RTTY DECODE & DISPLAY

The following describes an electronic design to decode and display radio teletype (RTTY) signals from a radio receiver.
It uses the popular Z80 8 bit microprocessor chip family to do so.
Prior to the computer age; RTTY reception was acheived using an electronic tone detector/slicer (many being vacuum tube based) with a mechanical readout device i.e. the Teleprinter.
My design dates from c 1980~89, is all electronic, self contained, and displays text as a 16 character rolling message format using 7-segment LED readouts !
See below for an example of the character set (quick brown fox text).
Your eye adapts suprisingly quickly to this presentation and is easy to view.
As this is a microprocessor based design it lends itself easily to decode of various standard "baud rate" signals (50/75/100 baud), variation in mark/space frequency shifts (170/425/850Hz), normal/reverse tone pairing, and provision for de-crypt of 'privacy' codes i.e. 1 or more inverted "bits".
As can be seen from the following pictures all circuitry was contained in two mating metal boxes (uProcessor module and Display module). Wire wrap socket construction is extensivley used giving maximum component density.
The unit is fully portable requiring only mains power and suitable audio input of RTTY signal stream.
The machine code sofware listing to run the Z80 chipset is reproduced below; from my article printed in the May 1981 AARTG (Aust Amateur Radio Teleprinter Group) bulletin.
Unfortunately the built example (pictured) no longer exists, it having been stripped to provide parts for other projects !
The design will decodes CCIR-CCITT, ITA 2, 5 bit start/stop radioteleprinter code, (AFSK) sometimes called "Baudot" which was once widely used for broadcast RTTY radio traffic.
As there is no form of error detection & correction a good strong signal is best, less a lot of erronous characters appear - caused by signal fading etc.
Nowadays (c 2010) no high power RTTY press/diplomatic/military transmissions can be found in the H.F. spectrum.
However during the 1980's I logged and "viewed" numerous stations; an example of my logging below :


Pictures & Diagrams: scroll down and across.

LINK TO SOFTWARE











THRANE RADIOTELEX c1980's
LINK TO uPROCESSOR MODULE CIRCUIT



















LINK TO DISPLAY MODULE CIRCUIT
LINK TO
ITA 2 CODE SPECIFICATION