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High Performance RTTY

from N4ZOU on April 24, 2013
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"Editor's Note: Due to the popularity of some of eHam's older articles, many of which you may not have read, the team has decided to rerun some of the best articles that we have received since eHam's inception. These articles will be reprinted to add to the quality of eHam's content and in a show of appreciation to the authors of these articles." This article was originally published on: 07/03/2004

High performance RTTY

Do you have a high performance Terminal Unit like a HAL ST-6000, Dovetron or a military HF DATA unit and find that you can no longer use it with your modern computer? Here is the way to hook it up to a PK-232 TNC and use it with a modern computer system.

The PK-232 and later PK-232MBX TNC has a port on the rear for hooking up an external modem. The header inside makes setting it up very easy. Any logging or terminal program that supports a TNC will work fine with the PK-232. With the Terminal Unit hooked up to the external modem port on the PK-232 you now have the TNC using the high performance demodulator instead of the poor one used in the PK- 232. The problem with the internal demodulator in the PK-232 is that it was set up for Packet mode that requires a wide filter and 200 Hz between the mark and space tones. This is fine for HF Packet and 200- baud Pactor modes but for RTTY, ASCII, and Amtor modes this demodulator is not optimized and suffers in performance. Even a sound card will work RTTY mode with the MMTTY program better than the internal PK-232 demodulator.

First we need to set up the PK-232 to accept the external Terminal Unit. Parts required are minimal and no soldering is required and the modification is completely reversible. You will need 2 Radio Shack mini clips found around there volt/ohm meters soldered to each end of an insulated wire about 8 inches long. This jumper wire is connected to the 7805 +5 volt regulated power supply and JP-6 pin A. This provides limited +5 volts for use with a buffer circuit between the PK-232 and the Terminal Unit. Look in your computer junk parts and retrieve a jumper. It's the same one used on computer motherboards, hard drives and cards. Radio Shack even sells them. Next set up JP-4 and JP-5 as shown below.

0x01 graphic

Setting the header up this way allows using a SPDT switch to select the terminal unit or the internal PK-232 demodulator for use. All high performance Terminal Units have very narrow filters on both the mark and space frequencies. This allows up to 110-baud rates. If you use HF/VHF Packet, 200-baud Pactor, and Morse modes you will need to select the PK-232 internal demodulator. At this point if you want to continue using the PK-232 without a Terminal Unit plugged in you can place another jumper across the external modem jack pins 1 and 2. The PK-232 will use the internal demodulator as normal and the jumper will prevent plugging in a 5-pin IDC connector in the external modem jack. In either case the PK-232 internal AFSK/FSK and PTT circuits are used.

Buffer circuits.

Some terminal units use TTL level logic at 0 and +5 volts and the PK-232 also uses TTL level signals. You could connect the TTL level output of the terminal unit to the TTL level input directly but this can allow the terminal unit to pass unsafe voltages to the PK-232 and damage U7 (8530) chip. A simple transistor buffer circuit will work fine and it's the reason we taped the 7805 +5-power supply.

0x01 graphic

Now for the circuit for a Terminal Unit that uses RS-232 level signals.

0x01 graphic

Some of the MAX-232 type TTL RS-232 chips require capacitors. Check the data sheet for the one you select and setup as required.

Audio supply.

You will need to properly feed audio to the PK-232, the terminal unit, and your sound card. This is critical for optimal performance of all three. The PK-232 will accept 8 to 600 ohms input and the line-in jack for the sound card is not very critical either but most high performance Terminal Units require around 600 ohms impedance at a relatively high drive level. You will need to build an audio splitter circuit to feed them. Here is the circuit I use.

0x01 graphic

I use an ICOM IC-756 PRO II and the audio output from the ACC-1 jack is just too low to drive the Terminal unit. In this case a HAL ST-6000. The sound card is very happy with the ICOM ACC1 level output and just to make sure a ground loop is avoided a 1:1 audio transformer (Radio Shack) is used to isolate it from the splitter circuit. Using a capacitor on the input to the LM-386 allows it to use 4 to 10,000 ohms impedance without distortion. C2 can be between 1 and 10 uF. I used a 3.3 uF as I had one already. The LM-386 is shown setup for 200db gain. Output of the LM-386 circuit is 8 ohms. An 8 to 1000 ohm center taped audio transformer (Radio Shack) is used to feed the PK-232 and ST-6000 Terminal Unit at 500 ohms each. Setup of the audio drive level is simple. Set R-3 at the mid point and use R-1 to set the required audio drive level for the Terminal Unit. Check your Terminal unit manual for this. Now use R-3 to set the proper audio drive level for the PK-232. Check your PK-232 manual for this.

