Brief overview of PSK31 and basic operating techniques.

Published in the WANSAC (Western & Northern Suburbs Amateur Radio Club) monthly club magazine  Vol 39 Issue April 2008 and May 2008

http://www.wansarc.org.au/

PSK31 by Peter Miles

PSK31 is a relatively new digital mode that is similar to RTTY (radio-teletype) and many claim that it is in fact the evolutionary replacement of RTTY. The creation of Peter Martinez (G3PLX), and released free to amateur radio operators in the late 1990s, PSK31 is a real time keyboard to keyboard mode based on the popular soundcard interface technique, featuring very narrow band width and high stability in a noisy and busy HF environment.

The name PSK31 says it all, Phase Shift Keying at a speed of 31.25 baud.

PSK31 comes in a number variants with BPSK31 being the most commonly used and the other less common variants are BPSK63 and QPSK31. Again it's all in the name BPSK31 means Binary (shifting 180deg) Phase Shift Keying at 31.25 baud. BPSK63 is the same as BPSK31 at a speed of 62.5 baud doubling its speed and bandwidth. It will in adverse conditions have a higher error rate. QPSK31 uses four phase angle positions called Quadrature Phase Shift Keying allowing it to achieve some very clever forward error correction that will produce near 100% copy in moderately good conditions and again with a 31.25 baud rate.

BPSK31 is the most used of the above variants and I have to say that in my limited activity with this mode that BPSK31 is the only mode that I have experienced.

BPSK31 bandwidth is very narrow (31.25 Hz), making it highly suitable for low power in a crowded and noisy band environment. The below screen dump from Spectran shows a typical VF (Voice Frequency) channel from 300Hz to about 2800Hz. The BPSK31 signal can be clearly seen at about 1100Hz occupying a very narrow band of the VF spectrum.

Note also the 50Hz and its third (150Hz) and fifth (250Hz) harmonic generated from my DC power supply transformer!

Screen dump from Spectran that shows a PSK31 signal within a typical SSB receiver's VF channel band-pass.

At 31.25 baud which converts to around 50 words per minute PSK31 is a bit slower that RTTY at 60 words per minute, but with a greatly reduced bandwidth. I find that 50 wpm as being quite adequate for what it is meant for, that is conversational key board to key board contacts. See the below chart from Wikipedia comparing data rate to bandwidth for some of the common digital modes.

I have to say that I love this mode and despite the fact that we are at the low point of the solar cycle and my antenna setup is very basic I have worked some good DX in the last few months with little difficulty. See extract from my log below. It has often been the case that after tuning across the twenty metre band and hearing nothing, that the PSK31 frequency of 14.070MHz is alive.

Listening to the band there is often very little to be heard just a weak wobbling tone however the PSK software is still able to pull out a useful print.

The below frequencies are recommended on the PSK31 official web site for general PSK31 activity. The vast majority of traffic is on the 14.07015MHz channel, which is often very busy and it is in fact rare to find no activity on this channel. I have also had a small number of contacts on the 7.03515MHz channel in the evenings. Note that all channels use the upper sideband.

3580.150 kHz USB
7035.150 kHz USB

10142.150 kHz USB
14070.150 kHz USB
18100.150 kHz USB
21080.150 kHz USB
24920.150 kHz USB
28120.150 kHz USB

For more details see: Operational PSK31 frequencies

There is a vast array of web sites devoted to PSK31 but the PSK31 official web site is not a bad start. http://aintel.bi.ehu.es/psk31.html

Software

There is a lot of software around for PSK31 and it is mostly pretty good and often free.

 I have chosen DigiPan 2.0 after trialing a small number of similar packages on offer. DigiPan stands for "Digital Panoramic Tuning" provides a panoramic display of the frequency spectrum in the form of an active scale extending the full width of the computer screen. It is possible to see as many as 30 or more PSK31 stations at one time in the waterfall display. Looks a bit like something out of the movie the Matrix!

Starting from the bottom (1) is what is referred to as the waterfall. The waterfall displays the VF band pass from 0 to 4000Hz. Signals will appear in the waterfall and it can appear quite busy. Selecting with the mouse a spot in the band pass between say 500 and 2800Hz will place a red diamond in that position this is where the print display screen (3) is receiving. It is also the transmit position.

Screen dump from DigiPan

Up to twelve PSK31 transmissions can be monitored simultaneously (4) in a scrolling fashion. Selecting anyone of these scrolling channels will populate the main receive screen (3) with entire content from when monitory began and align the red diamond at the appropriate position on the waterfall.

