Question. Is there a simple RF sampling device and software that will allow
the RTTY RF signal to viewed?
Thank You!
Dave Greig N3BUO
Phone: (682) 422-6667
Twitter: @N3BUO
Facebook: https://www.facebook.com/N3BUO
http://www.801tactical.com
Twitter: @801tactical
On Thu, Jan 10, 2013 at 10:14 AM, Kok Chen <chen@mac.com> wrote:
>
> On Jan 10, 2013, at 7:22 AM, Joe Subich, W4TV wrote:
>
> > 400 Hz is nearly the minimum bandwidth necessary for proper decoding
> > (minimum intersymbol interference). Narrower filters work by cutting
> > adjacent signals more than the desired signal but they impose a much
> > higher group delay than a filter that is "flat" across the necessary
> > 350 to 370 Hz bandwidth.
>
> That is perfectly correct.
>
> Lets assume the receiver uses a Raised Cosine filter which has been
> designed to be the narrowest possible demodulation data filter with no
> intersymbol interference (ISI). This means that the receiver's filter
> needs to be absolutely flat for 45.45 Hz on either side of the shift (45.45
> is twice of the keying sideband fundamentals, which is 45.45/2 Hz). I.e.,
> the receiving filter has to be perfectly flat and has no phase errors for a
> range of 170+91 Hz, or 261 Hz.
>
> Accounting for group delays and ripples from crystal filters, plus any
> tuning inaccuracy, and Joe's 350 Hz is completely reasonable. I bought 400
> Hz (INRAD) filters for both my FT-1000MP and my K3 specifically for narrow
> band RTTY use, and that is the narrowest I am willing to go. I complement
> my receiving system with high dynamic range sound cards to handle adjacent
> channel interference.
>
> This is not an abstraction either, one of the reasons 2Tone copies better
> than MMTTY is because it uses Raised Cosine filters (a little too tight
> when there is strong selective fading, but that will be rectified in the
> near future, if it has not already been). When you use a receiving filter
> that is too narrow ahead of it you are negating a lot of the advantage from
> good demodulators. The intersymbol interference will probably degrade
> 2Tone's performance all the way down to the performance of MMTTY's
> demodulator when both have a dual peak filter ahead of them in the receiver.
>
> The corollary is this: if you ever find that a modem performs better when
> you engage a receiver's dual peak filter, it is because the modem's filter
> is not optimal for 45.45 baud RTTY. This is nothing new, and I have
> mentioned it before on this reflector. When you place a narrow filter in
> front of K6STI's RITTY or ahead of cocoaModem, you will invalidate the
> Matched Filters that are in those modems.
>
> When there is selective fading, the data filter has to be broadened even
> more since the envelope of the RTTY signal is now also scattered by the
> ionosphere. If you have put an RTTY signal on an oscilloscope, when
> propagation conditions become less than perfect, the signal envelope takes
> the form that is not unlike a random AM signal instead of seeing constant
> power on the scope. This is even more obvious when look at the individual
> Mark and Space signals. The 261 Hz number I have stated earlier is the
> absolute minimum for a perfect 45.45 baud signal (what we call the AWGN --
> Additive White Gaussian Noise -- condition). Any narrower, and the signal
> interferes with itself.
>
> 73
> Chen, W7AY
>
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