Sound Card for SDR the Hump in the Middle

There is a screen grab elsewhere showing the ‘Hole in the Middle’ of my Softrock TX/RX display. I used YouTube as a resource for storing my videos to use here, now they are a collection in themselves, hence I have produced a video of the ‘Nothing’.

There are some hideous Humps displayed by some users. Using the Delta 44 Sound Card provided the construction of the RX avoids loops, it is possible to obtain a ‘Hole in the Middle’.

Packet Radio the TNC

OK then whilst I still have my guns loaded lets have a blast at Packet Radio then we can go a bit more positive with the posts.

I still run Packet Radio despite the fact that it has almost become the domain for a few cranks only. I returned from France this summer to find the regulars in an out and out character assassination of one person.

I was amazed at some of those involved, they instantly went down in my estimation, so far removed were they from the Gentlemen of Radio that I used to know. I don’t know how it all started but to me it resembled Jade Goody in the Celebrity Big Brother Pig Sty, I just had to post a message on the Bulletin Board to that effect.

I was a late comer to Packet. One thing that put me off was that awful thing the TNC. Once there was the possibility of a Software TNC I joined the throng. Why is the TNC awful? Well it is like the Soundblaster Interface totally unnecessary. I did let one into my shack once it generated so much noise in the 2m Band it rendered it unusable for
low signal work. It is mode specific to the extent that many Packet folk tie up a Radio and a Computer just for Packet. Many TNC’s are so old that the parameters have drifted and they cause many a frustrated operator.

When our local packet BBS went off air I set up a BBS and Node here with 2 TNC’s ready for installation elsewhere. All I can say is, it was interesting but I was really glad to see the back of it.

Me, with my Software TNC, well I can be doing 6m Meteor Scatter one minute on the same Computer and Radio, then Packet the next minute all on the same famous THREE BITS OF WIRE.
Hence I sit and scratch my head in wonder what this unit is for.

Right Click and select View Image for full size.


Is that not an FT817, it has Data socket, why do you have to unplug the Microphone?

It does not isolate the Computer Audio Input whereas the Computer Output is isolated by a 600 Ohm Transformer and the PTT line by an Optocoupler.
I was prompted to look at this unit when a Ham reported blowing both his Transceiver and Computer Sound Card when connecting one up. Intrigued I looked at the circuit diagram and the danger is obvious. If there is a PD on the Rig Ground and you touch the computer Input or Output jack centre pin to it whilst plugging it in you apply the nasty PD to the computer.
Fifty Quid well spent I would say, not.

SDR Flex Radio Software

I have never liked the Flex Radio Software, I looked at it long before I had an SDR receiver to put into it. I thought that perhaps given proper I and Q inputs it would perform more to my liking. Not so. Here I will show some of my personal dislikes. The Spectrum display is imprecise. Here I hunt down a CW signal and try to follow the Morse using Spectrum Display ‘real time’ and with Averaging. Using other software it is possible to read Morse Code directly from the pulsing of the incoming signal, and so you should. I find the lack of real time display unnerving. Here I hunt down an SSB signal zapping down the band with the Mouse like I do on Rocky software. I have seen another You Tube video of Flex Radio and it did the same as I find, tune rapidly and the display collapses, the very signal you are heading for has gone, you go past, stop and whoa it appears behind you.

QRP 80 meters JT2/JT65

Once again we are testing JT2 and also JT65 on 80 meters when the propagation is really poor between UK and South West France a path of 720Km.

The picture is self explanatory. Right Click and select View Image for full picture.


At 25mW level on this test taking QSB into consideration results were comparable usng both modes. Perfect decodes.

JT2 Tests Continue

Most reports I see regarding the experimental mode JT2 seem to mention the Sound Card in the computer being critical. My own tests with F6GUU have revolved around some strange problems which we have not come to any conclusions about.
One hundred percent perfect test sessions can be interspersed with fifty percent failure sessions at other times. Seemingly fine signals in the clear just do not decode. Lack of local stations able to help in the tests cause uncertainty as to whether problems lie at one end or the other or both.
Consistently, failed decode sessions at my end indicate 0 Sync for each RX window.

The mode still lacks some refinements as stated in a reply to me from Joe Taylor.

The JT2 decoder in WSJT 5.9.8 was cobbled together very quickly. I
wanted to get something working so that on-the-air tests could be made.
Many short-cuts were taken, and potentially important features left
out. (There is no AFC action, for example.) I am sure that on-the-air
performance can be improved considerably. I hope to be able to spend
some time on it again, in coming months.

We would just love to be able to identify positively the signals and conditions that will cause a decode fail. To this end I pushed the limits in some local tests.
Using un-measurably low power into a Dummy Load on TX and an Inefficient Antenna or a Dummy load on RX I tested weak and strong signal performance.

Test 1

Lower signal limits were reached when the RX noise level was set to -10dB 50 Ohm Load and a signal which read -11dB.


100% decodes every time.

jt10rx11Test 2

Next a correct setting of 0dB RX noise 50 Ohm load and a signal at -7dB.


