[RTTY] Dealing with High-Pitched Background Noise?

[Michael Rapp]

Interesting, Tim.

That is one thing I will play with.  I also need to work on getting a
better grasp of how adjusting the RF gain affects decoding.


On Wed, Jul 30, 2014 at 10:47 AM, Tim Shoppa <tshoppa at gmail.com> wrote:

> I think you are dealing with a psycho-acoustic phenomenon related to having
> a very narrow bandpass filter, where the narrowness is less than 10% of the
> center frequency..
>
> If a narrow 200Hz filter is engaged at 800 Hz center frequency, there is
> the psycho-acoustic impression of ringing artifacts. Exact details of the
> ringing depends on how poor the group delay response variation of the
> filter is - a 200 Hz narrow brick wall filter will have very pronounced
> ringing artifacts. Due to the low center frequency we notice the ringing
> artifacts and think of them as filter ringing in response to impulses.
>
> But if that same brick-wall narrow filter is engaged at 2125 Hz center
> frequency, the ringing artifacts sound different to our ears. It is the
> same phenomenon, but it can sound like a continuous noise and we don't
> really notice how it's impulse driven, but really the way I think about it
> is as if a bell is getting hit with hundreds of little hammers. Yes,
> psycho-acoustically, this drives me nuts!
>
> I've generally found that for RTTY, the MMTTY and (even more so) 2Tone
> decoders work much better with 500Hz or broader receiver filters. Enabling
> narrow brick wall filters usually does not work out very well. Yes I had
> some battles over run frequencies in NAQP RTTY too! 2Tone can work very
> nicely with wide receiver bandwidths, as long as you disable AGC or turn
> the RF gain knob down enough, such that strong adjacent signals don't pump
> receiver gain.
>
> Tim N3QE
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-- 
/*/-=[Michael]-=/*/