm a knob guy so I like the Signalink for the manual controls on it's front instead of accessing windows on the laptop to manipulate sliders controlling the soundcard functions.
Basically the Signalink and it's soundcard are like a modem. The information encoded into audio tones is fed to the transmitter by this device and ultimately radiated out into the ether. The shielded USB connection cable is not visible but it's there plugged into the back of the unit and drops down behind the table top. Here is where things get interesting and more confusing. The TX knob on the Signalink controls the amout of audio "drive" to the radio. To provide the most efficient and "clean" signal this drive has to be carfully controlled. The TX knob on the Signalink works in conjuction with the radio's ALC or Automatic Limiting Control. ALC limits the RF drive level to the power amplifier during transmit to prevent distortion. To adjust for proper operation I switch my rigs output coax to a dummy load. This is just a giant resistor that absorbs the RF signal and bleeds it off as heat instead of sending it on to an antenna. While transmitting a PSK signal I watch the ALC meter on the rig. If the TX knob on the Signalink is turned up too high one or more bars will show on the meter. Backing off the TX knob until no indication of ALC is evident on the meter lands you in the sweet spot and the ideal balance of drive power is met. This is why I like the Signalink USB. My laptop's operating system is quirky enough and I think it would be a real pain to minimize fldigi (digital mode software) and go to the control panel window and jump through those hoops just to make a small adjustment to the soundcard drive.
That's it! Once the ALC conditions are met and the output is switched back to an antenna all systems are go for digital radio fun.
Click on the picture for a close up view and look at the yellow signal on the waterfall display. You will notice two fine red lines that straddle the yellow strip. This is the tuning indicator and is movable by using the mouse. If you see a signal and click on it the two red lines will jump to that spot and the software will begin decoding. The same applies when transmitting. Find a clear space on the waterfall, click on it and where the red indicator rests shows the exact frequency of the transmitted PSK signal.
Last night I spotted a bright PSK signal on 30 meters. I copied the callsign that belonged to the trace and when the QSO was over I gave Jean VE2GHI in St Georges, Quebec a call. He responded and we had a pleasant textbook PSK-31 QSO. I am very proud of the unsolicited comment that Jean made concerning my signal. The tricky part with the digital modes is that you cannot see your own signal on the waterfall so you can't tell if it is overdriven. This is why monitoring the ALC is so important. With the system properly adjusted 15 watts is more than adequate to send a good clean signal just about anywhere.
Note that there is no yellow stripe under the red indicator on the waterfall. I took the photograph after our QSO ended. The indicator shows where Jean's signal was. To the right are two bright signals and a very faint one just visible.
PSK Facts
PSK stands for Phase Shift Keying.
A PSK-31 signal bandwidth is 31.25 Hz wide. Hence the name PSK-31.
Because of the very efficient and narrow bandwidth PSK-31 is well suited for low power operation and less than ideal antenna systems.
PSK-31 contacts can be conducted with about 100 Hz separation between signals without interference. This means that twenty hams can carry on PSK-31 contacts in the same amount of bandwidth required for one sideband voice contact.
PSK-31 is probably the most widely used and popular of the digital modes in amateur radio.
One more thing to add to my list of Wants. As of right now, I just want to be able to sit with a real radio in front of me and hear voices come back at me when I call CQ. But I must say, to hook a computer into an HF/VHF/UHF transceiver and type on a keyboard and see a response on the screen wouldn't be so bad. I've had enough of computer to computer. Back to the air.
ReplyDeleteI hear you Norm. I have a love-hate relationship with computers but because of ham radio I have learned a great deal about them in a short time. Once I discovered the digi-modes I was hooked. Still many I have not yet tried.
ReplyDeleteYour right though in your situation with the station building phase just beginning you have plenty on your plate worry about.
For fun once you are operational on HF you can simply download and install some freeware and just stick your computer's mic in front of the rig's speaker and it will decode the sigs. You just can't transmit without an interface of some kind. This is what the short wave listeners do.
Good luck Norm and vry 73
This is really interesting. I'm scared. I can't afford any more hobbies... I have been away from radios since 1996 (and then only used CB's) but it's great to see that today's modern technology is being used for radios as well. It's so interesting!
ReplyDeleteI had no idea this digital stuff was being done with ham radio when I first got started. It is amazing.
ReplyDeleteCool thing is today there is no code test period. Just pass a simple multiple guess test and your in.
I hear you on the hobbies. My hobby is collecting hobbies, I gotta stop!