ZL2AFP MSK-HELL

A narrow-band weak signal chat mode for LF/MF

Updated 3 Nov 2010

Introduction

PSK and MSK are spectrally efficient modes, i.e. use very little bandwidth for a given keying rate. MSK (Minimum Shift Keying) is very similar to PSK (Phase Shift Keying), but instead of changing the phase 180 to signal data bits, the frequency is increased or decreased a very small amount, sufficient to exactly achieve a 180 phase shift in one bit period. The total frequency shift required is only one half the baud rate (for example 52.5Hz at 105 baud).

Because the resulting phase shift is produced without sudden changes in phase, the MSK signal does not require other measures such as amplitude modulation to reduce the keying sidebands. The spectrum is virtually the same as PSK, but the phase relationship between the carrier and the data is different. MSK is little used on HF, but has been widely used on LF, is now used on MF for DGPS beacons, and is used for VLF submarine communications.

The HUGE advantage of MSK over PSK is that because there is no amplitude information on the signal, the transmitter and amplifier need not be linear.

At the receiver, MSK is technically difficult to demodulate, and especially difficult to synchronize. The AM envelope of PSK does not exist, and conventional AM sync recovery (as in PSK31) is not possible. In the context of MSK-Hell, this isn't important since demodulation can be asynchronous and still be very sensitive. Synchronous demodulation is achieved using a Costas Loop approach (similar to ZL2AFP CMSK). While a synchronous demodulator isn't truly 'Fuzzy' in the traditional Hellschreiber manner, it offers text clarity advantages on less stable reception paths.

History
Hellschreiber is a simple text facsimile system, where dots in a matrix representing a character are transmitted serially, scanned up and then across each character. The conventional method is to use a CW transmitter, sending key-down for black dots and key-up for white spaces. Hellschreiber was first used in 1927, and we should remember that, for at least 30 years, Hellschreiber was the preferred mode on LF for press broadcasts and other telegraphic transmissions.

It seems appropriate, both technically and historically, for Amateurs to return to LF some 80 years later, with a high-performance narrow-band Hellschreiber-style mode!

The Hellschreiber technique uses considerable redundancy (as many as 98 symbols per character, compared to say RTTY with only 7 symbols per character). This feature, combined with the ability of the reader's eye and brain to recognise patterns in noise, gives the technique a considerable advantage over conventional digital modes, where just one bit lost to noise can create havoc. Of course the advantages of the extra redundancy come at the cost of greater bandwidth or slower sending, or both.

In this application we reintroduce MSK as a modulation technique for Hell-style visually interpreted ('Fuzzy') modes. This method was first suggested by Nick UT2UZ, as being a better approach than PSK-Hell suggested at the time by the author and developed by Nino IZ8BLY. MSK-Hell was first applied in IZ8BLY Hellschreiber (then called FM-Hell) in 1999, and was soon followed by a version in MIXW by Nick UT2UZ and Denis UU9JDR. It now also exists in FLDIGI and other multi-mode programs. More than ten years have gone by since then! The new software presented here is intended mainly for LF and MF, where extreme sensitivity and narrow bandwidths are the over-riding necessities of communication. Consequently it offers slower, lower bandwidth options, while maintaining compatibility with IZ8BLY Hellschreiber at the standard baud rate.

MSK Modulation of Hellschreiber
In an MSK implementation of Hell, direct modulation is used - a black dot is represented by one frequency and white (i.e. no dot) by another. The shift is so narrow that conventional FSK techniques will not demodulate the signal. The phase sensitive detectors used have a major advantage because they are quite insensitive to AM noise, and the demodulated MSK-Hell text always has excellent contrast.

Most sensitive modes for these frequencies are very slow - but here we have a typing-speed system which is sensitive, easy to tune and easy to use. Each character contains up to about 50 dots; because the eye reconstructs the received information, and because the phase-sensitive demodulator gives excellent contrast between text and background noise, the modes are very immune to QRM. You'll find that (at the higher speeds) it also works very effectively on HF, and the software is (at the highest speed offered) compatible with other HF MSK-Hell ('FM-Hell') software.

