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No Switch TNC/Sound Card Interface
Do not contact (N4ZOU)
on
January 5, 2004
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No switch TNC/Sound card interface
Here it is! That's right, an interface without a DPDT switch for selecting your TNC or sound card. This allows you to hide it and the cables out of sight. You also no longer need to worry about which device is selected. Now you're thinking, what's the catch? You will need a modern transceiver with Data and FSK ports. The newer versions of sound card programs now support transceiver control from within the program. This includes PTT control so that eliminates the PTT circuit from the interface. The transceiver will also need a FSK port which will eliminate AFSK audio between the TNC and transceiver so that only the sound card speaker or line out jack is connected to the transceiver audio input jack for generating a AFSK signal when using a sound card program. Doing this removes the requirement of having a switch separate the transceiver audio input and PTT circuits. You will also need two serial ports on the computer you plan to use. One serial port is connected to the Data port on the transceiver and the other serial port is connected to the TNC (or Hamcomm type modem). The interface will require your sound card interface circuits. This can be a simple resistor network for a 100 to 1 ratio using one 10 K-ohm resistor and one 100-ohm resistor which is shown in the drawing or you may use isolation transformers in the sound card to transceiver audio in line. This allows both the TNC and sound card to share the audio from the transceiver. My transceiver is an ICOM IC-756 PRO II and the interface works fine with it as shown in the drawing. You may also need an isolation transformer to replace the 1-uF capacitor feeding audio to the sound card. It would be very easy to use a commercial sound card interface. The TNC PTT and FSK cable do not need to go through the interface box. A single wire with shield cable can be taped at ether the TNC connector or the Transceiver connector and connected to the commercial sound card interface.
Now we need to get control of the transceiver by the computer so that we can have the sound card program send a command to key the PTT. There are about as many different ways to do this, as there are companies that build modem transceivers! Most transceivers use TTL level data, which must be converted to RS-232 level data using a level converter for connection to your computer. Many articles on building your own have covered this or buying this accessory from your dealer and so I wont go into it here. Just do a search and you will find many articles on building your own or buy one. These show up on Ebay all the time. After you obtain one you will need your manual on setting it up. Also the help files in the sound card programs go into great detail on setting it up. I use Logger 8.07, which has a very good data terminal for use with a TNC and supports accessing the data port on transceivers. You don't really need transceiver control while using the TNC so you could use your old favorite terminal program even in DOS mode. You could also replace the expensive TNC with a Hamcomm or Volksrtty type modem and build it in your interface box. For the sound card modes I use Digipan, Zakanaka, Hamscope, or MMTTY. All support transceiver control as well as many other sound card programs. Another nice feature of this interface box is that it stops that happy little Microsoft “Start Windows” music from being played on whatever frequency your transceiver happens to be set to when Windows checks for comports. You know we have all heard it! Hope to catch you on one of the many digital modes we have available.
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No Switch TNC/Sound Card Interface
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by W8JI on January 5, 2004
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Any interface should include isolation transformers on audio lines.
Not doing so is a very common cause of problems with purity of emissions.
73 Tom W8JI
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RE: No Switch TNC/Sound Card Interface
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by W8JI on January 5, 2004
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There also is another problem with this simple interface. The audio out of the soundcard is directly in parallel with the audio out of the TNC. Depending on how those devices are manufactured, the unused device can clip the audio of the device being used and generate harmonics or intermodulation.
The very least that should have been done is using an addition high value series resistor to each device being combined.
I think this is an example of something being "too simple" to the point where it shouldn't be used.
If it works without problems, it is just a lucky combination of devices.
73 Tom
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RE: No Switch TNC/Sound Card Interface
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by W8JI on January 5, 2004
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Never mind that last post,I had a brain fart. The audio lines that are paralled are speaker outputs in the shack, so they won't affect on the air signals.
It still should have a isolation transformer on the microphone lead though!!
73 Tom
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RE: No Switch TNC/Sound Card Interface
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by AA4PB on January 5, 2004
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I disagree that it is mandatory to have an isolation transformer in the audio lines. It may very well be necessary on any particular installation but then again it may work just fine without it. Until the advent of sound card modes it was the "norm" to connect a TNC or multi-mode controller without the use of isolation transformers in the audio lines. I've run a number of rigs and a lap top computer using nothing more than a resistive voltage divider and had no problems with hum, distortion, or RFI. I've run Tono, MFJ, SCS, and Hal TNC/Controllers over the years with no isolation transformers.
Of course there is nothing wrong with including an isolation transformer and if I were building units to be used in multiple stations I would certainly include them to reduce the chances of having a ground-loop or RFI problem in any particular situation. For ones own station however the isolation may not be necessary - just be sure to check your transmitted signal to be sure (which should be done anyway).
