[Elecraft] Re: Binaural CW Reception

[Jerry Volpe]

I have done a couple of Binaural CW reception projects and found the
results very appealing. Let me begin  by sharing a few thoughts:

1. With binaural reception you normally would NOT use tight CW
bandwidths as the object is to allow more audible information to
be processed by our brain. I think that you would want at least 600Hz or
greater. I typically use a 1000Hz type filter or wider but generally
nothing tighter. A stereo 'soundscape' is created within your head with
the primary signal (the one centered in the Binaural filter) appearing
in the center of your head, and the ones higher in pitch progressively
off to one side and the ones lower in pitch off to the other side.
2. Due to the sound dividing by frequency you notice only the lower
frequency background 'noise' component in one ear and the higher
frequency component in the other. It is surprising how much LESS in
intensity the noise is overall than what it was with combined energy to
both ears. This alone is worth the price of admission.
3. Many of today's CW operators have not trained themselves to use their
own brain as a CW filter and instead rely on very sharp band-pass
filters for single-signal reception. Binaural CW reception provides
multiple signals which many find too confusing.
4. Binaural CW reception should be great for NET, roundtable, and even
contesting where you often need to hear many stations on slightly
different frequencies.

My first project was a stand-alone Binaural CW filter using a pair of OP
Amp ICs. Basically, the audio input passed into both a low pass filter
and a high pass filter with the low and high frequency cutoffs at the
desired center frequency.... 700 Hz for example. The output of the each
filter was further amplified (as needed) and then applied to either
stereo speakers or to stereo headphones.
PRO: Small package. Simple approach. Low cost. Easy to build. Works well
enough for a single design cross-over frequency.
CON: I could have used better quality OP Amps for lower distortion. The
ones I got were from Radio Shack (sigh). Still not bad. Can't change
cross-over frequency. This is OK if your receiver has a fixed CW offset.
Myself, I like to change my offset to minimize long term listening
fatigue. But then again only a couple of my transceivers allow for
that.... most don't. Another CON was the fact that I had to build this
filter. It was amazing to me how many hams were interested in the
project (I posted the info on the Ten-Tec reflector last year) but were
unwilling or unable to assemble a project without a kit.

I think it would be wise to use two pass-band filters rather than a
low-pass and a high-pass configuration. That way you can also take away
the unnecessary lows... say below 300 Hz and the unnecessary highs
perhaps over 1500 Hz. These filters should not have sharp slopes as that
will add ringing.

My second project began with the following in mind:
1. Using band-pass filters rather than low-pass and high-pass.
2. Include the ability to move the combined filter cross-over for
different CW offsets.
3. Get the lowest distortion possible in the filtering.
4. Use something commercially available rather than 'build your own'.

My first thought was to obtain two SCAF audio filters (highly
programmable as far as band-pass characteristics, no ringing, low
distortion). SCAF filters are not too expensive (you need two) when
found used. I have seen them sell for around $40 to $60 each. However,
before I found the two filters I decided t use another approach using a
62-band, two channel, Pro Audio equalizer. The one I obtained (for $65
used!!!) was a practically new Crate LS3-231. With this approach I can
move my cross-over frequency as desired and have good control of the
high and low frequency roll-offs. Distortion is negligible. I would not
recommend this approach with a typical home stereo equalizer as the
filter quality is not good..... the band separation is poor.... and you
don't typically have near enough bands (the Crate has 1/3 octave bands)
to allow selection of the appropriate cross-over frequency.... and I
doubt they hold up well in a high RF environment. Anyway, I am currently
using the Crate solution and it does a great job. Trouble is I want to
use this equalizer for other types of reception so I am continually
reprogramming it. For that reason, and for a bit better cross-over
programmability, I am still planning the simpler approach using two SCAF
filters (as soon as I find what I am looking for at a price I want to pay).

About DSP filters in this application:

I know of one commercial manufacture, TimeWave, that includes Binaural
CW reception in their high end DSP filter. I had my suspicions regarding
DSP signal path delays and QSK CW operations. I obtained a TimeWave
DSP599zx and found that the resultant binaural audio was very good but
that the delay was as bad as I had anticipated and unacceptable for QSK
over about 15 wpm. Your mileage may vary. :)  Anyway, if you don't use
QSK or you are very casual at speed and you don't mind spending a
sizable sum for one of these filters go for it. Please note that the
actual firmware version in the DSP-599zx is important and that only the
most recent firmware's have the Binaural CW function (or so I was told).

Lastly, I do know that there are at least one current amateur radio
transceiver with binaural CW reception built in.... perhaps more than
one. I am talking a fairly expensive transceiver (not named) so for me
it wasn't a reasonable path to just for this extra capability when I
would make or configure binaural reception for a lot, lot less. Try
before you buy!

73,
Jerry, KG6TT
Fairfield, CA