lpsk31 Manual

  1. Introduction
  2. Features
  3. Compilation
  4. Command line options
  5. Operation
  6. Bugs and annoyances
  7. Version history
  8. Copying

1. Introduction
lpsk31 is a ncurses-based Linux console application for Amateur Radio communications in the PSK31 digital mode. lpsk31 detects the carrier phase changes that make a PSK31-encoded character by reading audio samples from the DSP in the computer's sound card, connected to the output of a receiver. The audio signal is processed by a software phase detector which, for BPSK, detects '1' or '0' states ('0' signaled by a 180 deg phase reversal) and the resulting stream of BPSK31-coded bits is decoded into ASCII characters for printing to the Receive window of lpsk31. In QPSK the carrier phase changes are processed by a soft-decision Viterbi decoder which provides error correction in bad receiving conditions.

Transmission of PSK31-encoded characters is realized by writing pre-computed audio samples to the sound card's DSP and feeding the resultant audio signal to the transmitter's input. The transmitter must be in the SSB mode during transmission but for reception a narrow-bandwidth mode like CW-Narrow is preferable since PSK31 theoretically only occupies about 32 Hz of bandwidth. lpsk31 has built-in CAT control capability for the Yaesu FT-847 Transceiver which, when enabled, can change the operating mode automatically.

Usage: lpsk31 [-hv]

-h: Print usage information and exit.

-v: Print version number and exit.

2. Features

Sound-card set-up
lpsk31 automatically prepares the sound card and sets the line input level at the mixer to about 50%. For proper operation the level of the input signal must be adjusted from the receiver, if volume control is available, or by changing the value of the Recording/Capture level in the ~/lpsk31/lpsk31rc run time configuration file, if audio is taken from a fixed-volume source such as the "data" i/o port. The output audio level is also set to 50% although again this will need to be set-up properly for low-distortion transmission. Please see the Operation section for details on how to set-up the sound card.

Software phase detector
lpsk31 was designed from the beginning to be simple and efficient, decoding PSK31 signals using only integer arithmetic and simple processing methods. It uses the computer's sound card to read the '1' and '0' elements that make the PSK31-coded characters by implementing a software detector of the received audio signal's phase shift. This software phase detector is centered on a nominal frequency of 500 Hz and it operates by integrating the output from an in-phase and a quadrature detector over the period of a PSK31 element (32 msec). The carrier phase is then calculated by an integer arctan function implemented using tables to calculate the arc tan (in degrees) of the (quadrature/in-phase) signal ratio. Phase changes are calculated by comparing consecutive values of the carrier's phase after every PSK31 element.

Phase/Tune Indicator
lpsk31 has a simple visual aid for tuning the receiver accurately and displaying the carrier's phase changes. At the lower left corner of the ncurses screen there is a box with three 'graphical' indicators of the carrier phase, phase slip rate (= tuning error) and squelch (signal) level. For correct tuning the squelch bar graph must be at a maximum, the phase slip at zero (green sector) and the phase indicator should show phase changes with the 'o' pointer moving to opposite dots on the circle for BPSK, or to positions about 90 or 180 deg from each for QPSK.
More on this in the Operation section.
In the center of the carrier phase display the phase detector channel in use (A or B) is shown.

Macros in Transmit mode
lpsk31 allows 9 pre-recorded Macros to be bound to the F1-F9 Function keys for easy transmission of repeated messages like call-sign exchanges, station info, BTU, sign-off etc. Theses Macros are stored in the 'lpsk31rc' run time configuration file (in ~/lpsk31/) and are invoked by keying the relevant Function key. Short labels for these Macros are also stored in lpsk31rc and are displayed in the 'Commands' boxed window at the bottom right of the screen, for easy identification. For even more operating convenience lpsk31 allows the use of 'tags' embedded in the Macros for automatic insertion of certain basic strings in the transmitted message: call-sign, RST, name, QTH and QTH locator, for both the operator and the remote station, and the current UTC date and time - e.g. 11 tags altogether in this version. More on this in the Operation section.

QSO Recording and Logging
lpsk31 can save details of activity during its operation in three separate files on the disk:
1. In ~/lpsk31/record.txt all characters printed in the Transmit or Receive window including Macros are recorded 'raw' e.g. in real time and unedited. This makes it possible to check everything transmitted or received during operation if something goes wrong.
2. In ~/lpsk31/log.txt all details of a QSO entered in the QSO Record form (under the Receive window) are saved in text form with the addition of date and time to form a standard station log.
3. In ~/lpsk31/log.adif an ADIF-formatted version of the QSO Record is saved for direct uploading to eQSL.cc or other sites that can accept ADIF formatted logs.