Designed for the Conditions

The LF and MF Amateur bands (2200m, 600m and 160m) are characterized by relatively stable carrier phase on received signals, accompanied by low Doppler shift. These bands have very strong lightning interference, but mostly local, not the background of random impulse noise typical of lower HF. While there is multi-path reception, especially on 160m, the path changes are slow. The slow fades can be very deep, and signals are generally quite weak, and frequently accompanied by man-made interference; so in order to have a conversation at typing speed on these bands we need a mode that is very sensitive, has excellent impulse noise and QRM tolerance, but need not have extreme Doppler shift or carrier phase instability tolerance.

It is no accident that MSK is used by military and commercial services on VLF and LF, or by the MF DGPS beacons around 300kHz. A well designed MSK mode can have all the features just described. We demonstrated this in the ZL2AFP CMSK digital mode program. In the ZL2AFP MSK-Hell program however, we concentrate on traditional dot-matrix transmissions that are reconstructed by the user's eye and brain. Several important design features have been added to make a robust and very sensitive chat (QSO) system totally suited to LF and MF.

The following modes are offered:

Mode Baud Rate Bandwidth Typing Speed ITU Definition
MSK-Hell 31 31.25 50Hz 7 WPM 50H0F1B
MSK-Hell 63 62.5 100Hz 15 WPM 100HF1B
MSK-Hell 105 105 170Hz 25 WPM 170HF1B
Feld-Hell 31 31.25 150Hz 7 WPM 150HA1B
Feld-Hell 63 62.5 300Hz 15 WPM 300HA1B
Feld-Hell 105 105 500Hz 25 WPM 500HA1B

The program offers symbol rates 31.25, 62.5 and 105 baud, in both MSK-Hell and ASK-Hell (conventional Feld-Hell). The default mode (in bold text with pink background in the table) is MSK-Hell 63. The default speed, 62.5 baud, has a leisurely typing speed, and sensitivity is so good that you can generally copy any MSK-Hell signal you can see on the waterfall display.

The 105 baud MSK-Hell mode has a transmitted bandwidth of under 200Hz, so is easily within the bandwidth limit which applies in New Zealand. This is not the case (as you can see from the table) with the much wider Feld-Hell modes, which are offered here mostly to enable users to prove to themselves the advantages of MSK.

Compatibility

MSK-Hell and FM-Hell
The MSK-Hell 105 baud mode is compatible with the 'FM-Hell 105' mode offered by IZ8BLY Hellschreiber, MIXW and FLDIGI. The only difference in this respect is that the ZL2AFP MSK-Hell software offers a choice of asynchronous and synchronous receiver demodulators.

Feld-Hell 105 and 122.5
The 105 baud Feld-Hell mode offered by ZL2AFP MSK-Hell is also completely compatible with standard 122.5 baud Feld-Hell, despite the lower baud rate. This is because the reduced resolution font used by this program for MSK-Hell is also used for Feld-Hell. This is done in order to limit bandwidth. The font has less detail (only 12 rather than 14 pixel-pairs per column), and the baud rate required to achieve the standard column rate is 12/14 or 86% of the standard dot rate. Thus the column scan rate remains the same as standard Feld-Hell.

Lower Baud Rates
The 31.25 and 63.5 baud rate MSK-Hell and Feld-Hell modes in ZL2AFP MSK-Hell currently have no equivalent in other programs. These slower speeds are ideal for beacon and weak-signal QSO applications, but generally only appropriate on LF and MF.


