Feld or Field Hellschreiber or Hell Writing machine. Was designed 1929 it use human interpertation of the data being presented to the user via scanned text. No synchroization
FM Hell is a Hellschreiber mode. The Hellschreiber modes FELD HELL, PSK HELL , FM HELL and HELL 80 are graphical modes where characters are drawn and the interpretation done by the user.
For FELD HELL, the transmission is done by On-Off keying (OOK) as in CW, by BPSK for PSK HELL and by AFSK in HELL 80 and FM HELL (MSK in fact).
Generalities: Feld Hell is a Hellschreiber mode. All Hellschreiber modes are "Fuzzy" modes. The rules to be "Fuzzy" are:
- the transmitter uses no coding,
- the receiver does not decide when data is present,
- the receiver does not decide what data is present.
These modes are only human readable. They cannot be read by computers unless sophiticated neural system are used. In fact, hypothesis that human eye/brain character recognition in its context was better than a computer could do,
Created by : Rudolph Hell in 1929
Baud rate : 245 bauds but, in fact, it is a pseudo-122.5 bauds (one "pixel" is 8.163 ms long)
Reading direction : column by column, from down to up and from left to right,
Matrix : height: 7 pixels et width: 7 pixels…so a total of 49 pixels per character
Speed : 25 wpm
Modulation : On-Off keying of an audio tone
Reception mode : indifferent (USB or LSB) but in general USB
Character set : all ASCII printable characters except small letters, carriage return (+ line feed) and error correction character. The used font (FeldHell) is derived from the font of G3PLX (Peter Martinez),
Shape of pulse : raised cosine profile
Bandwidth : about 300 Hz,
Synchronization : no need, each column is displayed vertically 2 times (but transmitted once)
Pmean/Ppeak : 0.25
Lowest S/N : - 12 dB
Note: pieces of information about Hellschreiber modes can be found on the WEB site of ZL1BPU (Murray Greenman) . Pascal