Recognized characters The alphabetic characters, punctuation marks and special codes that lpsk31 can currently recognize are in the header file lpsk31.h. The table of characters has the ASCII symbol on the left, the Varicode bit pattern in the middle and the equivalent HEX code on the right.

3. Compilation
Before compiling lpsk31, it is possible to customize some entries in the header file lpsk31.h. These are the colors used in the ncurses interface and can be changed by replacing the colors in the five #define statements as follows:

  #define LABEL_COLOR      CYAN            /* Color of window labels   */
  #define BORDER_COLOR     CYAN            /* Color of window boxes    */
  #define KEYBD_COMMANDS   MAGENTA         /* Color of key commands    */
  #define STANDOUT_COLOR   YELLOW          /* Standout (bold) color    */
  #define FORM_COLOR       YELLOW_ON_BLUE  /* Color in QSO record form */

These colors can be replaced with the color pair names, defined below these entries in the header file lpsk31.h.

A list of all characters, punctuation marks and special signals recognized by lpsk31 are listed in the lpsk31.h file. Different characters can be entered in the table in the lpsk31.h header file before compilation, following the instructions in there.

Compilation is very simple, just run 'make' in the lpsk31 source directory to produce the executable binary 'lpsk31'. This can then be copied to a suitable location, usually /usr/local/bin or /usr/bin. There is only this hypertext documentation file which you can copy to a location of your choice. Also the default lpsk31rc configuration file in the default/ directory must be copied to a directory named lpsk31 in the Home directory and then edited for proper operation.
You can also type 'make install' to install the 'lpsk31' binary into /usr/local/bin, this hypertext documentation in /usr/local/doc and the default lpsk31rc run time configuration file to a directory named lpsk31 in the Home directory. If ~/lpsk31 does not exist, it will be created.
Type 'make uninstall' to remove the 'lpsk31' binary, the 'lpsk31.html' documentation file and the ~/lpsk31/ directory.
Type 'make clean' to remove all files produced by compilation except for the 'lpsk31' binary, from the source directory.
Type 'make distclean' to remove all but the files that were included in the original source directory.

4. Command line options
lpsk31 can be invoked with the following options:

-h: Print this usage information and exit.

-v: Print version number and exit.

5. Operation

lpsk31 User Interface
lpsk31 is a ncurses application for the (Linux) console. On starting the following are presented to the user:

1. The Transmit window: At the top of the screen the Transmit window prints all characters typed by the user in the Transmit mode including Macros when activated by keying one of F1-F9 function keys. If the "Record QSO's" ('R' keystroke) command is enabled, all characters printed to this window are also saved in the ~/lpsk31/record.txt file.

2. The Receive window: Below the Transmit window, the Receive window displays all characters decoded form the PSK31 signal. If the "Record QSO's" ('R' keystroke) command is enabled, all characters printed to this window are also saved in the ~/lpsk31/record.txt file.

3. The Phase/Tune window: At the bottom left corner the Phase/Tune window displays some 'graphical' aids for correctly tuning the receiver. At the bottom of this window, a bar graph indicates the squelch (signal) level. This is derived from the averaged vector magnitude of the in-phase and quadrature signal detectors and it peaks when the audio (carrier) frequency is at the required 500 Hz.
Above this another bar-graph displays the phase slip rate of the psk31 carrier and is derived from the phase detector's averaged output. The phase slip rate is equivalent to a frequency error in the PSK31 carrier relative to the 500 Hz nominal center. This bar graph must be centered for correct tuning.

Above this is the PSK31 carrier phase indicator. It is a circle of 16 green dots, each representing a step of 22.5 degrees anti-clockwise, relative to the middle-right reference dot. An 'o' is used as the 'pointer' and it shows the carrier phase at the end of each PSK31 element. For a BPSK31 signal, ideally it should jump between two opposing dots on the circle, indicating 180 phase changes. For a QPSK signal, the indicator should jump to dots at 180 or +-90 degrees to each other.

4. The QSO Record form window: This is a single-row ncurses form just below the Receive window and is opened by the F10 key command. In this, the remote station call-sign, RST-in, RST-out, remote operator's name, QTH, Locator and the operating frequency are entered during a QSO and become available to Macros after a TAB or RETURN keystroke. The entries in the form can be saved to ~/lpsk31/log.adif and ~/lpsk31/log.txt with the F11 keystroke.

