The HAL RTTY-1 Tuning Display

              The RTTY Tune is a device marketed by HAL Communications to displays standard split teletype tones in a traditional cross display that mimics the “Crossed banana” displays of oscilloscopes.  In its standard configuration, the RTTY-1 takes raw audio from the receiver or other source and filters out all but signals near the 2125 Mark and 2295 Space tones used for old school baudot teletype.  Of course, ASCII and AMTOR/SITOR tones near these same frequency pairs will also be indicated.  The RTTY-1 does not decode anything, but rather just indicates the amplitude of the signal found within its passbands.

              When properly tuned, the arms of the cross will be equal length and the stronger the signal, the longer the arms.  They will alternate back and forth at the baud rate indicating proper reception of the signal.  If the signal is not centered on the standard tones, the arms will be of differing lengths and a slow, but sure alteration of the receive frequency should bring the arms into alignment and decoding should begin.

              The RTTY-1 is small and a far easier display to use than its predecessor the RS2100 oscilloscope.  The solid state RTTY-1 is much smaller and more power efficient while giving a much larger display as well.  The RS2100 was an actual CRT oscilloscope requiring 120V input in a large cabinet and substantial current draw.  The RTTY-1 is about an inch thick and runs off 12-13V DC.  The RTTY-1 also only requires a single audio feed via a phono plug, while the RS2100 requires decoded mark and space inputs from a terminal unit, a feature missing from most modern equipment, and difficult to pull from a DB25 port on other units.

              The standard RTTY high tones are the default of the RTTY-1.  However, if you change certain resistors, alternative audio frequencies can be selected.  Unfortunately, this is more of a permanent modification and does not allow for rapid changes.  The idea is to be able to use the indicator with marine radios or other equipment that uses lower frequency pairs.  This limitation makes the unit less useful for modern amateur RTTY operation where signal selection is usually made on a waterfall display by clicking the mouse and the tones can be anywhere from 500 Hz to 2500 Hz. 

              Where the unit holds the most promise is older hardware that relied on small LED indicators of various quality.  I can see this unit being very helpful for PK-232 or older KAM and SCS units that used bouncing LED’s to indicate tuning.  Of course, old terminal units such as the HAL ST-6 would benefit greatly from this display if a vintage RS2100 isn’t available.   For display purposes, it also makes a nice light show for when you are listening to RTTY.

              Typical of most HAL products, the power connector isn’t standard so if you get one missing the HAL specific coaxial power plug, it is worth a note to Urbana to acquire a new one.

              HAL did publish a paper on how to modify the tones of the RTTY-1 to either Low tones or Marine tones.  The former centers at 1,360 Hz (1275/1445) and the latter around 1,700 Hz (1615/1785).  Unfortunately, both modified tones still don’t help with most waterfall displays which like to center around 1500 Hz.  Still, the low tones are far closer to the center of the audio passband than the standard high tones.  I have not modified mine, since I tend to listen to internet teletype which is still sent on the high tones and all the scopes in my shack are calibrated to these tones.  However, if you would like to modify an RTTY-1 to the low tones you must replace some resisters.  The following is the table


Low Tones


Change     from   to

R1         6,980    11,000

R2         6,490    10,500

R12       7,500    12,400

R13       6,980    11,820

R6 adj 2295 Hz to 1445 Hz

R20 adj 2125 Hz to 1275 Hz



Marine Tones


R1         6,980    8,870

R2         6,490    8,450

R12       7,500    10,000

R13       6,980    9,310

 R6 adj 2295 Hz to 1785 Hz

R20 adj 2125 Hz to 1615 Hz


              Of course, my mind always goes to the place where you could add 6 potentiometers and have a variable resistance for the six resistors and then you can adjust the tones to whatever you’d like.  Of course, you’d have to mark each control to ensure that each is at the proper value for the tone set you’d like to see.  You would also be able to adjust it so that you can have different tone spreads for commercial/military teletype.  This would take some engineering of course as well as a plan to wire up a control panel with the new controls.



                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      

Front Panel controls of the HAL ST-8000A

              The ST-8000A was designed for the military and as such, the controls are simple to use and robust in construction.  There is a single toggle switch that powers the unit on, and the rest of the controls are membrane buttons.  There is a “2^nd” key that allows for most buttons to be used for two purposes.  There are also LED’s used for tuning, status indicators, and data output for settings such as tone and baud rate that have been selected.  Once quirk of the controls is that once the selection has been typed in, a green LED will blink until the enter key is selected to finalize the input.

