[TenTec] 400 Hz CW

[Bill Tippett]

N6KB:

 > Low frequency audio into the ear canal is a bit like VHF RF into a
microwave waveguide. Energy transfer from outside to the ear drum is no
doubt much more efficient at higher audio frequencies.

         Indeed this is correct.  The ear's actual peak
response is around 3 kHz as shown in Fletcher-Munson
curves:

http://www.silcom.com/~aludwig/images/Fletcher-Munson.jpg

However this is very different than the ear/brain's
response to signals in noise.  This gets into an
area called psychoacoustics which is fascinating to
research.  Here's an interesting summary:

How "wide" is your brain?

You may be reading some of this and say to yourself, "Self, this 
doesn't make sense - I can hear a weak signal as well with the CW 
filter as I can with the SSB filter.  What's the deal here?"

We are getting into a field referred to as Psychoacoustics, or "How 
the brain perceives sounds."  As it turns out, the "trained ear" can 
fairly easily resolve a bandwidth of less than 30 Hz - assuming the 
presence of random noise in the background.  This means that if you 
have a single CW signal amongst 2.4 KHz of white noise - or even 
other CW signals that are at roughly the same strength but at a 
different pitch, then your brain/ear is perfectly capable of picking 
it out, being able to "ignore" the 2.4 KHz of noise and the other 
"dissimilar" signals:  Under these conditions, narrower filters won't 
always improve the "copy" for a skilled operator.

Put this same CW signal in amongst other similar signals - some of 
which are much stronger and very close to the same frequency- and 
even the trained ear is hard-pressed to make out the "buried" 
signal.  Under these conditions, the brain isn't able to reject those 
"other" signals and a our hypothetical 30 Hz filter may make a world 
of difference.

A simple detection/indication circuit - like an audio level meter - 
does not have any means to be able to pick out that 30 Hz of signal 
among the 2.4 KHz of noise, so if you were to look at the meter 
alone, you would never be able to see the meter deflect in sync with 
the keying of the signal.  Take the output of a 30 Hz bandpass filter 
- one that passes only that range of frequencies occupied by the CW 
signal- and feed it into the same meter and then you will likely see 
the meter deflect with the CW signal.  Why?  Instead of 80 parts 
noise and 1 part CW signal bandwidth, we are now feeding it only that 
part of the bandwidth containing the CW signal.

http://www.ka7oei.com/qrss1.html (from box at right)

                                         73,  Bill  W4ZV