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.