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
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