Inexpensive Audio Diversity Spatial Reception: CW in Virtual Head Space
© 2006 Jozef Hand-Boniakowski  WB2MIC

In 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.

I 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.

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The 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. 

There 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.

I 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? 

The 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.

I 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 resulted.

Hams 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 unnecessary.  This considerably attenuated background white noise.

The 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.

The 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 ranges.  Graphic 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 try.

In 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.

73, Jozef Hand-Boniakowski WB2MIC

Licensed 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 for any questions or comments.