The scrolling channel screen may also be set up to recognize various strings of characters and upon recognizing that string highlight the channel. For example I have set it up to alert me to any transmission that has CQ in it and highlight that channel with an orange back ground or my call sign with a yellow background.

A small vector scope (2) will help identify the transmission type and the quality of the transmission.

To contact a station on one of the scrolling channel rows (4), just click anywhere on the row and then double-click on the callsign in the single channel window (3) to capture that call sign and use it with the macros and log.

Screen (5) is for transmit text either directly key in or dumped in from one or more of the macros along the top of the screen (6)

Macros are an integral function of DigiPan and the first thing that is needed is to set up your personal data as a number of the commonly used macros will refer to this information.

Pre-loaded macros of commonly used text such as CQ, name, location, station equipment etc can reduce the amount of typing. Commonly used in contests or working foreign station where English is perhaps not the first language.

DigiPan contains a macro language and 24 Macro keys, each of which is associated with one of the Function keys. Twelve are associated with F1 through F12 and another twelve associated with CTRL-F1 through CTRL-F12. Right mouse key allows the editing of the macro. The below screen dump shows the example of the CQ macro button after the right mouse button is keyed. First is the macro button label (CQ), below the label is the text box. Any text in this box will populate the text to be transmitted when the macro is activated. Too the right of the text box is the selection of macros.

The below example shows the CQ macro button which when keyed will first detect the <TX>macro in the text box and key on the transmitter. Transmit the CQ CQ CQ text then detect the <MYCALL> macro and insert the call sign from the personal data, continue with the rest of the message and finally detect the <RXANDCLEAR> macro, un-keying the transmitter returning to receive and clearing the text from the to be transmitted text box on the main screen.

My count shows that there are more that 50 macros that may assist in the operation of the DigiPan package. The eight below are some of the more straight forward examples!

<TX>                 Places DigiPan in Transmit mode

<RX>                Places DigiPan in Receive mode

<CALL>            The remote station's call

<MYCALL>       The Call entered in DigiPan's Personal Data dialog

<MYNAME>      The Name entered in DigiPan's Personal Data dialog

<NAME>           The Name of the remote station

<MYQTH>         The QTH entered in DigiPan's Personal Data dialog

<QTH>              The QTH of the other station

DigiPan can be down loaded from http://www.digipan.net/

Connection

This has been covered in the SSTV article, but it is important so hear it is again!

The interface between the radio equipment and the computer is very straight forward. The audio output and input from the radio can be directly plugged into the relevant input and output of the computer's sound card. While many operators have had success with hard connection method I have chosen to use isolation transformers to ensure that stray R.F. can not find its way into un-wanted areas by way of ground loops. 

Feeding the signal into the microphone socket of the transceiver will require a substantial attenuation of the PSK31 generated signal to prevent over driving the transmitter. This can be achieved by feeding the signal via a simple resistor network and fine tuning with the transceiver mic gain while monitoring the RF power level.

A search of the internet will find a large range of circuits to achieve the interface between computer sound card and the transceiver. There are also a number of commercial interfaces available for the task.

The critical interface issue to watch for when transmitting is over driving the transmitter which will result in a lot of adjacent channel interference and possibly damage to the transmitter.

Duty Cycle

The PSK31 generated signal is a continuous phase modulated tone causing the transmitter to work hard. Therefore it is recommended that the transmit power be wound back to around twenty watts for the average 100W HF SSB transceiver or to about the AM rating of the transceiver.

Operating

Spend a bit of time listening (Watching) to get a handle on the operating practices, if you have used RTTY or even CW before many of the abbreviations will be familiar.

Something that is cute about the real time keyboard to keyboard contact is that if you type out a spelling mistake and instinctively back space and retype the word that is exactly what happens on the screen of the station receiving the text. Everybody does it!

PSK31 is a fantastic addition to the arsenal of radio communication modes particularly when band conditions are less than optimum.

Also see related pages:

Computer to Radio Soundcard Interface Computer Sound-card Interface for TS-430S

For more information on sound card interfacing. See: http://www.qsl.net/wm2u/interface.html

For more information on PSK31. See: http://aintel.bi.ehu.es/psk31.html  

ARRL article on  PSK31. See: http://www.arrl.org/psk31-has-rtty-s-replacement-arrived

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