On Apr 9, 2011, at 4/9 7:44 AM, RLVZ@aol.com wrote:
> Based upon the information below that basically contends that
> "software
> based dual peak filters outperform the a rigs dual peak DSP filter"
> can I
> assume then that I'd do better during RTTY contests by turning my
> K-3's dual
> peak filter off" and turning on the MMTTY dual peak filter?
In general, that is correct. Especially when you change baud rate.
That being said, I have no experience with MMTTY, so I have no idea if
its filter is a true matched filter that improves SNR for weak RTTY
signals.
Keep in mind that matched filters are weak signal weapons (they
optimize the output SNR under a given noise condition); they are not
QRM weapons.
In fact, because an RTTY's matched filter have the shape of sin(x)/s
in the frequency domain, their envelope fall as 1/f, they are by
themselves quite wide when you are far away from the tones, but very
narrow when you are near a tone. So they are good for nearby QRM but
not good for far away QRM. You can think of them as filters that have
a lot of "blow-by."
The "dual peak filters" that I have seen tend to have sharp skirts.
So they look more like a cascade of a moderately narrow band pass
filter with a matched filter.
An analogy is the Audio Peak Filters for CW. They too are purposely
designed to be very narrow near the desired signal and then flare out
as you go further from the center frequency. You wouldn't use them
alone when there is loud QRM nearby -- under such a circumstance you
would supplement the APF with a narrow bandpass filter. One filter
improves SNR relative to noise and the other filter rejects QRM.
The key point though, is when you are using digital modes, and you
have a desktop computer sitting there anyway, and latency is not an
issue (a character may be delayed by an extra 10 ms before being
printed), software that are written by RTTY enthusiasts to run on the
computer can be better, perhaps much better, than the DSP firmware
that a rig manufacturer gives you.
If you have an Icom or a K3, you can check out the disparity between
what the manufacturer gives you and what RTTY software running on a
desktop can do by comparing the built-in RTTY decoder in those rigs
with a software decoder that runs in a computer. I don't know about
the Icoms (I have never owned an Icom HF transceiver), but my K3
really does not decode weak RTTY very well.
Once you can get the sound card into a state where it does not clip
(that is the reason why I keep alluding to I.F. filters as being the
"roofing filters" for the sound cards), you can do pretty much
everything you need for RTTY demodulation on a modern computer.
73
Chen, W7AY
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