[RTTY] Feld Hell

[Kok Chen]

On Mar 27, 2009, at 9:13 PM, rick darwicki wrote:

> Is there any one program that stands out for weak signal work on PSK/ 
> Feld Hell etc like RITTY does on RTTY?

Unlike RTTY and PSK31, Hellschreiber is a facsimile modes.

Instead of using a demodulator and decoder to guess a character to be  
printed, it uses the human visual system to help "fill in the gaps  
caused by noise," much like CW uses the human auditory system to help  
fill in the gaps with imperfectly received Morse code.

IMO, because of that, unlike RTTY, more of the "weak signal"  
capabilities rests on your own eye-brain system and not as much on the  
demodulator.

Just like CW, it is better to display all the analog nuances of  
Hellschreiber when SNR gets worse, much like it is better to copy weak  
Morse as received, instead of hard clipping it into  single tones that  
are only fully on or fully off.

The original Feld Hell inkers were bilevel devices  that were not  
capable of gray scale, but they were still capable of producing  
partial "pixels," thus giving it the "fuzzy" decoding capabilities (do  
a Google on "fuzzy" and ZL1BPU).   I saw my first Hellschreiber inker  
in the early 1950s when I was very young; it was producing a tape of  
characters from an SX-28 -- something I didn't realize was quite an  
engineering feat until I grew older and wondered how one even transmit  
Chinese characters to be printed over radio!

Modern software implementation does better than the mechanical inkers  
in that the computer display is better at printing gray pixels  
(assuming it is implemented as such) instead of only different sizes  
and shapes of glyph elements that the mechanical inkers were capable of.

The weak signal "fuzzy" nature of Hellschreiber is almost like  
receiving RTTY using a dozen different TUs and then letting your brain  
decide which TU(s) has (have) produced the correct output.  In this  
case, Hellschreiber makes use of the 2D spatial correlation that we  
humans are very good at.

You can do a bit better with gray scales by using modern variants of  
Hellscreiber, like FM and phase modulated Hellschreiber.  In  
cocoaModem, I had for example used a linear FM discriminator to  
demodulate FM Hellschreiber instead of using an FSK discriminator.  FM  
Hell also has the advantage of better "crest factor" (peak power  
versus average power) than Feld Hell (the original Hellschreiber mode).

The unfortunate part of Hellschreiber (for the implementor) is that it  
is difficult to test a demodulation completely objectively by going  
through an HF Channel Simulator as you can with RTTY -- your own eye- 
brain system is always a part of the Hellschreiber reception chain.

73
Chen, W7AY