ZL2AFP EXChat

This is a computer program for Amateur Radio transmission and reception using a computer and radio transceiver. The program equips your computer with a one sentence at a time chat-mode for operation on the HF bands. You use it in the same way as you would Skype™ or cell-phone TXTing. This facilitates rapid-fire QSOs and especially makes nets easier.

As usual, sound card techniques are used to generate transmissions using tones at audio frequency, and to receive and decode the incoming signals, also at audio frequency. An SSB transceiver translates these signals to and from the HF Amateur Bands. The EXChat mode is a development of DominoEX, and is completely compatible with other DominoEX versions. In fact operators without EXChat, but who have another version of DominoEX, can take part in a QSO, if in a slightly clumsy manner. What is different about EXChat is that it operates in Sentence Mode.

To read more about DominoEX in general, visit the main DominoEX web page.

The ZL2AFP EXChat program offers the usual six speeds:

	MODE	BAUD	BW	SPEED	TONE SPACING
	EXChat 4	3.90625	173Hz	~25 WPM	Baud rate x2
	EXChat 5	5.3833	244Hz	~31 WPM	Baud rate x2
	EXChat 8	7.8125	346Hz	~50 WPM	Baud rate x2
*	EXChat 11	10.766	262Hz	~70 WPM	Baud rate x1
	EXChat 16	15.625	355Hz	~100 WPM	Baud rate x1
	EXChat 22	21.533	524Hz	~140 WPM	Baud rate x1

Table of DominoEX modes. * Default mode

The speeds quoted for EXChat are approximate for typical text in English. For speed measured in Characters per Second (CPS), divide the above values by 10. Speed varies slightly with content. Speeds and parameters have been chosen following extensive on-air and simulator testing. Their use can be best summarised by the following table:

In this context, 'DX' means different things on different bands. For example, on 80m DX means anywhere outside groundwave range; on 20m double-hop or long path; on LF, MF and VHF, any weak signal application. There's a general principle for EXChat: it's a bit like driving a car - when the going gets tough, change down a gear! Here are some suggestions:

• 8 baud for weak signal MF and transition periods (sunrise/sunset) on 80 and 40 metres.
• 11 baud, for general 80 (night) and 40 metre (day) use.
• Faster 16 baud for daytime 40 metre use and single-hop 20 metre QSOs.
• High speed 22 baud for local ground-wave use, including VHF and UHF.

Sometimes (especially on higher bands) faster speeds give better copy. In a net situation, go with the speed which works for everyone.

Download (see below) the archive and unzip it into a new folder called "EXChat" on your computer. This will provide you with all the programs, the help and related documents.

In that folder, locate the executable EXChat_xxx.exe (where xxx is the version number), and create a shortcut to it by right-clicking, and then drag this shortcut to your desktop or task bar. It will have a cute Domino icon.

Very little setting-up is involved. When you first start the program , read the HELP information about setting up, or read it here before you start.

ZL2AFP EXChat Installation V0.00 (03 November 2014)
ZL2AFP EXChat Help

Tuning Signals

Tuning in signals is no different to DominoEX. You can tune the receiver and/or slide the waterfall. (if you hold the mouse over the waterfall, you'll see a 'CLICK LEFT TO DRAG SCREEN' note tied to the mouse). Aim to place the received signal in the centre of the area between the yellow lines. If your transceiver is stable and has no offset between transmitter and receiver, just use the main tuning on the rig, and leave the waterfall alone. This is especially important in a net situation: otherwise stations are forced to continually chase each other around.

You'll need to be quick with tuning, as the transmissions can be quite short. The good news is that if you set up a calling frequency, and tune accurately, everybody will be able to copy everybody else without any retuning.

Make sure you have sufficient audio from the receiver. The waterfall background with no signal should look grey with just a few noise dots and stripes. If the signal tends to disappear during fades, just add a bit more level.

The waterfall display (see picture) has a slider control on the scale, so you can tune the receiver slightly to compensate for receiver/transceiver netting errors. Use this sparingly, especially in a net situation. The scale changes when you change speeds, but always starts at 1000 Hz at the bottom. If, when tuning in a signal, it looks furry, and is either smaller than you expect or too tall, you probably have the wrong speed setting.