PK-232 to your transceiver.

Hook up the PK-232 to your transceiver as shown in the manual except the audio in will be taken from the splitter. If you want to also use a sound card for software modes there are two different ways to hook this up. The best way is to hook the PK-232 up using only the FSK and PTT circuits. The sound card is connected for AFSK with the required interface circuit and the PTT control is via transceiver computer control with the software commanding PTT ON and OFF. This separates PTT and Microphone level audio and all operation becomes automatic. If you do not have FSK mode on your transceiver or software control of PTT simply use a DPDT switch to select PTT from the PK-232 or the sound card PTT circuit and AFSK tone generation from the PK-232 or interface audio output from the sound card.

Setting up the Terminal Unit.

After you have your interface between the PK-232 and the Terminal Unit built you will need to set up the receive tones to match the PK-232 and the FSK tones produced by the transceiver. The center AFSK frequency of the PK-232 is 2210 Hz with a 200 Hz shift. This is compatible with stations using a 170 Hz shift. Just make sure that your Terminal Unit is set up for "High Tones" and a center frequency of 2210 Hz also. The default center frequency used by MMTTY is the same. If you have questions about setting up the Terminal Unit simply tune in a RTTY signal with MMTTY and see if the Terminal Unit tuning display is the same. Be sure that you have AFC turned off in MMTTY and using the HAM default setting. The tuning indicator on the PK-232 should also be working and showing the correct in tune display. You do not need to worry about the Terminal Unit transmit circuits as there not used at all.

Operation of the PK-232

Start your terminal program and set it up for operation with the PK-232. A note to remember here is that any USB RS-232 serial adaptor will work fine with any terminal program running under any operating system that will accept USB standard devices. Windows HyperTerminal as well as N1MM and Logger programs have been tested and work fine with a USB serial adaptor. You might also download a program called Winwarbler. This is a free program and uses the MMTTY engine but has receive windows for the sound card and also the PK-232 at the same time. Tune in a RTTY signal and watch the text displayed from both the PK-232 and sound card. Here you can also flip the internal/external switch and compare the received text from a weak station. Also note that the Threshold control works for both the internal demodulator and the external Terminal Unit. When I am using the Terminal Unit I will turn the Threshold control all the way down and allow the start and hold circuits in the Terminal Unit to control print to the screen. This control is much better than the squelch circuit in the PK-232 and the squelch set up with the sound card system. When you have a fade the squelch can stop text print even thou there are still tones that can be copied. A good Terminal Unit will continue to print as the start and hold circuits work with the tones and not the signal strength level as used in a squelch system. There is no special setup required with Baudot and ASCII modes. You may use Pactor 1 mode with the Terminal Unit engaged but you must lock out 200-baud operation. You do this by setting PT200 OFF. For Key2Key operation you will never know the difference, most of us can't type that fast! If your going to use Airmail or Winlink you do not need to perform this project as it will be a waste of time for you. For RTTY operation in poor or contest conditions this setup will amaze you! Not only that you get to use your modern computer and latest software. As for Amtor with my ST-6000 the link will hold and pass text even when you can no longer hear the tones in the monitor speaker and the S-meter is just sitting on 0. If the tones are still there even if they are very weak a good Terminal Unit will continue to receive and produce text on the screen. For the other modes available like Morse, Packet, 200-baud Pactor 1 and FAX simply select the internal demodulator and set the Threshold control as required.

I have been using this system for several months now with a computer with no real serial ports at all. I wanted to use the later model computer but really missed using the HAL ST-6000 Terminal Unit. I could really tell the difference between a PK-900 which is better than the PK-232 or a sound card and the performance of the ST-6000. I had the PK-232MBX as a backup as I enjoy working the TOR modes, which you can't do with a modern sound card and remembering that it had an easy to access modem port I decided to see if I could come up with an easy way to use the ST-6000 with the PK-232MBX and still retain all the other modes as well. It only took me a few days to figure it out and make an interface for the system and best of all it works great.