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RE: No Switch TNC/Sound Card Interface
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by K0BG on January 5, 2004
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Tom may have had a brain fart as he says, but I'd be leary not using an isolation transformer regardless. Some rigs have isolated speaker connections (neither side of the speaker grounded i.e.: SGC) and grounding either of the speaker terminals could distroy the ADSP circuitry. I'm with Bob on this one, it may work okay, but I'd still use the transformer.
Alan, KØBG
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RE: No Switch TNC/Sound Card Interface
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by OBSERVER11 on January 5, 2004
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I must disagree about the isolation transformers. I have used my home made interface with three different brands and four different models and I have not had any problem.
TS-440SAT
FT-817
IC-703
IC-706MkIIG
All connected from the sound card to the ACC connector on the transceiver.
I do pad the TX audio using a 100:10 voltage divider.
Proper grounding/shielding is needed to limit stray RF.
I do not use an optoisolator to key the transmitter either, I use a 2N2222A and a 1N4148 to clamp transients.
Very simple, very inexpensive.
Why put up road blocks? DEMANDING isolation transformers keeps the casual builder from finally taking the leap into the sound card modes. If you do not require parts you cannot find at your local Phone Shack on a Sunday afternoon, all the better.
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No Switch TNC/Sound Card Interface
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by N6JSX on January 5, 2004
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This circuit is potentially trouble waiting to happen.
1. I've seen some PC sound cards putting DC out over the speaker lines to power up external speaker amps. This circuit could put hot DC into the radio Audio IN.
2. I hope you don't use this circuit with a radio microphone plugged in - the loading of this circuit will cause you severe MIC level degradation - I found this to be the case on my FT-767GX.
3. For safety all around - inserting 1:1 Radio Shack isolation transformers is HIGHLY recommended! This will also assist in insuring you minimize dirty PC power supply humming coupleing directly into the audio chain. I don't care how much RF grounding you do - audio grounding falls into a whole different world than RF grounding.
4. I would also use opto-isolators on all switching signals - isolate and keep the units/signals seperate as much as possible. Ask some of the EchoLink board users how much trouble they have seen due to NO isolation transformers on the board. But putting in isolation transformers and opto-isolators fixes 99% of the problems. There is a HAM (can't remember who) that has a web site on PSK interfacing/software - look at it - it has good info.
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RE: No Switch TNC/Sound Card Interface
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by AA4PB on January 5, 2004
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If your sound card puts out DC power on the speaker line then you should be using a blocking capacitor rather than a transformer to block the DC. Secondly the 100:1 voltage divider would in all likelyhood reduce the DC level to an insignificant amount. Thirdly, you really should take some measurements to know what you have to deal with rather than just plugging things in and hoping for the best. Personally I have yet to run across a sound card that outputs DC power for the speaker on the signal line. Not saying that there aren't such animals but I doubt they are very common. One thing is that the DC power would likely cook any "normal" speaker that was plugged into it.
In all likelyhood the circuit will work just fine in most cases, although one should always check the quality of the transmitted signal, transformers or not. If your particular application needs the transformer then by all means add it.
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RE: No Switch TNC/Sound Card Interface
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by W8JI on January 6, 2004
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Isolation transformers should ALWAYS be used on audio lines. The rest like opto-isolators on control lines is optional and almost always not even necessary.
Even radio manufacturers are smart enough to not ground shielded audio cables at more than one point INSIDE the radio!
Even if a system works, it does not mean you aren't puting out needless hum and noise along with the good signal.
Suitable transformers are a matter of two dollars in cost for brand new units, and are as close as a Radio Shack store.
I can't imagine why anyone who understands how low level audio systems like this work would NOT want to keep the audio grounds isolated from ground loops, or why anyone would think an extra two dollars to ensure a clean signal is a hardship!!!
But then maybe the idea you don't need to float audio grounds is common, and that's why so many PSK signals come along with extra spurious emissions.
73 Tom
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RE: No Switch TNC/Sound Card Interface
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by AA4PB on January 6, 2004
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Do you realize that one of the most popular interfaces, the original Rigblaster, has no transformer isolation for the receive signal? That the Small Wonder Labs PSK series of PSK31 transceivers have no transformer isolation in the audio lines? That the Hal P38 has no isolation transformers? That the SCS PTC-II has no isolation transformers?
To keep total size down I wanted to interface my lap top to my K2 without using any external boxes. I was able to do it with a total of two resistors inside the radio, no transformers and the signal was clean - no hum, buzz or other distortion.
