Audio Diversity Spatial Reception: CW in Virtual Head Space
2006 Jozef Hand-Boniakowski WB2MIC
December 2005, after 26 years of using the Icom IC701 transceiver,
almost all on CW, I upgraded to the IC756 PRO III.
Talk about technology shock!
Though the IC701 served me well, the new innovations, bells
and whistles of the IC756 PTO III are awe-inspiring.
The Icom’s IC756 PRO III with its DSP, SSB /CW filters with
variable twin passband tuning, notch filter and two types of noise
reduction is very impressive. Having
a small antenna farm, the IC756 PRO III’s ability to switch
between two SO239 ports and its two separate VFOs got me to
thinking. Perhaps there
was a way to enhance the CW signals the IC756 PRO III was receiving
even more. Would it be possible using a pair of good quality stereo
headphones to listen to binaural, spatial CW?
I decided that it was and embarked on an audio diversity
spatial reception project.
could have taken a number of routes, but I decided to pursue one
that was inexpensive, simple and easy to put together.
I had been seeing ads in QST for the Idiom Press SCAF-1 audio
filter. I wondered if I could achieve my goal by using two of them,
one for the left earpiece and another for the right earpiece fed
into good quality stereo headphones.
The SCAF-1 audio filter had a good write-up, but I recalled
that my ham friend, Joe, in New Jersey, K2JT, had a 1970’s Autek
QF-1A audio filter he was willing to part with.
Since the Autek QF-1A has more notch and control options then
the SCAF-1, I opted for using it.
I bought the Autek QF-1A from Joe for $25.00 and began
looking on eBay and QRZ.COM for another. It did not take long before I was buying the second filter
for $30.00 shipped from Gordon, W4GLM, in Florida.
Both filters arrived within two weeks.
They looked good and worked well, just as I remembered from
back in the 1970s.
two Autek QF-1A filters have a selectable switch for choosing
between PEAK, NOTCH, LOW PASS, and HIGH PASS.
There is a selectivity and frequency control that works in
conjunction with each one of these controls.
Each filter also has a separate auxiliary notch filter.
This allows for peaking with the main control and notching
interference out with the auxiliary control.
The number of combination and setting possibilities between
the two filters should be enough so that any differences between
them, after each makes its way to separate left and right earpieces,
winds up placing multiple CW signals in different places in virtual
space, that is appear to be coming from different places inside the
operator’s head. At
least, I hoped so.
were a few other considerations.
I wanted to be able to move the signals around in my head.
I wanted to be able to move a single signal around, and to
move multiple signals in relation to each other.
It seemed to me that independently controlling the left and
right ear volume output of each Autek QF-1A was a must.
I thought about building my own little control circuitry with
a potentiometer, perhaps a little meter or LED bar graph
(for effect). And
then it struck me. Why
not use a stereo graphic equalizer?
Not only would the graphic equalizer provide independent
channel (earpiece) volume control and hopefully, add spatial sound
kinetics, it would offer a multitude of independently adjustable
additional frequency controls and enhancement (boost and cut off)
per earpiece. I started
looking on eBay and within a few days found a brand new Radio Shack
(model 32-2059) dual 15-channel graphic equalizer.
I bought it for $44.00.
I was tempted by the Nady dual 31-channel model, but that I
concluded was both unnecessary technically and an additional
expense. I was getting
quite excited about the prospects of the project.
The graphic equalizer arrived within a few days and I was
ready to put my audio diversity spatial system together.
hooked up the system by taking the audio output of the IC756 PRO III
going in parallel to the two Autek QF-1A filters inputs.
I used good quality shielded cable with ¼-inch phone plugs
at both ends. I fed the
outputs of each of the two Autek QF-1A filters using similar
shielded cabling into the left and right inputs of the Radio Shack
graphic equalizer. Each
of the three units was independently grounded to the station ground
buss. The two graphic
equalizer outputs were cabled into the left and right earpiece of a
pair of Panasonic stereo headphones.
Would it work?
first time I turned the system on was late Saturday evening on 160
meters during the very busy CQ World Wide DX Contest.
