[RTTY] Intrigued

[Kok Chen]

On Jan 26, 2007, at 3:30 PM, Ian wrote:

> Definitely not FeldHell. Might be Olivia.

Yes, definitely not Feld Hell, and not Olivia either unless you feed  
the Olivia modulator with some non-standard input (i.e., not the  
usual Varicode and definitely with no FEC nor any interleaving).

It is though, probably some variant of M-FSK, i.e., FSK with multiple  
tones (the class of which MFSK16, Olivia and DominoEX all fall under)  
but with more than one tone being active at any time slot.  The usual  
encoding from Olivia as far as I understand, will only output one  
tone at any time slot.

A waterfall display is usually implemented as an FFT (fast Fourier  
transform, an approximation of the spectrum) of the audio signal from  
the receiver.

Now imagine that you want to project some raster scanned text image  
directly onto the waterfall -- each pixel of the text would just be a  
point in the frequency-time space of the waterfall.  Each column in  
the text raster is just be a discrete frequency in the waterfall, and  
each successive text raster row is just a different vertical time  
slot on the waterfall.

So, to get text projected onto the waterfall, all you will need to do  
is to feed each row of the raster image into an inverse FFT to create  
the temporal sound samples (i.e., the raster image is in frequency  
space).  You could do this with a baseband analytic signal which is  
the complex output of the inverse FFT, and then mix it with a  
quadrature LO into an audible audio passband which you then feed into  
an SSB transmitter.

It would be interesting to find out what emission type this would  
fall under FCC Part 2.201 and if it is even legal for an amateur to  
transmit such a form of signal in the US.

It might fit under some fascimile/image classification using multiple  
tones.  Perhaps emission J2C?  We know it is not encrypted, and it  
can be made to fit into a finite passband, and many amateurs already  
have equipment that can decode it --- so it could be legal if  
documentation is published.

Transmit first and ask Riley for forgiveness later?  Not me.  I'm not  
going to risk my license. :-)

73
Chen, W7AY