On the late 20's and Rudolf Hell invented the HellSchreiber, an ingenious fac simili teletext printer system, that was extensively used during WWII and later until the 60's.
The HellSchreiber transmitter consists of a typewriter machine connected to a spinning drum with contacts that creates a digital (binary) modulation of the 'pixels' of the typed character. The receiver consists of a spinder wheel activated by a solenoid that creates marks on a reel of paper that continously run. It's simple, it's brilliant, but with advance of telecommunications the Hellschreiber was abandoned.
In the early 90's the home computers got more advanced which allowed the Radio Amateurs to rediscover this method of communication by performing the encoding and decoding of the signals in software. Indeed, several variants were created and up till today it has several users.
Even in the 80's the computers could have done that, with a proper software. That's what this project is about.
The audio signal is input to the EAR socket of ZX spectrum.
'9' changes reception speed preset (105, 122.5 and 245 baud)
'5' tune down (decrease) reception speed
'8' tune up (increase) reception speed
'0' when pressed plots black pixels to check present rendering position
Text Editor with Z80 assembly Highlight
The choosen editor was "Context"
It was used PASMO, that can generate .tap files to be loaded on an emulator
ZX Spectrum Emulator
Can be either Fuse, RealSpec, Eighty One, or Spectaculator. Fuse was used due to its timing accuracy
Finally I could get my hands again on a real ZX spectrum (TK90X). The first tests showed that a single sample for each bit produced bad results (video).
A new sample routine was added within the delay loop for each bit. If there is signal a counter is incremented. At the end, if at least 1/4th of the samples were active, then a symbol (bit 1) is recognized and plotted as a black pixel. Otherwise it plots a white pixel .
This sampling routine was tested with the sound sample found in the Wikipedia article about Hellschreiber (video) as well as with some text generated by fldigi (video) and behave very well. It makes possible to use the volume control to change the 'intensity' of the black ink, which may be very useful when testing with live reception signals or recorded from live radio. This was tested using the sample in WB8NUT website.
Source code in .asm as well as .tap image available.
This is an Early Release for the project. The Decoder is working on the emulator and it's ready to be tested on a real machine. It can toggle between 3 basic speeds and allow the tweaking of the speed. The input shall be performed by key 0 or "RIGHT" on a Kempston joystick