I wrote this article to myself and a friend told me to go ahead and post it on here. June 7, 2004 I had an MRI and a Brain Tumor the size of a golf ball was found in my head. My doctor told me I could drop dead at any minute! I have major brain surgery scheduled for June 24 so by the time this is posted I will be dead or recovering from the surgery. The doctors tell me I have an 80% chance of a complete recovery with my memory intact. 10% chance of a limited recovery with disabilities and a 10% chance I die or will be brain dead. So this is going to be printed and put with this setup so I know what I did later if I don't remember or for who ever gets it when my wife sells it. Do not E-mail me about this article. If I make it I will post a note that I survived. Anyway, 73 to all and God bless. Scott N4ZOU

Member Comments:
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High Performance RTTY  
by VK3DWZ on April 24, 2013 Mail this to a friend!
Thank goodness I've never bothered with computers, and RTTY. Seems like a very great deal of work for very little reward. Yes, we have RTTY here--a HAL ST-6 we built in 1983 connected to a Siemens Teleprinter machine. All went together easily with very little fuss!
RE: High Performance RTTY  
by K2CBI on April 24, 2013 Mail this to a friend!
Glad that you made it Scott!

73, Mike
High Performance RTTY  
by VE3TMT on April 24, 2013 Mail this to a friend!
A simple keying circuit using a 4N25 into my FT990's RTTY port seems a lot simpler. But glad you recovered from the surgery Scott.
RE: High Performance RTTY  
by K9MHZ on April 24, 2013 Mail this to a friend!
Judging from his call sign lookup, it looks like he had a bad experience with some people here, I'm guessing in the past.

Not sure what that all means, but I hope he's OK. Nice article.
RE: High Performance RTTY  
by AA4PB on April 25, 2013 Mail this to a friend!
The HAL and other military TUs with L/C or switched capacitor filters were "high performance" back when they were designed. I wonder however, how they really compare with the current crop of software that is using DSP filtering.
RE: High Performance RTTY  
by N0IU on April 26, 2013 Mail this to a friend!
First of all, good luck with your surgery.

And I am with Bob AA4PB. It just seems like a lot of trouble to go through all of this for RTTY when MMTTY is pretty much the "gold standard" for RTTY these days. I built an "interface" "dead bug" style with one transistor and one resistor for the keying line and PTT line on the back of a DB9 connector and it all fits neatly inside the shell.

And Bob, I built the sound card interface from your QST article back in 2002 and I still have it! It worked great with my 756Pro's and KAM+. I don't use it any more since my current ICOM uses a single USB connection and I sold my dear old KAM+ years ago.
RE: High Performance RTTY  
by K9MHZ on April 28, 2013 Mail this to a friend!
The article is 9 years old.

I'm guessing and hoping the author came through his ordeal OK, which is the best news.
RE: High Performance RTTY  
by KL7AJ on May 1, 2013 Mail this to a friend!
One nice thing about having access to QST articles going back to the dark ages is that you run across some real gems of obscure methods for doing things.

There were some amazing advancements made to RTTY terminals back in the early 1960s. One of these was the recognition that RTTY (FSK) is actually FM and that by using a limiter you could greatly improve the noise also exhibited CAPTURE EFFECT, effectively blanking weaker signals on exactly the same frequency. (Not always a good thing for DX or contesting, but for reliable traffic handling, it couldn't be beat).

Another advancement was mark and space filters with actual consideration given to the time domain properties....this was pretty skookum advanced stuff before DSP was around. It's one thing to build a very narrow's quite another to build a very narrow filter with no RINGING!

RE: High Performance RTTY  
by K9MHZ on May 5, 2013 Mail this to a friend!
That true FSK adaptation to the SignaLink USB in the latest QST was thought-provoking. The gray sidebar explained the FSK/AFSK differences, albeit subtle as they are. Reading it, I couldn't help but flash back to hamfests of long ago, where people would swamp a guy who had a TTY printer for sale. I seem to remember a "snob effect" that went along with the RTTY "capture effect".

Then came sound cards for the masses. Maybe the old days weren't so great after the risk today of a few unwanted SSB products, but hopefully lots of dB down.

High Performance RTTY  
by WC2C on May 18, 2013 Mail this to a friend!
I hope you are having a complete and speedy recovery. this is a very interesting article. I do RTTY but this is far to complex for me to attempt, however I really enjoyed reading it. It reminds me of the articles that were so common in the old "73" magazines years ago.
Best 73 de Aaron - WC2C
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