There is no school like the old school and this fax technology from the 20′s using psycho-visual perception for image synchronization is just that.   A fax mark is represented by a carrier; and if there is no carrier there is no mark.  After a period of time, 7/122.5 seconds for the Feld-Hell standard, the tape is advanced and a new column is started.   By double printing each column the decoder lets the mind visually synchronize the image.   So not only are the decoding and transmitter requirements kept simple but it works amazing well, allowing communication over noisy channels that otherwise might not be able to support it, say, using morse code.

AVR Keyer

This AVR keyer, written for the ATmega328, has both a key down signal to turn a carrier on and off and a keyed 1000 Hz tone for driving a modulator, a speaker, or the line-in of a sound card.     There are two key down pins for ease of interfacing.  One pin provides an active high key down signal (PD7/pin 13) and the other an active low (PB0/pin 14).  The keyed 1000 hz tone is provided by timer 0′s OCA pin (PD6/pin 12).

The message to be sent is actually hardcoded in the source code.  To change it you have to modify and recompile the source code.  The keyer will also send out any characters received over the AVR’s UART.  In addition you can change the speed of the transmission.  While Feld-Hell is ordinarily transmitted at 150 characters per minute, there is a much slower version of it called Slow-Feld which is used by beacons.  This mode can be enabled by changing the characters per minute value in the source code to 1.6455 from 150.

To decode standard Feld-Hell on Linux, Fldigi (http://www.w1hkj.com/Fldigi.html) works very well.    IZ8BLY’s Windows software (http://xoomer.virgilio.it/aporcino/Hell/index.htm) is also recommended.  It not only decodes Feld-Hell but also Slow-Feld, plus it can be run in Wine.  Note, Fldigi’s faster version of Slow-Feld is incompatible with IZ8BLY’s version.  (see http://www.qsl.net/zl1bpu/HELL/Modes.htm about Slow-Feld and http://www.qsl.net/zl1bpu/HELL/Index.htm for an Intro to Hell)

Unzip and to compile type “make MCU_TARGET=atmega328p feldhell.hex”