One topic often ignored, or at least often misunderstood, is the
received audio.
Simply put, the whole goal of receiving DX stations is to actually
understand
what is being said. Anything that degrades understanding degrades
reception.

First, there is no good substitute for having a professional hearing
test.
My speech pathologist friend agreed that out of the many that I found,
this one makes no claims and might spot "gross problems".
http://www.freehearingtest.com/test.shtml

The speech pathologist pointed out that "if your on line friends are
even half
way serious about listening to and understanding weak voices in a noise
environment, they really should have their hearing test by a
professional".
She pointed out that subtle hearing deficits can have a major effect on
one's
ability to understand. I have learned that the further an accent is
from the version
I hear every day, the stronger, as in better SN/N, the signal has to
be. And for
the foreign languages that I understand, even less noise is allowed.
Since I can't
do very much about the noise, I try to not degrade the received audio
any more
then I must.

From my perspective anything that degrade or distorts the received
signal simply
adds noise. And some of what I am saying appears to contradicts other
parts.
For instance I feel that under ideal conditions there should be no
bandwidth limits.
But I am a strong believer in Dallas Lankford's elliptical low pass
audio filters. And
I love narrow crystal filters for some situations.

Quoting form Kiwa's "Tips for Improving Receiver Performance",
http://www.kiwa.com/rxtips.html:

"All capacitors from the point of detection to the speaker output
should be examined.
All ceramic capacitors within the audio path (from disc to SMT types)
should be
replaced with a polyester type such as the Panasonic V or B series
(available from
Digi-Key) or WIMA MKS-3 types. Low pf values (less than 10,000pf) could
also be
replaced by polystyrene types (available from Mouser7). Ceramic
capacitors,
especially the general purpose disc capacitor types are notorius for
creating bad
audio. Probably the best description as to what they do to the audio is
that they
create a smearing sound which reduces the overall clarity to the audio
quality.
The polyester types are much more transparent and they will improve the
audio quality."


There have been several threads on the effects various capacitors types
have on audio.
Instead of reinventing the wheel I will offer some links to pages that
detail and display
these effects.
http://members.aol.com/sbench102/caps.html

Walt Jung has a very interesting group of very informative articles
regarding amplifiers.
And one especially useful pdf showing a "real time capacitor checker"
http://waltjung.org/PDFs/A_RealTime_Signal_Test_For_Capacitor_Quality.pdf


To be fair, Ron Elliot has another viewpoint.
http://sound.westhost.com/articles/capacitors.htm

Mr. Elliot has some fine designs and I love his simple MOFET power
follower.
Very clean and very simple.
http://sound.westhost.com/project83.htm
Mr. Elliot also has what he calls a "Sound Impairment Monitor"
http://sound.westhost.com/sim.htm
http://sound.westhost.com/project57.htm#top
I found his first unit to be very usefully in evaluating amplifiers and
amplifiers
that showed significant "difference" also degraded weak, poor S/N,
audio
signals. Hi unit also shows the effects different capacitors have on
audio.


Spoken language is made up of small pieces called phonemes.
Anything that causes phonemes to be misunderstood reduces
intelligibility.
Since most readers will speak English as their primary language, I will
use
English words for examples. Other languages will suffer from similar
effects,
though naturally the exact sources of confusion will vary from language
to
language.

It is important to realize that even if you can not tell sailing from
failing due
to audio defects, you can often get at least the gist of a conversation
from
context.

These are links to interesting information about audio intelligibility.
I used a similar set of words to those in the first three links.
While these were designed for sound reinforcement studios, they were
the best I could find and have been very useful in my ongoing quest
for improved reception.

http://www.meyersound.com/support/pa...ch/drtlist.htm
http://www.meyersound.com/support/pa...ech/pblist.htm
http://www.meyersound.com/support/pa...ch/mrtlist.htm
http://www.meyersound.com/support/pa...h/section3.htm
http://www.meyersound.com/support/pa...eech/appen.htm
http://www.meyersound.com/support/pa...ossary.htm#drt

"Communications and speech intelligibility"
http://www.isvr.co.uk/spe_hear/comint.htm

Thoughts on "HiFi" SSB,
"What is Lo-Fi - Mid-Fi - Hi-Fi and Extended SSB ( ESSB ) ?"
http://www.icycolors.com/nu9n/essb.html

"The Effect of Bandwidth on Intelligibility"
http://www.icycolors.com/nu9n/images/Sound.pdf

This is a quote from
http://www.meyersound.com/support/papers/speech/section2.htm:

"One of the most obvious aspects of sound system performance that
affect intelligibility
is frequency response. Severely band-limited systems deliver speech
poorly. For instance,
telephones are generally limited to a 2 kHz bandwidth, and this makes
it hard to distinguish
between "f" and "s" or "d" and "t" sounds."

This is a link to charts that show the effect of increasing noise to
intelligibility.
http://www.meyersound.com/support/papers/speech/mbn.htm
http://www.meyersound.com/support/papers/speech/mf.htm

And this chart is usefully in that it shows the effect of a
"competing", and
unwanted, interfering voice(s).
http://www.meyersound.com/support/papers/speech/mv.htm

Little things can matter a lot more then I would have believed 18
months ago.

With every minor improvement I have made in my system, there has been a
direct and corresponding increase in my ability to understand weaker
and
weaker signals. This is more then just my opinion, it is the result of
careful
measurements, over a 12 month period using the ISM/HiFer beacon that
I have already mentioned.

I hope some find this useful.

Terry