[RTTY] Dual Peak Filter question

[Kok Chen]

On Apr 9, 2011, at 4/9    7:44 AM, RLVZ at 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