Wednesday, 22 July 2015

Data modes - 1. Getting started

This posting is supposed to be one of about three - this one dealing with the basics, one with CAT-type rig control, and one with the software and modes. There might be others in the series too. I will try to link them all together too! Just like World Peace, it will be great once we finish building it.. 

Since my first data contact - RTTY on 20 metres using a Creed teleprinter 35 years ago - to my latest - 4 metre meteor scatter using computer generated FSK441 on 20 July 2015 - nothing fundamental has changed.

That may sound odd, but I think that although I have ditched the mechanical teleprinter and replaced it with a computer, the basic idea remains the same. You generate tones, you insert them into the audio section of your rig, and then you take the received tones, insert them into the audio section of your decoder. So at its simplest, you only need an audio cable. Then you need to find some way to switch the rig from receive (rx) to transmit (tx), but not if you just want to listen.

It really can be that simple. An audio lead is all that is standing between many amateurs and the world of WSPR, moon bounce, meteor scatter, PSK, JT65 data QSOs, photos beamed from the International Space Station, slow scan TV, ...

Back in the 1970s it seemed a lot simpler, you just made up the audio lead. These days you are bombarded with fancy solutions, many of them very expensive. Recently, when I received my Yaesu FT-450, I started off using it on data modes with just a single audio lead. So I am writing this to try to dispel some myths that the whole subject is too complex to deal with.

One thing I will not deal with here is packet radio. Packet was (or is?) a sort of amateur version of the packet switching formats behind Internet communications. We got there first in many ways, but the telecoms companies quickly adopted packet switching to offer Internet over the phone system, so I will not be dealing with that. Nor will I deal here with any system which uses switching rather than audio tones. For a time rigs offered ways to send RTTY direct from the teleprinter, and these still hang around, but they are not really practical for many modes we use now.

Conveniently, a lot of modern rigs have a standard socket for data tone input/output. It is a six pin mini-DIN, shown here on an earlier picture I took of the back of my FT-450 ...
It is the one on the bottom left marked, handily enough, "DATA".

Many rigs made in the last 20 years have this more-or-less standard 6pin DIN socket. Check in the handbook of your rig to make sure. For example, the Yaesu FT-817 handbook has this diagram of the pin connections:-
As we are not looking at a tx/rx control yet, (PTT for push to talk), we only need to think about the data tones in and data tones out, and indeed only one data audio out connection (there are two). Again from the FT-817 manual:-
And now we see a difference between my simple audio lead and what Yaesu show. They show the rig connected to a terminal unit with a PTT connection. For the FT-817 you need to switch it between tx and rx if you want to transmit (but not if you just want to listen).

However, we are dealing with the FT-450 to start with. It has a "Digi-VOX". Digi-VOX is like microphone VOX, so when you have it turned on, it monitors the digital input via the mini-DIN plug for an audio signal like the tones you are trying to send, and if it sees them, it switches to tx. Simple (but you have to get the level and sensitivity right). If there is a tone you are sending the rig, you are swtiched to tx, if not, then you are in rx. Perfect. All you need to send PSK or whatever is the audio lead from your computer, and when the rig detects the audio tone it will go into transmit automatically.

Here is the audio lead I used to get started with the FT-450:-
You can make them up yourself though I bought this one on eBay. Not exactly high tech. You plug the mini-DIN into the rig and the 3.5mm stereo plugs into the sound card of your computer. There are two 3.5mm stereo audio plugs as your computer sound card has separate audio in (sometimes marked "mic" or "line in" at different signal levels) and audio out (sometimes marked "speakers" or "headphones").

Set the audio levels right and you are now ready to download one of the data programs from the Internet (FLDigi, HRD, WSJT-X etc) and off you go  ... provided that your rig has digi-VOX.

The FT-450 has digi-VOX, so has the FT-897. I seem to recall that the TS-590 has too. But the FT-817 does not, and neither does the IC-7100. So for them you will need some sort of switching for the PTT as well as the audio lead.

Digi-VOX is not really perfect. It works and I have used it many times, but the problems which arise can make life difficult. For example, any transient click on the audio line will cause the rig to go into transmit. If you forget that you were working data and decide to play your latest tunes on iPlayer, they will get transmitted on 20m or wherever you are. There are ways round this, but it is probably better in the long run to take control of you PTT.

You can buy data interfaces which have built-in data-VOXs in them. So all you need to do is to plug the audio leads into the computer and the mini-DIN into the rig, and the switching is taken care of automatically. This turns a rig without digi-VOX into one with digi-VOX, but does not really solve any of the problems. Simple though.

At this stage you will also have noticed that you are plugged into your computer's sound card, in the case of a lap top you have silenced the speakers, and in the case of a desk top you have had to unplug the speakers. To solve this you can install a second soundcard (I have 3 installed, and two "virtual" ones - more on that later). You can buy cheap USB sound cards for a few £££ on eBay. Then you plug the sound card into a USB socket and plug the rig into that. Here is an example of what a USB sound card can look like :-


This is a simple cheap one, there are other designs, and you can get all sorts of expensive and bigger ones. The principle is the same. I have compared more expensive ones and I have to say that I cannot find any real difference in the better ones, and the cheap ones seem OK for data purposes. You might need a USB-USB extension lead to separate it from your computer as it is very wide, but apart from that the cheap ones seem fine. You can of course connect speakers to the cheap USB sound card and use your in-built sound card on the computer for the rig.

If you add another sound card you will need to select it for data purposes in your software, and adjust the audio levels to suit. More about that in our third posting on data modes (I hope).

