Minimum Discernible Difference
radio, as with most things in life, is always filled with
and compromises, particularly when it comes to matters of signal
strength. Is that bigger antenna worth the extra cost? What
few more watts buy me? Should I replace my feedline with
that has lower loss? How much more readable would I be if I
added a few feet to my tower, or added a second antenna at some lower
height, in order to gain a few db at the takeoff angle of interest?
In my case, it was the antenna question
that first led me to try to quantify the difference in readability
between two signals of comparable, but unequal strength. I was
to decide between two antennas that differed by roughly 2 db in forward
gain and by several hundred dollars in cost. My gut reaction said
2 db wasn't worth the extra cost, but as a DXer and contester I changed
my mind after I generated some audio files similar to the ones I've
included here ... especially when I considered the probable reduction
in background noise that the tighter pattern of the larger antenna
would provide. No doubt others with different motivations or
constraints would have reached a much different conclusion, but
hopefully these examples may offer some small assistance to anyone
faced with similar choices.
Any comments (favorable or
otherwise) would be warmly received via email sent to ... dave (at)
ab7e.com. I am not an audio professional so be gentle with
of the files on this page use background noise recorded from a
moderately noisy and otherwise dead 20m band within approximately a 500
Hz bandwidth (roughly 350 Hz to 850 Hz with some rolloff in the 70 Hz
segments at each end). The CW Signals were generated and recorded to
.wav files using CW Player, a nice freeware application from F6DQM.
Various tones and speeds were used depending upon what I was trying to
accomplish, but in general I tried to choose those that I thought were
mid range for the majority of people (roughly 550 Hz and 26 wpm). All
audio fles were converted to .mp3 format and mixed with appropriate
level shifts using Goldwave, a reasonably competent audio editor.
files were generated with decent fidelity (32 kHz and 64 kbs), but
they may "play" differently depending upon individual situations such as
headphone or speaker response, software equalization, user hearing
response, user tone or speed preference, etc. Your mileage may vary
... a lot.
Each of the files must be downloaded from the links
and played locally .... they won't play directly from this website.
Embedding a player here would have been possible but wouldn't have given the
user much control over playback, at least not with my limited knowledge
of web crafting. If you fear that these may not be legitimate .mp3
files, my recommendation is that you ignore them.
Stepped Signal Strength
a first analysis, and as could be inferred by the title of this page, I
was curious what the minimum discernible difference was between two
weak signals of approximately equal strength. Clearly the answer to
that is a function of how close they are to the background noise
level. A difference of a few db might be unnoticeable for two
relatively strong signals, but if one of the signals is beneath the
noise threshold and the other is above it the comparison would be more
striking. I was interested in the weak signal case, so I generated
this file with CW text stepped in six 1 db increments beginning more or
less beneath the noise and rising above it.
In my case, I could reasonably discern a difference in volume for each
one db step, and my threshold of easy readability was at the third
level ("two db" sent text). Your results might be considerably
Here is the same set of signals, except in reverse order from loudest to weakest.
The file above tries to display readability purely with
respect to background noise. A somewhat different
consideration is how two concurrent signals of unequal strength compare
other ... how much difference in volume does it take to be heard first
when both are near the noise level. To explore that I created
four files with different tones and different volumes, , each with two callsigns sent at 26 wpm.
The multiple files were an attempt to provide enough tone difference
between the callsigns to distinguish them from each other, but with
mirror image tone assignments to compensate for possible personal
Two of the files have a one decibel difference in volume
between the two callsigns and the other two files have a two decibel
difference. Each file starts out with Callsign A louder than
Callsign B and then changes midstream to make Callsign B louder than
Callsign A. Version 1 files use 550 Hz for one callsign and 520
Hz for the other, while version 2 files use the opposite assignment.
Here are the four files:
In my opinion, one decibel difference is at least noticeable and two decibels represents a competitive edge.
Effect of Speed
In the presence of spikey background noise (my best attempt at a
suitable scientific term) it is sometimes helpful to use lower CW
speeds so that our brain can integrate each element over a longer
period and thus more effectively sort out the noise. To
investigate that effect I created the following file. It contains
20 different callsigns ... five each sent at 20, 25, 30, and 35 wpm ...
all of them using a 550 Hz tone. The volume of each callsign is
the same as the louder of the two callsigns in the Dueling Callsigns
I wouldn't try to take this illustration too literally, though.
Slower character speeds may help the dits bridge noise spikes but
faster speeds will slip in the gaps between static crashes more
effectively. Faster speeds may also facilitate copy during
conditions of rapid QSB. Slower speeds may get you ignored by
impatient operators or by those who naturally copy better at faster
speeds. Still ... if you're trying to make contacts with an area
of the world when the band is opening or closing, or if you know the
other end of the path is in an area of high QRN, cranking down the
speed might make a difference. If the other operator keeps asking
for repeats it might not be because his/her code speed is
necessarily poor ... it might be because the QRN spikes on his end are
obliterating your dits.