Screenshot of the ZL2AFP MSK-Hell software
(Click on image for full-size view)

Guided Tour

Text Panes
Check out the screen-shot above. There are two text panes - the large graphics receive pane at the top, and the pale yellow transmit text buffer pane immediately below it. Received text shows in the receive pane as constructed from tiny black or grey dots. Although each character is only transmitted once, the text is always displayed as two rows of text one above the other in traditional Hell fashion. This is because the receiver has no knowledge of the timing of the scanned and transmitted characters, so by displaying them twice the text joins up and remains readable even if the phase is arbitrary (see example below). It also means the text remains readable even if the stations have a gross (1% or more) baud rate or sampling rate difference.

   
Double text printing - misaligned text (right) is still readable

You can type in the transmit buffer even while receiving ('type-ahead' feature), so when you press F9 Send, what you have already typed will start the transmission. The program also features a row of programmable Function Keys which you can click on, or use the keyboard equivalent, which is generally more convenient. A special key used to control TX/RX can be defined to allow automatic return to RX at the end of a transmission.

The program window size defaults to a size suitable for a Netbook PC (as in screenshot above). The height of the window can be stretched to suit other computers by dragging the top of the window up, or the bottom down. This enlarges only the receive pane. The width is fixed.

Modes
Two modes are offered: MSK-Hell and on-off keyed Feld-Hell, both at three speeds, 105, 62.5 and 31.25 baud. The MSK mode has a choice of two receiver detectors.

The default reception mode is MSK-HELL (Sync) (synchronous detection of MSK-Hell), which offers slightly clearer text when conditions are stable. In this mode, the detector used is the classic Costas Loop (also used for PSK), which allows the data clock to be recovered. The use of this clock (which, like the data has slightly varying timing due to propagation changes) means that text is displayed in straight lines with better clarity. Under conditions where timing changes markedly (early evening on 160m, for example), occasional letters may not print clearly because the sync is lost momentarily. This mode is not truly 'Fuzzy' since the computer makes pixel timing decisions for you.

The MSK-HELL (Async) detector (for asynchronous detection of MSK-Hell) is less affected by multi-path, since it operates and is timed completely independently of the received data bits. This detector simply measures phase difference from one sample to the next. The phase differences are plotted directly on the screen for the user's eye to decipher (i.e. it is a 'Fuzzy' mode). This mode is not fooled by timing variations, but propagation can cause the text to be blurred at times, and the text tends to wander up and down slightly in sympathy with propagation changes. It has superior sensitivity - about -18dB S/N for modest copy at 31 baud, and 100% blemish-free copy at -15dB S/N. The 63 baud mode achieves about -12dB S/N for blemish-free copy.

Feld-Hell - To reinforce the advantages of MSK-Hell, the program also operates the traditional Feld-Hell (ASK-Hell) mode at the same three speeds. There really is no comparison for weak signal operation - Feld-Hell has much more noisy reception, is less sensitive, prone to multi-path effects and lightning, and uses much more bandwidth.

Software

The Windows™ software is compatible with Win2000, WinXP and Win7. It may work with Vista on some computers. The program requires at least a 1GHz processor, SVGA display and a 16-bit sound card. One serial port (or USB equivalent) is required for PTT control. Memory requirements are minimal, and the program size is only 200kB. The program consists of just one file, creates just one setup file (will operate without the included help information), and no changes are made in the computer's registry or anywhere else. To remove the program, simply delete the files made during installation.

The 'beta' version below has I&Q mode and a resizable receive window. It works great on Win XP and Win 7.

Download:
ZL2AFP MSK-Hell transceive program (Version 1.0 02.11.10 beta)
* If you have not previously used your rig with Digital Mode software, you will need to set up a digital interface which provides PTT control via a serial port, and handles the audio between the rig and the radio. For more information see:

Getting Started with Sound Card Digital Modes        Simple Digital Interface Design

If you have any questions or useful feedback about this program, please contact Con ZL2AFP (ZL2AFP at xtra dot co dot nz).


Copyright © Murray Greenman and Con Wassilieff 2010. All rights reserved.