Tech Net July 27, 2001

Click To Hear PSK31

PSK31 (and SSTV, MFSK, etc.) Interface and Information for the "multimedia" EPARA Tech-net

PSK31 Basics (very basic):
PSK31 is a digital mode of communications, designed for efficient and reliable keyboard-to-keyboard communications via radio. This mode uses the soundcard in your computer (and freely available software) as a type of modem - to convert your typewritten messages into an audio modulation signal, and to convert received audio signals into type-written messages.

To take advantage of this fun mode, you need to first connect your radio to the soundcard in your computer. The audio output of your rig must be interfaced to the MIC or LINE-IN input of your soundcard, and the SPEAKER or LINE-OUT of the soundcard must be interfaced to the audio or MIC in of the rig. Lastly, the rig must be "keyed" by the software for transmitting. This can be accomplished by using VOX on you rig, or by using a serial port of your computer. We'll discuss a couple of circuit options to to this too.

The next section will deal with the interface details. Additional information and resources for PSK31 (and SSTV) are listed after the interface information.

By the way, this same interface can be used (along with freely available software) to receive and transmit SSTV images. Software available today performs the audio encoding/decoding and displaying of SSTV images right on your computer. So, by building this simple interface, you open up at least two additional modes of operating!

There are many PC-soundcard to rig interfaces available, such as the RigBlaster, which work just fine. If you are interested in a simple plug-n-play solution they will work fine. But, if you are interested in building something yourself, saving a little money, and getting that great feeling of accomplishment - then stay tuned...

PSK31 to PC/soundcard interface:
As mentioned above, there are two or three simple circuits that need to be constructed for the interface:
1. Receive Audio: Radio --> Soundcard
2. Tranmsit Audio: Soundcard --> Radio
3. Tx Keying: PC serial port --> Radio (or use VOX)

Just about any soundcard in a modern PC will work just fine for PSK31. It is preferable for the cards to have LINE IN and LINE OUT ports for the audio interfacing. The LINE interfaces are desirable because their input and output levels are basically fixed, which eases the interface. If your soundcard doesn't have the LINE IN and LINE OUT, you can use the MIC IN and SPEAKER OUT ports. When using the microphone and speaker ports, it may be necessary to adjust your audio output level and recording input level (windows mixer controls) when operating PSK31. Either way, the same interface configurations work.

The interface on the RIG side can also take many forms. Many folk simply use the SPEAKER or HEADPHONE output for the receive audio output to the soundcard, and the MIC input for the transmit audio input from the soundcard. Using these ports, you'll probably have to make sure the AF GAIN and MIC gain are adjusted properly to avoid overdriving the transmitter and soundcard inputs. Some rigs have separate audio input and output ports that are designed for packet, etc. These ports may be more convenient to use if your rig has them because you won't have to change your AF and MIC gain settings when you change from phone operation to PSK31. Again, the same rig-soundcard interfaces work with either configuration.

So, here are some ideas on these interfaces:

1. Receive Audio: The audio output of the rig is generally larger than it needs to be to drive the input of the soundcard, particularly if you are using the MIC IN on the soundcard. The simplest interface is to simply use a resistor divider to knock the amplitude down. The picture below shows this simple interface. A simple resistor dividing of 100:1 will drop the audio level into the millivolt level required by the MIC input. If you are driving a LINE IN, such as a packet input, then a 10:1 divider (or even 1:1) may be more adequate. It depends on how you adjust your audio output level from the rig.

2. Transmit Audio: Generally, the audio output from the soundcard, whether its from the speaker output or the line output, is much larger than it needs to be to modulate the transmitter, particularly if you are driving the MIC input of the rig. The simplest interface is a just a resistor divider to knock the soundcard amplitude down. The picture below shows this simple interface. A simple resistor dividing of 100:1 will drop the audio level into the millivolt level required by the MIC input. If you are driving an auxiliary audio input, such as a packet input, then a 10:1 divider may be more adequate.

If you are using the LINE IN of the soundcard, you can likely get away with no attenuation at all. A very simple interface, when using LINE IN and OUT of the soundcard:

A simple way of interfacing to the LINE IN and OUT of the soundcard and isolate the ground at the same time is shown in the following circuit. If you are using the MIC IN for the soundcard, you can put a resistor divider, adjustable if desired, on the right side of the top circuit.

Any of the above circuits should be used as guidelines. You should consider your input and output signal levels, and your desire to isolate ground, in the design of your interface. Feel free to modify the above ideas for your purposes.

3. Transmit Keying: If you are using the MIC input on the rig, you can simply put the transceiver into VOX mode to control transmitter keying. You'll probably have to adjust the VOX sensitivity for reliable operation.

There are a couple of reasons why you may not be able to use VOX. Certainly, you can't use VOX if your rig doesn't have it! Second, if you are using an auxiliary audio input to the rig, such as a packet audio input, the rig may not respond to VOX. In these circumstances, you could simply hit the SEND or TX button on the rig when you are ready to transmit, or you can build a simple keying interface. Most rigs include an auxiliary port that includes a remote keying line that will key the radio when shorted to ground. In the very least, you could use the PTT line on your microphone input.

The PSK31 programs can be configured to toggle the RTS or DTR line of one of your computer's serial ports during transmit. The simple circuits below illustrate how to use this signal to key the transmitter.

I used one of the thin, flexible wires from a discarded computer mouse for the interface to the serial port on the computer, since it had a DB9 plug already molded onto one end. This simple interface can be used to drive most PTT lines and remote KEY or TX ports. This circuit does not isolate the ground between the rig and the computer.

The circuit below is slightly more complex, but offers ground isolation (not necessary in most applications)

All of the above circuits provide the basic interface needed for rig-soundcard interface for PSK31, SSTV, etc. There are many ways to create an interface, so these circuits should be considered as good starting points. Most applications won't need anything more elaborate than this. There is a lot of additional information in the links below.

Frequencies

1838.150 
3580.150 
7035.15 for region 1 and region 3, and 7080.15 for region 2 * 
10142.150 
14070.150 
18100.150 
21080.150 (although most activity can be found 10 kHz lower) 
24920.150 
28120.150

PSK31 Fundamentals

PSK31 Information and links:
The links below just scratch the surface of the wealth of information that is freely available on the web for PSK31 and other computer-soundcard communication modes for Amateur Radio.

PSK31 Interface Page from Bux CommCo. - Offers kits for the interface. The site also shows full rig-specific schematics of the kits for your homebrewers. Of course, they sell the components for the kits. They list just about every radio manufacturer... They also sell all of the different connectors that you are likely to need (MIC connectors, AUX connectors, etc...)

WinPSK: This is the homepage for the WinPSK program, a really great free WinPSK31 program written by Moe Wheatley, AE4JY. The user manual has interface ideas...

DIGIPAN: Here's another great PSK31 program that is FREE. Some interface information on this page too.

TONS of good info on PSK31 is available from WM2U on this page. I 'borrowed' some of the pictures presented here from him. He's got an entire page dedicated to interfacing for PSK31.

More interfacing ideas and details are available from W5BBR on this page.

www.psk31.com: MEGA-TONS of information and links about PSK can be found here!

The Official PSK31 Page: or so it says! Maybe it is, who knows. Lots of PSK31 info and links.