160 meters was hopping and this would be a good test of the
project. As soon as I flipped on the graphic equalizer, the IC 756
sounded like its normal self as signals were being passed straight
through the audio filters and graphic equalizer.
I then pressed the on-button for the graphic equalizer’s
left-channel and CW signals shifted in my head.
Then the right channel and they shifted again.
I was on the right track.
adjusted each and then both Autek QF-1A filters.
All I could say was “Wow!”
There was a CW signal dead ahead.
There was a signal to the right of center.
And, there was one far away off in the distance to the left.
I continued playing with the settings of both the graphic
equalizer and the Autek QF-1A filters.
I set all the 15 slider pots on each channel so that they
enhanced the range between 800 and 1200 Hz which I surmised were the
best for CW reception. I
enhanced these frequencies by advancing the associated 4 sliders up
above the midrange setting for both left and right channels.
With each change in the settings a different spatial effect
who have been around for a while may remember the Autek QF-1A filter
with its on-board 120 V.A.C. power supply.
When using high quality stereo headphones, the Autek filter
instruction sheet says that a hum will be heard when using
full-range low impedance headphones. The slight, but annoying 60 Hz hum was indeed there.
The instruction sheet suggests that adding a 47-ohm resistor
in series with the audio would help reduce it.
I added two 47-ohm resisters inside each Autek QF-1A.
The hum was significantly reduced, but not completely gone.
Cutting the frequency response pots on the graphic equalizer
on both channels to suppress low frequency response helped a great
deal. What did the
trick, however, was pressing the low-frequency cut-off button on
each channel of the graphic equalizer.
No hum whatsoever remained.
I also set the graphic equalizer sliders to cut out the high
frequencies above the CW note. For CW communications purposes these frequencies are
considerably attenuated background white noise.
Autek filters and the graphic equalizer have audio amplification.
While playing with the IC756 PRO III, Autek filters, and
graphic equalizer settings, a very strong signal appeared while I
had the rig and equalizer volume turned up.
I was not prepared to see the graphic equalizer fire a very
bright blue LED on each channel.
Apparently, music buffs, bands and DJs want to see measured
audio level a distance. This is one bright LED!
The LED dutifully blinked
in step with the received CW.
This threw me back to the days of my youth when around age 10
I taught myself Morse code using flashing light.
Radio Shack model 32-2059 graphic equalizer has 4 LEDs that serve as
channel output level indicators.
They fire at a range from –10db to +6db.
Only the first (blue) LED fires occasionally as the output of
the IC756 PRO III is not high enough to fire the others, or to
shatter eardrums. Each
of the graphic equalizer’s slide pots is capable of cutting or
boosting a band of frequencies plus or minus 12 db. The 15 sliders on each channel are centered around the
equalizer control bands centered around the following frequencies: 25,
40, 63, 100, 160, 250, 400, 630, 1K, 1.6K, 2.5K, 4K, 6.3K, 10K, and
16K Hz. Using a
different graphic equalizer would, of course, provide different
equalizers with built-in bar graphs would add more visual drama to
the ham shack. The
graphic equalizer also provides audio output for tape recording or
for sending audio to a computer for either further processing or
direct digital recording (to .MP3 files, etc.).
Using newer audio filters instead of the 1970s Autek QF-1A
might be interesting. The
Idiom Press SCAF-1 audio filter comes to mind as a possibility to
the end, the small amount invested in my audio diversity spatial
reception project was well worth the results.
The system is easy, fun to use and inexpensive.
With fine tweaking of the transceiver RF gain and system
controls, it is possible to make the CW signal being received
approach that of regenerated CW.
I would like to hear from hams who try my scheme and their
results and any improvements they have achieved.
Jozef Hand-Boniakowski WB2MIC
since 1963, Jozef Hand-Boniakowski, WB2MIC, is an ARRL Life Member
and is active on all the HF bands, 99% on CW.
After spending 33 years teaching mathematics and science, and
after 6 years in telecom R&D, Jozef is now fully retired.
He can be reached at 45 Lamb Hill Road, Wells VT 05774, or firstname.lastname@example.org
for any questions or comments.