              The 8000A is a complex modem that can be configured for a wide variety of inputs.  However, I use it only as an audio filter between the radio and the decoding modem, currently a HAL DSP-4100.  In this case, the important settings for operation are the setting up the filters so that the signal gets processed, and ensuring that the audio path is correct.

              There are two methods to tune the filters of the 8000A, both get to the same place, varying only in the parameters that are being utilized.  The traditional way to set the filters is to have the unit display the mark and space frequencies along with the baud rate.  First select the M/S display and enter each frequency in turn.  The difference between the mark and space will be the shift and the average of the two numbers will be the center frequency.  Since I am feeding a DSP-4100, I set the M/S at the standard Amateur RTTY tones of 2125 and 2295 at 45 baud.  This gives the standard shift of 170 Hz.  Since I am feeding the audio to a DSP-4100 that has it’s RTTY filters set at these tones, it is easy to match the settings.  The Radio RTTY mode outputs are also set at these tones, so the M/S display and setting method makes tuning amateur RTTY easy.

              The second method is to have the 8000A display the center frequency, shift, and baud.  This method is more used when monitoring commercial or military RTTY that use alternative shift and baud rates.  Since commercial and military stations are not as worried about bandwidth, the shift is often around 850 Hz, even with a relatively slow baud rate of 50.  In this case, the radio will be set to USB and to more easily utilize the radio passbands, the 8000A and DSP-4100 tone selections need to be changed.  To make things simple, I change the Cf to 1500 Hz, and select the shift  to the appropriate amount and change the baud rate, all by direct entry.  I then switch the display to M/S where the tone frequency are automatically displayed.   I then go to the terminal software and use the configuration commands to align the DSP-4100 to the same M/S settings.

              For an example of copying commercial RTTY over the air, German weather station DDK broadcasts weather over RTTY using a 425 Hz shift and 50 baud.  I dial in the frequency on the radio set to USB to 10.099.315.  This centers the tones at 1500Hz in the passband.  I can then use the various radio filters to reduce noise and interference.  I set the 8000A to 1500 Cf, 425 Shift, and 50 baud.  Switching to M/S display, I see the tones at 1287.5 Hz for Mark and 1712.5 Hz for Space.  I then go into the Hal Software and adjust the baud on the front panel, and go to the configuration page to change the M/S frequencies.  Using these settings, audio passes through the radio filters into the 8000A where it is filtered and then passed to the 4100 where the audio passes through the DSP board and finally to the decoder where it is converted to serial data for display on the screen.

              Traditional RTTY is sent on LSB but new amateur convention, which is nearly all soundcard modems now, uses USB for digital work.  To correct for this problem when using legacy equipment, everything has to be set to Reverse.  Since I am using FSK settings on the radio, the 8000A is set to REV as well.  I keep this setting for commercial RTTY and reverse it again in the software rather than change the modems around.

              There are some articles on the internet about using the HAL ST-8000 and ST-8000A as regeneration units to supply audio to a second decoder for conversion to serial data that is fed to a computer for display.  This is known as regeneration.  Basically, audio from the radio is filtered and decoded by the primary modem.  Instead of outputting serial date, the modem generates high quality Mark and Space audio tones that are fed to a second decoder for conversion to serial data.  The second decoder can be pedestrian in performance because the quality of the output from the regenerating modem is extremely high, even if that output is full of errors.  In effect the signal is decoded twice, but any errors in the first decoding are carried through to the input of the second decoder.  This is a stop-gap method to use the older TU with newer equipment.  My setup does not work like this.  In my system, the regeneration is on, but the pin that has the output signal is not connected to anything.  In my set up, the audio is filtered in the 8000A but never decoded.  The filtered audio is extracted before the demodulator and fed to the DSP-4100, where it is further processed and then decoded using higher end computers rather than just the analog decoders in the 8000A.  I have a ST-8000 in line that directly decodes the same audio that is fed to the 8000A.  The decoding of the 8000A/DSP-4100 is superior by a significant margin than the old “gold standard” that was the ST-8000.

              The final article in this series will see how radio teletype is utilized at N1ZZZ, and it certainly is, if not unique, at least highly unusual.