5. The Keystroke Commands window: Most of the important key commands recognized by lpsk31 are displayed in this window. The Function keys F1 to F9 are reserved for starting Macro transmission. Each Macro has an associated label which is displayed next to the Function key that starts it and these labels are user-defined in the ~/lpsk31/lpsk31rc file together with the Macro text.
Please note that Macro transmission is interrupted with the SPACE key. ESC has no effect on Macros.

The other commands are as follows:
F10: Opens a new QSO record form for entering call-sign, signal reports, QTH, Locator and Frequency. If the transceiver in use is an FT847 and CAT is enabled, then RST-out and Frequency are obtained automatically from the receiver via CAT commands.
If unsaved data form a previous form exist in the buffer, a confirmation request to save to file is displayed.

The TAB key makes the current field available to Macro tags and moves the cursor to the next field.

The RET(ENTER) key enters all fields to a buffer and closes the form. Note that the following fields are validated as follows:
Call-sign: Alphanumeric characters and / only.
RST: Minimum 336 and maximum 599.
Locator: A-S, A-S, 0-9, 0-9, A-X, A-X
Frequency: 0.05-9999 (MHz).
If validation fails, the form will not close! Make sure your entries match the validation criteria before Entering the form data.

E or e: Opens the QSO record form for editing existing entries.

F11: Saves current form data to ~/lpsk31/log.txt and ~/lpsk31/log.adif after requesting a confirmation. This key works only when a new form has been edited in some way and Entered. F10 cancels the Save.

F12: Toggle Transmit/Receive mode. If the transceiver in use is an FT847 and CAT is enabled, then it will be switched to SSB in Transmit and CW-N in Receive. This key will initiate a Pre-amble signal when going to Transmit and a Post-amble signal when going to Receive. Pre-amble is a steady tone followed by phase reversals and Post-amble the opposite. The duration of the tone and reversals is user-specified in ~/lpsk31/lpsk31rc.

M or m: Toggle PSK31 modes. There are three supported modes, BPSK, QPSK-USB and QPSK-LSB. If an FT847 Transceiver is in use and CAT is enabled, lpsk31 will switch the Transceiver to the right mode (USB for BPSK and QPSK-USB and LSB for QPSK-LSB).

Ctrl-W: Initiates transmission of the call-sign in CW/Morse. This command work only when lpsk31 is in Transmit mode.

R or r: Toggles the raw recording of all characters printed in the Receive and Transmit windows including Macros. This option is ON by default since it is quite useful.

T or t: Toggles CAT control for the FT847 transceiver. Do not activate if other rigs are in use since it will cause problems!

K or k: Capitalize letters in the Transmit window. All letters including text from Macros are capitalized before printing and transmission.

Esc: The Escape key will cancel most current operations like manual transmission, the QSO record form etc. When in Receive mode it quits lpsk31 so watch it!

There are some other key commands used mainly for testing and debugging but please note that their operation may not be very reliable:

W or w: Toggles writing of the audio samples that are sent to the DSP to the file lpsk31.bin. This file can then be read using the command below for testing and debugging. Please note that there is no indication of the status of this command and that it has to be toggled OFF to enable reading of lpsk31.bin.

F or f: Enables reading and decoding the DSP samples from lpsk31.bin for test purposes. When finished, lpsk31 reverts to its normal mode of reading directly from the DSP.

S or s: Toggles the recording of DSP samples of the audio signal to the file lpsk31.bin. The file can then be read with the command above when recording is toggled OFF. Please note that normal operation of lpsk31 is suspended during signal recording.

Configuration and set-up
After compilation and installation the ~/lpsk31/lpsk31rc run time configuration file MUST be edited for proper operation! To do this, load ~/lpsk31/lpsk31rc into your favorite editor and make sure the you check ALL the configuration fields in the file and that you properly edit all the ones that must be edited. There are four sections, Sound card, lpsk31 Run time, Operator/QTH info and Macros fields. Please check ALL and EDIT as needed!! There are more detailed instructions in the file itself so please follow them for a proper set-up. In particular, station details like Call-sign, operator's name and QTH etc must be edited for Macros to be usable.

Next, the sound card must be set up. For this to be done, the receiver's audio output should be connected to the computer's sound card (usually to the 'line' input) and the receiver tuned to some stable carrier signal with no am modulation. Modern HF transceivers have general coverage receivers with DSP audio filters down to 25 or 50 Hz bandwidth and this makes it possible to use a local broadcast station as a signal source.