True that isolation transformers can eliminate ground loop problems and if you want to be on the safe side it is certainly reasonable to include them. The current "you've got to have isolation transformers" craze however has only appeared since the advent of the recent sound card modes. You may or may not actually *need* them to get a clean signal.
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RE: No Switch TNC/Sound Card Interface
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by KC8VWM on January 6, 2004
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>>>If you do not require parts you cannot find at your local Phone Shack on a Sunday afternoon, all the better. <<<<
"If you got questions... We have blank stares"
KC8VWM
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RE: No Switch TNC/Sound Card Interface
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by RFSOAKED on January 8, 2004
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BLANK STARES, HI HI!!!
That sounds like my local rat shack, any of the ones i have ever been to actually as well.
I use a Rascal interface from Bux Commco, plug everything up and forget about it. I run all digital modes available, echolink when i'm at work and have the time to call to the home rig, and fsk and hard-keyed cw (what they call it opposed to running sound card cw) all using the Rascal.
It's only $29.95 for the kit form right now and he includes one free interface cable for your rig, doesn't get much easier, and it's a fun item to build. Hey, i'm not making money off of plugging his creation, i just wanted to let you guys know that there is a heck of a lot easier way to do things.
My setup is simple, the Rascal is on COM1, the Cable to the CAT port on my Yaesu Rig is on COM2, and the audio lines from the Rascal go to Line IN, and Line OUT on the sound card. So, only four connections at the computer, but you don't really need the CAT connection if your radio has a DATA port. I made it rather simple by adding a junk box item, the Rascal has a 5 Pin Din jack for plugging in the radio interface cable, problem is that i wanted to use more than one cable. On the FT-847 (my beloved Yaesu) it has the HF Data I/O port which is an 1/8 inch stereo plug that provides RX audio, TX audio, and PTT connections for all AM/SSB/CW (soundcard sent) use. Then the radio has the packet port, which is used for echolink and packet and any other modes of operation that you want to use on FM. And then there is the Key jack for keyed CW, which the Rascal also uses via a 1/4 Stereo plug and Din plug on the other end.
All three interface cables are plugged into an old ABCD switch box that was used back in the days of AT keyboards and Serial mice. Allows me to switch between cables going to the radio and Rascal without unplugging the darn things all the time. And to keep control of the rig i also put in a toggle switch which breaks the connection of the PTT line with the Rascal, this is VERY important to have a transmit inhibit inline. The reason is simple, if you have the Rascal or other similar interface hooked up and you boot up the computer you will notice the TX led flicker a few times on boot, that is because the PC detects the Com ports and pulses them a couple times during post. You may also run into a problem with other programs changing the Com port status, i have seen it happen before, and also heard it with an unsuspecting ham transmitting without knowing it. One funny incident was a ham listening to music on his computer and the radio was transmitting it, oops!
I run a simple Y cable on the Audio OUT, which allows me to keep the computer speakers and Rascal both hooked up at all times, no cable flipping. When i want to run PSK or SSTV, two of my favorites, i just flip the switch on the speakers turning them off so the audio is only going to the Rascal. On another note, i use a SB Live sound card which is nicely suited to this use because the Audio OUT lines, it has front and rear, are unamplified line level outputs, and it also has the seperate Line IN which is much better suited for this use than a MIC IN, at least in my opinion after using other setups with the MIC jack. That is important because it allows me to set all the audio levels in the control panel and leave them, i then adjust the audio level of music or games, or anything else using the volume control on my amplified surround sound speakers.
Its the best setup for everyday use for me. Once it was all hooked up and the audio levels adjusted i just left it all as is and havent had to touch anything yet. And if you have access to a sig scope to check your transmitted signal please do, its annoying listening to the ones that overdrive the transmit audio, not to mention that you notice them right away on the waterfall, they are usually the signals that are twice as wide as the receive marker.
73
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No Switch TNC/Sound Card Interface
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by N0VUB on January 10, 2004
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I have always used transformers in all my audio interfacing between sound card and radio. It's not so much that it won't "work" but matching audio output of the sound card to the mic input of the radio can make a difference on levels and clean audio. I use them in broadcast radio for those very reasons, why not use in ham radio.
73
Mark
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RE: No Switch TNC/Sound Card Interface
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by AA4PB on January 15, 2004
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In low level audio work it is not necessary to match impedances. As a matter of fact, standard professional audio equipment with a line level interfaces use an output impedance of 100 ohms or less and a load impedance of around 10K. Even audio power amplifiers generally use a very low (under one ohm) output impedance to drive an 8-ohm speaker in order to have an impoved damping factor. The audio stages inside equipment often uses op-amps with a 500 ohm or so output to drive the high input impedance of the next stage. Impedance matching provides maximum power transfer but that is not needed for low level audio circuits.
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