Posted by
Joe Subich, W4TV-4 on
Aug 09, 2009; 6:26pm
URL: http://elecraft.85.s1.nabble.com/K3-Filter-Ring-with-Noise-tp3410069p3413566.html
Jim,
> While Joe is rarely wrong, he is wrong about this, and Lyle
> is right. In nature (that is, at the input of your K3) the
> noise voltage is spread out over frequency. When you narrow
> down the IF, you receive less of that noise. As you widen
> the filter, the noise rises in direct proportion to the
> bandwidth. So when you narrow the filters, you are improving
> the signal to noise ratio.
You miss the point ... while there may be less noise power at
RF (or IF) as you narrow the filter, humans listen at AF.
At AF and generally increase the audio gain - with a combination
of AF Gain, RF Gain and AGC - to maintain a constant "desired"
signal level in their ears. When the lower noise power is
amplified more - and concentrated closer to the desired signal
- when the desired signal is at or even slightly below the
average noise level a narrow filter makes it MORE difficult
to copy the weak signal because the noise begins to become
coherent and sound more like a signal.
Haven't you ever noticed it is sometimes easier to copy a weak
signal in the presence of noise if you turn down the AF gain
until the noise itself is barely at the threshold of hearing?
This is much the same thing ... it's a psychoacoustic phenomenon
that is difficult to explain until you've experienced it.
However, the ear-brain system is relatively good at finding
a coherent signal in a mass of incoherent noise but once
the noise begins to become coherent (band limited and narrow)
it is like adding discrete interfering signals.
73,
... Joe, W4TV
> -----Original Message-----
> From:
[hidden email]
> [mailto:
[hidden email]] On Behalf Of Jim Brown
> Sent: Sunday, August 09, 2009 1:33 PM
> To: Elecraft List
> Subject: Re: [Elecraft] K3 Filter Ring with Noise?
>
>
> On Sat, 08 Aug 2009 22:35:42 -0700, Brett Howard wrote:
>
> >I liked Joe's comment about it being spaced over more
> bandwidth. Thats
> >an interesting thought but based on the whole logarithmic
> scaling stuff
> >it does make sense.
>
> While Joe is rarely wrong, he is wrong about this, and Lyle
> is right. In
> nature (that is, at the input of your K3) the noise voltage
> is spread out
> over frequency. When you narrow down the IF, you receive less
> of that noise.
> As you widen the filter, the noise rises in direct proportion to the
> bandwidth. So when you narrow the filters, you are improving
> the signal to
> noise ratio.
>
> The fundamental problem of narrowing the filter is a
> psychoacoustic one --
> that is, the ability of the human ear/brain being able to
> separate signal
> from noise. When you narrow the filters, the only noise you
> hear has the same
> frequency components as the signal, so the brain is trying to
> separate signal
> and noise that are very close to the same pitch.
>
> BUT -- human hearing is logarithmic, both in pitch and
> amplitude. Our ability
> to separate frequencies is based on proportional differences
> in frequency, so
> setting the CW pitch to a lower frequency gives the ear
> greater ability to
> separate signals (or signal and noise) that are close
> together in frequency.
> That is, if the pitch is set to 500 Hz, 100 Hz bandwidth is
> 20% of the signal
> frequency, whereas at a pitch of 1 kHz, 100 Hz bandwidth is
> only 10%. I know
> some really good CW operators (N6RO is one) who regularly
> work at pitches
> below 500 Hz for this reason.
>
> BTW -- I know exactly what the original poster of this thread
> is talking
> about -- I really suffered from it when I lived in Chicago.
>
> 73,
>
> Jim Brown K9YC
>
>
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