With the narrowest bandwidth selected, start lpsk31 and fine-tune the receiver for a 500 Hz beat frequency output, using the Squelch level (the bar-graph at the bottom of the Phase/Tune boxed window) as an approximate guide, e.g. tune for a maximum indication on the bar-graph. If this goes full-scale then reduce the receiver's volume level until a maximum indication of 80-90% is achieved. If the audio output is taken from a fixed-level outlet (like the 'data' port) the only way to set the correct input level is to edit the Recording/Capture level value in the ~/lpsk31/lpsk31rc file by trial and error for the correct setting.

To make sure that the receiver is accurately tuned for a 500 Hz audio output, slowly tune the receiver while watching the carrier phase indicator (the circle of green dots in the Phase/Tune window) until the 'o' indicator holds a steady position or moves around very slowly. This indicates that the phase slip rate (= frequency error) is nearly zero, and this should be confirmed by the phase-slip bar-graph, below the phase indicator, showing its pointer in the center of the green sector. Please note that a tuning step of 1 Hz or less if available is needed for this very tricky operation! Such are the demands of the PSK31 mode.

For PSK31 transmission, the DSP output level needs to be set up for minimum distortion in the transmitter. The Volume/PCM level setting in ~/lpsk31/lpsk31rc can be edited to adjust the audio output level but if the setting needs to be below approximately 20% then a simple resistive attenuator will be needed ahead of the transmitter's audio input. In each case the best way to set the sound card output level would be to have the transmitter's settings (Mic gain, ALC or other appropriate control) at an average or 'normal' level and adjust the audio level so that the RF output power is about 50% or less of the rated maximum power. This will ensure that distortion is kept to a minimum and the dissipation of the final stage within its ratings.

Receiving PSK31 signals
To receive BPSK signals, start lpsk31 in a console and tune-in a BPSK31 carrier as follows: Set up the receiver for a narrow-band mode (normally this would be CW-Narrow, and if available a DSP audio filter with a bandwidth of 100 Hz or less should be used) and then slowly tune the receiver until the squelch level bar-graph peaks. Then carefully fine-tune the receiver until the phase slip bar-graph centers and the BPSK31 signal starts printing in the receive window. For an accurately tuned signal, the carrier phase indicator should jump between opposing dots in the circle, indicating a 180 deg phase shift.

However all this is easy to say, since in practice the carrier phase indication is affected by path length changes, fading, noise, QRM etc. Also, any frequency errors in tuning the receiver, will appear as a steady drift of the pointer (phase slip) around the circle, confusing the issue further. A lot of practice is needed before the user becomes familiar with these (very limited) tuning aids and able to tune in a PSK31 signal quickly. This is a limitation deriving from the simplicity of the lpsk31 phase detection scheme (no FFT or waterfall or signal spectrum display available).

It is possible to record and decode a PSK31 signal from a file. This is done by toggling the 'S' key command to record the PSK31 signal to a file that will be called lpsk31.bin in the current directory. The format is plain binary so the file can become quite large for a long recording. To decode the recorded signal, toggle 'S' to OFF and key the 'F' command. Please note that all this is mainly a debugging facility and hardly useful, and that there is no status indication for these commands.

Receiving QPSK signals is similar to BPSK but requires even more accurate tuning of the receiver. Please note that the operating mode of the Transceiver during transmit and receive must match that of lpsk31 as selected with the 'M' key command.

Conducting a QSO
As usual to make a QSO with another station using PSK31 you may call CQ or reply to a CQ. Calling CQ would normally be done using a prepared Macro bound to a Function key (F1-F9). Hitting any of these keys will change lpsk31 to Transmit mode and start sending the Macro. When the Macro is finished, lpsk31 will revert to Receive mode unless an up-arrow '^' is the last character in the Macro in which case it remains in Transmit so that a manually typed message can follow the Macro. Reversion to Receive would then be done by hitting the F12 key.
Please note that Macro transmission can be interrupted only with the SPACE key. ESC has no effect on Macros.

When a station replies to a CQ it would normally be netted closely and characters transmitted should appear in the Receive window. If there is a significant frequency error it will cause errors in decoding but correcting this error from the main dial may cause the remote station to loose reception so using the RIT control may be advisable. This can also cause problems since lpsk31 requires accurate tuning and using RIT can confuse things.

Once two-way contact is established, at least the station's call-sign, RST, and the QSO frequency must be entered in the QSO Record form before it can be saved to ~/lpsk31/log.adif and ~/lpsk31/log.txt. Any information entered in the form is available to any relevant 'tags' in the Macros when the TAB key is hit. If any fields are not edited, lpsk31 enters blanks in place of tags! Please note that during transmission no entries can be made to the QSO Record form.