100% reliable decodes.

jt0rx7Test 3

Next 0dB RX on an antenna -8dB signal giving the Icom IC706 some real signals causes lots of visible spurii.


100% reliable decodes.

jt0-8Test 4

Badly adjusted RX level of +8dB and signal level of -5dB seemed to be about the upper limits. Just look at the mixer products and the PSK31 stations nearby, if you want to stress a Digi system use an IC706.


100% reliable decodes.


No signs of any critical sound card input levels here. Vast maladjustments still produced reliable results. The test reproduced a true on air path as near as possible.

Sound Card Interfacing for Digi Modes

Over the years I have been asked to write an article on Sound Card Interfacing, I have always resisted.
On the air I admit that I taunt those who have spent hundreds of Pounds/Dollars on commercial Interfaces instead of using “3 bits of wire”.
I have been astounded to see the inconveniences endured by those who have paid out the small fortunes too. Unplug the Microphone from the Rig and plug in the Interface when you want to change from Phone to Digi Modes, wow that is well worth paying good money for.

Recently I ended up on some pages on the web dedicated to an interface design, I think there may even have been a Yahoo group. Was there really a Lecture Circuit too at one time? I began to wonder just how many Billions of words have been written on the subject, how many thousands of videos, how many Millions of pounds profit made by companies. Yet I cannot remember one mention of Safety.

We and our neighbours go out and purchase a Hi Fi Amplifier a Recording Device, Computer and a Radio Tuner and think nothing of connecting them together Output to Input etc. Why? well they are designed to be interconnected. Well so are our Radio Transceivers, particularly the modern ones which have a data socket.

Why do Radio Amateurs panic when they want to connect their rig to a computer. Well you can ‘Blow up the Sound Card and the Radio’ yes that is true but that is due to lack of Good Practice. That lack could kill the Amateur.

Also you could get RF Feedback into the computer Audio, yes true, but will 600 Ohm transformers prevent it, not necessarily, whereas a common mode choke, (winding the screened lead on a Ferrite Core) probably will.

What the transformers will do is drastically limit the Audio Response, may be a cause of some problems in the past, certainly a disaster in modern radio SDR systems.

Anyone familiar with the Icom IC706 (the rig that receives on the frequency on the dial and many others at the same time and sucks more current on RX than 3 other radios put together; but that is another story), will be familiar with what I call the Shaving Brush. A Din Plug for the Data Socket, supplied ready wired with a couple of inches of multi core screened cable, outer cover removed for an inch or so which looks just like a brush with multi colour bristles.
Some years ago there were many IC706’s locally, owned by new Licensees. I was asked to provide details of utilising this for Digi Modes.

I photographed everything provided drawings, parts list and write up. I handed the draft to one keen user who was technically knowledgeable. One criticism, came the verdict, you spend more time, a whole A4 page, writing about Safety before you get down to the Interface, we wanted you to write about Interfacing.

There is my point.
Good Practice will prevent nasty voltages which will destroy the Computer Sound Card or Radio, as a small bonus it may save the Operators Life.
Good Practice will eliminate Ground loops and the need for Transformers.
Good Practice will prevent RF feedback into the audio circuit.

The big difference between the Domestic set up mentioned above and the Computer in the Radio Amateurs Shack is the fact that the Amateur is most likely going to be bringing Real Earth into the Shack and the Radio. He then is going to connect this Earth to a Computer which in the UK is most likely to have its ‘Earth’ connected to the Neutral of the Mains AC Supply. This Neutral is the centre point of a 415 Volt 3 Phase Supply. Under fault conditions, which I have experienced at this QTH when the underground cable Neutral failed, this can be lethal.

For Safety do not have both Mains ‘Earth’ and True Earth anywhere in the Shack, I leave you to implement this, or get in a professional to help. I want neither wrecked Equipment or dead Hams on my hands.

Pay 100, 200, 300 Pounds for your commercial interface and it may or may not protect your Sound Card and Radio. It will not protect your or others lives. You still have a potentially Lethal Shack.

For this reason I have not and will not mention Interfacing without dire warnings.

OK I simplify just a little with my 3 bits of wire statement, only the other day someone thought for a minute and said, not 3 bits of wire you mean 4. No I don’t, the wires are screened lead. Think about it though, you do not want to produce earth loops.
Also there is a need to level change the Computer Serial port for the Rig PTT so we must add a Transistor 2 Diodes and 2 Resistors. This circuit is already published in dozens of places on the web. I just build it into the 9 pin Serial Port plug and forget it. For portable use with my Lap Top computer I required a low profile arrangement. I therefore potted the circuitry onto a skeleton plug. The top of the Transistor can just be seen through the potting.


Off you go then down to the Ham store, pay half the price of your Transceiver for a commercial interface, most modern Hams do.
Whilst you are out be sure to get some good Rubber Gloves and thick Rubber Sole Shoes, always wear them in the shack, have extra pairs ready for any visitors and invest in a good Fire Extinguisher.

Yes some of my postings are meant to promote thought and discussion.