[Elecraft] RTTY filter for K3

[Alan Bloom]

Good point.  It seems like an adaptive equalizer should work so long as
propagation hasn't totally garbaged the signal.  However, it would
greatly increase the complexity of the demodulator.  But maybe not if
the equalizer assumes a simple model of the transmit filter and makes
the a priori assumption that the channel does not change with time (i.e.
ignores propagation).

Al N1AL

On Mon, 2008-05-19 at 14:59, David Woolley (E.L) wrote:
> Alan Bloom wrote:
> 
> > 
> > The problem is that different RTTY transmitters use different amounts of
> > filtering.  However I suspect that a "Nyquist" receive filter designed
> > to compensate a typical/average transmit filter probably would have
> > pretty good performance on most received signals.  Does anyone know if
> > this has ever been tried?
> 
> With digital filters, one ought to be able to use the sort of adaptive 
> filters used in modems, where the filter coefficients are adjusted by 
> comparing the actual signal against the expected signal.  Modems send a 
> known training signal, to get fast initial convergence, but if the 
> signal is at least marginally decodable, I would have though it would be 
> possible get the filter to converge.  One could probably start by making 
> assumptions about the transmit filter and using known information about 
> the receiver.
> 
> This tactic will work better for synchronous systems, like PSK, but I 
> would have thought it must be possible to train a filter off an 
> asynchronous signal as well, although one might have solve for the 
> signalling unit centre position after receiving the whole character.
>