Right, so we have our audio connection made. How can we control the PTT if we do not have or do not use digi-VOX? Well, the Yaesu diagram above showing the RTTY terminal unit connected to the FT-817 shows a handy terminal on the terminal unit marked "PTT". Sadly, our computers do not come with a PTT terminal. So we need to add one. This can be done simply by using a switching chip connected to a USB socket on the computer. The will turn your rig to tx as required via the mini-DIN plug. That is the simple way, but the more complex way is to do it via CAT control (which is the subject of part 2 of these postings).

So let us stick with simple PTT switching. For a long time you could buy simple combined audio connections and switching controls on eBay. You can still find ones using CAT control, which allows you to do more things. But for now we are sticking to non-CAT. The sort of leads you found were these:-

This still shows the two 3.5mm audio plugs, the 6 pin mini-DIN data plug for the rig, but also another USB plug for the computer. Inside the box is a chip to convert DTR or RTS signals from the computer into a simple switch connecting the rig PTT to earth. That will switch your rig to tx whenever required by the software. The chip shows up as a peripheral device on the computer in the "COM Ports" list on the "Device Manager" screen. So it might appear as COM3, for example. You then set up your software to send (DTR or RTS) PTT signals to "COM3" (or whatever number Windows gives it, if you are using Windows) and that turns the rig to tx. Now you have automatic tx/rx switching, and if you try to play music after you have finished operating, the rig will stay in rx and you will not transmit the music.

What we have done now is go from the top drawing to the bottom one:-
The photo of the lead shows the chip mounted in a box along the lead, though you can get leads on eBay with the chip at the USB plug end (note though, once again you may need a USB-USB lead to separate the wide plug body holding the chip from the USB sockets).

This just gives us simple audio in/out and PTT. It does not give us CAT control. CAT is useful as it allows you to control the rig frequency from the software. CAT opens up all sorts of possibilities, but that is for the next posting in this series. CAT can be really tricky to set up, but once set up, it runs well.

If your rig does not have a CAT connection, or your software does not support one, or maybe does not need one, then this is as far as you need to go with connecting up the rig to the computer (but see below about earth loops and isolation).

Now that we have a working system to convey audio and PTT between the rig and the computer, there are still a few issues to clear up.

Firstly, the use of a simple audio lead can bring some problems. The audio lead can pick up RF from the rig, so it needs to be well screened (and ideally short too). Also, and more tricky to resolve, it can help form an "earth loop" between the rig and the computer. This allows mains hum to be picked up and it is very difficult to fix. I have had all sorts of issues with earth loops over the years, but only while dealing with audio equipment. I have never had the problem with data links for radios. However, it is a good idea to isolate the rig from the computer. It is also a good idea to isolate the PTT line too, so that neither has a direct DC link to each other.

Isolation can be done in various different ways, but using audio transformers is popular, as are opto-isolators for data isolation. Here is a look inside the box of the lead shown above, and you can see the chip and the audio isolation transformers:-

The transformers have yellow insulation on the cores and sponge on top to locate them physically. The chip is between them to the left.

Some of these data leads were pretty badly made. Most used chips made by Prolific. The chips may or may not be real Prolific chips: some are fake chips. You can still get the fake ones to work if you know how, despite Prolific's reasonable attempts to render them useless. Then again, if the audio transformers are not good quality you find that the audio bandwidth is small which is limiting for some modes like JT65 or PSK where you are offered a waterfall. With poor bandwidth you get narrow waterfalls. They do work though.

Secondly, given the poor bandwidth, you might want to replace this composite PTT/audio lead with just simple audio lead and a "CAT" lead. You can buy simple CAT leads on eBay for most rigs. Here is one for the FT-817:-
In this case it has a USB on one end with the chip built into the plug, and an 8 pin "CAT" plug for Yaesu rigs on the other end. I have various versions of this one, with D9 or 3.5mm plugs, depending if it for Yaesu CAT or Icom CIV fitted rigs. But whatever, this is still all you need for data operations. As the drawing above showed, the advance beyond just an audio line at the the top of the drawing, is an audio lines and something to control PTT (CAT just controls PTT and some more things). BUT CAT is often troublesome to set up, so more of this in a later posting.

Thirdly, I am talking about simple set-ups. The sort of audio lead you can make yourself, or even the data lead if you feel adventurous (yes, I have made one myself). But you can buy all sorts of things cheaply from eBay and even go to ham shops and spend a fortune. Nevertheless, I am trying to point out that all this hides a simple concept of two leads, one for audio, and one for data. Yes, you can buy a box that does it all for you, but it is useful to know how it all works.

Finally, very recent rigs such as Software Defined Radios (SDRs like my Flex 1500) or less basic radios (like my Icom IC-7100) now have USB plugs which allow direct digital connection to a computer. This saves both the audio lead and the PTT or CAT lead. Both of those rigs create "virtual" audio ports inside the computer. Then you get digital audio right from the rig into your processor, and it saves issues with bandwidth, earth loops, USB sound cards and the whole palaver. Still, it is good to know what "virtual" stands for, just a software version of the sound card.

Beware of some rigs which have USB connections but which do not set up virtual sound cards. I think that Alinco make one like this, which allows you to control the frequency of the rig, but you still need to take the audio out down an audio lead. Although you miss the extra quality and ease of a digital audio line, you just work with an audio lead instead. Just like I did with RTTY back in the 1970s. Nothing changes, really.

More on CAT and modes/software later, I hope.

See you all again soon.

73

Jim
GM4FVM

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