The waterfall is much sharper and more informative than you'll see in other programs, because it operates in synchronism with the received signal. As a result it is also much more sensitive. Each of the little white stripes is a received tone or symbol. With NVIS reception, they can be briefly less distinct due to Doppler and timing changes.

Note: When you drag the waterfall to tune in the signal, you ALSO move the transmitter frequency.
The software acts like a transceiver, with the transmitter following the receiver.

If the speed is right (and you will get used to the sound, so be able to guess quickly), the signal will occupy about the centre two thirds of the area between the yellow lines, and the two little 'metrics' displays below will show stable pictures. If the signal strays outside the yellow lines, reception will quite quickly deteriorate.

The little indicator on the right is the instantaneous 'sync triangle', indicating where the software has determined sync to be by measuring the received symbols. The height is arbitrary, and the width is that of one symbol. The red line indicates the current symbol sampling point, and it moves about to track the signal. Sampling in the exact centre gives the most noise-free results and therefore lowest error rate. This display won't have a clear triangular peak if the speed is wrong. The peak tends to move around while receiving, as the ionospheric path changes the signal's time of flight. This is especially true during NVIS reception. The software (indicated by the red line) tracks these changes.

The little display on the left is 'sync history', and displays symbol timing (vertically) against elapsed time (horizontally). Imagine that this is a historical record of the position of the peak of the sync triangle, sampled at every symbol. You should easily be able to see the effects of multi-path and fading here. These little indicators are useful in determining the best operating speed for the prevailing conditions.

Operating

In order to tune your rig, antenna tuner, or adjust power level, you can simply press ENTER on the keyboard (or press the SEND button), and the transmitter will operate for a few seconds while you make adjustments. If the transmitter doesn't operate, check your software and rig settings (did you press APPLY?). You will never need to use high power. 10 Watts gives good results on 80/40 metres, because the receiver is so sensitive.

To send a sentence, simply type what you want to say, into the Transmit pane (see below), check that the receiver is quiet (there will be no signal on the waterfall), and then press ENTER. The message will be sent, and your text will be placed in the receive pane. The squelch system should prevent you from sending while another station is sending. You can't send until the squelch is closed - until the blue bar on the signal-to-noise (SNR) meter display sinks below the red line. (See the picture to the right).

Because of delays and operator reaction times, you may find that you press ENTER just as a message from another station comes in. You will unfortunately 'double', and the only way to avoid this as much as possible is to listen carefully and watch the metrics. If you spot a 'double' event, just send 'Agn Pse' and wait for the response. You can also cancel an on-going transmission immediately by pressing the RECV button.

You can continue to type while the transmitter is sending the previous sentence. It's a good idea however to wait for the other guy to reply first, as otherwise you could easily be talking at cross-purposes, or have to scrap your sentence to first reply to something he said.

You can edit transmitted text before it is sent by moving the cursor and using Backspace, Delete and the usual editing functions. You can also copy (Ctrl C) and paste (Ctrl V) from the receive pane to the transmit pane if you are quick. This feature is especially useful for repeating what you said when your last over was missed.

The Squelch attack controls are designed to minimize the amount of junk text which appears on the screen. The junk accumulates in the receive buffer at the end of a transmission (before the squelch closes) and typically appears on the screen at the start of the next received signal. The Fast setting will leave more junk, but you won't miss anything you need. Slow may cause the beginnings of transmissions to be lost. Medium is a good compromise for most conditions. Experiment to find the most suitable setting for the prevailing conditions and the speed you are using.

Squelch operation is also affected by the threshold setting. This is made by clicking on the signal-to-noise meter (vertical bar graph) to move the red line. The received text is shown on screen only when the signal SNR exceeds this point. An appropriate setting will depend on the band and conditions, but somewhere around 10 on the (arbitrary) SNR scale is about right. This setting also affects the transmit lockout previously mentioned.

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This program and associated documentation are respectively © Copyright Con Wassilieff ZL2AFP and Murray Greenman ZL1BPU 2003-2014. Please do not copy, alter or publish without permission. All Rights are Reserved.