To reply to a CQ, tune in the station accurately and type its call-sign, signal report and frequency in the QSO Record form. Then prepared Macros can be used to reply to the call and conduct a QSO. Have a look in ~/lpsk31/lpsk31rc for some typical prepared Macros.
NOTE: You will need to edit at least some of these!!

6. Bugs and deficiencies
I have fixed whatever bugs I came across testing lpsk31 but there may be some hiding, waiting for the right conditions to appear.

The PSK31 phase detector algorithm may need further improvements to make it more tolerant to fading and noise. The biggest problem seems to be adjacent strong signals raising the AGC and reducing the level of the PSK signal being decoded.

The use of a simple integer-only DSP decoding process makes it necessary to run the software phase decoder/synthesizer at a 500 Hz center frequency. This also makes it necessary to run the receiver with a CW pitch setting of 500 Hz. This well below the usual 700-800 BFO pitch setting and will probably make it necessary to change BFO settings for CW work.

The lack of AFC, Waterfall or Spectrum display makes it necessary to manually tune the receiver accurately for reception. This can be difficult in poor signal conditions at least, if not always!

7. Version history

Version 0.1 First beta release of the basic lpsk31 decoding command line tool.

Version 0.2 Added the ability to work in either stereo or mono mode so that more than one audio sources can be connected to the sound card without switching or removing plugs.

Version 0.3 Changed the phase detection scheme from a tunable PLL to a fixed frequency phase detector with no AFC. Due to the nature of PSK31 the phase lock or AFC schemes required the tuning to be within +- 4 Hz before a lock was achieved and this was little help to the tuning process. Also the continued phase changes made the PLL/AFC loops difficult to keep stable in noisy conditions.

Version 0.4 Added a full ncurses user interface including a tuning/phasing indicator, separate Transmit/Receive windows, a lot of keystroke commands, macros for automated transmission of some fixed message sequences, a Morse identifier and a CAT interface specific to the Yaesu FT-847 (that's the only rig I have!).

Version 0.5 Implemented QPSK operation with the help of Peter Martinez who kindly provided Pascal source code for the Viterbi decoder which I ported to C and incorporated in lpsk31.

Version 0.6 Divided some large functions into smaller ones to streamline the source code and improve readability.
Further edited the Viterbi source code so that it integrates seamlessly with the internal workings of lpsk31 (changed the accepted range of phase angles from 0-256 to 0-360).
Changed the phase detector scheme so that carrier phase and phase change are in the range 0-360 deg instead of +-0-180 deg. to match the Viterbi decoder scheme.
Fixed some minor bugs and streamlined some code.

Version 0.7 Fixed a bug that caused a seg fault when a new macro was selected without returning to receive after the previous one.

Version 0.8 Changed the sound card set-up function so that the Recording Level device (igain or reclev) is not selected automatically since this can be unreliable. Instead, the Recording Level device is specified in the ~/lpsk31/lpsk31rc file. PLEASE NOTE that this makes this version incompatible with existing lpsk31rc files! If you want to keep your original lpsk31rc files, then you need to add the Recording Level device name after the Recording Source name entry. Please see the defaults/lpsk31rc file entries for sound card set-up.

Version 0.9 Fixed buggy 'strncpy()' statements that could possibly cause segmentation faults when reading the lpsk31rc file.

Version 1.0 Added two additional tags that can be used in Macros:
<op-freq> is replaced by the current operating frequency.
<app-version> is replaced by the name and version of this application.

Version 1.1 After a bug report from Juha Vierinen regarding seg faulting of xnec2c, my graphical adaptation of NEC2, I changed all "sprintf" commands to "snprintf" to avoid buffer overruns. Following on the above changes, I revised all similar situations in lpsk31 source code and changed all "sprintf" commands to "snprintf" just in case. While going through the lpsk31 source code, I also fixed some minor bugs like typos and tidied error messages.

Version 1.2 Changed the source tree files to a more appropriate arrangement, according to commonly adopted practice. Also edited the source code to remove warnings by cppcheck and clang analyzer.

Version 1.3 I changed the phase detector code to correct the discontinuity in calculating phase change around +-180 degrees.

Version 1.4 Fixed a bug reported by Richard Shaw KF5OIM and also some warnings produced by cppcheck static scanner.

8. Copying This software package is released under the GNU Public License. Please see the COPYING file for more details.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

July 22 2002.