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Path: utzoo!watmath!watnot!watcgl!dmmartindale
From: dmmartindale@watcgl.UUCP (Dave Martindale)
Newsgroups: net.aviation
Subject: Oregon Avionics portable intercoms
Message-ID: <2558@watcgl.UUCP>
Date: Wed, 25-Sep-85 01:58:49 EDT
Article-I.D.: watcgl.2558
Posted: Wed Sep 25 01:58:49 1985
Date-Received: Wed, 25-Sep-85 03:59:42 EDT
Distribution: net
Organization: U of Waterloo, Ontario
Lines: 130

In early August, I posted an article comparing all of the portable
aircraft intercoms that I knew about (the article was
2316@watcgl.UUCP).  The article included comments about an intercom
family sold by Oregon Avionics, but at that time I hadn't been able to
use one.

Last week, I was visiting Portland, and dropped in to Oregon Avionics.
The company's address is really someone's home.  It looks like one guy
designed the intercoms, and handles inventory, testing, and sales,
while he has someone else actually build the circuit boards.

I talked to this man for a while about the design of the intercom, its
features, and so on.  I was quite impressed.  He has put a lot of
attention into details: using isolation resistors on the headphone
outputs so 8-ohm headphones could be used, using diodes and capacitors
to isolate the mike output to make sure it works with various radios
with the radio or intercom on or off and with various sorts of mike
keying setups in the radios.

The circuit board is absolutely packed with parts - I'd estimate that
it has twice the parts count of the RST intercom, which is probably one
of the simplest around.

Now for a more formal review:

Oregon Avionics has slightly redesigned their intercom line since
sending out the info that my previous report was based on.  There are
now a "simple" and a "complex" intercom, each available in both 2-place
and 4-place versions.  There are also some options that can be added.
(There is also a panel-mount version that I'll ignore.)

The primary feature of all of their intercoms is the squelch
circuitry.  All other intercoms that I know of use some sort of
comparator to decide when to squelch the audio and when to let it
pass.  OA, however, uses an op amp with finite gain driving a CMOS
analog switch.  Thus the squelch opens up and closes down over a period
of time rather than instantly.  The time constants seem to be
well-chosen too.  The effect is that when you start speaking, your
first syllable can be heard; other intercoms that I've used (RST,
Sigtronics) lose the first part of the first syllable.  If you stop
making sound instantly, the squelch turns off after about a half
second, but if your last word trails off into nothing, the delay is
extended.  This explanation sounds unconvincing on paper, but it really
does work - the squelch turns on and off in a manner that is much
closer to the way you really want it to than with other intercoms I've
used.

The audio from the microphones is filtered before being fed to the
squelch circuitry in order to make it less sensitive to triggering by
noise.  The mike audio is also filtered by a bandpass filter (I think)
to reduce the audibility of wind and engine noise before it is fed to
the audio amplifier.  There is a trimmer pot inside the box to adjust
the gain of the mike preamp to compensate for the noise level in the
environment and the sensitivity of the mikes used.  Getting at the
trimmer requires a bit of work to open the case.

The connections to the headphone jacks use isolating resistors that
allow you to use an ordinary 8-ohm audio headset for some passengers
without damage and without reducing the audio level available to the
150-ohm aviation headsets.

The 4-place versions of the intercoms have a second box for the 3rd and
4th headset connections, like most intercoms.  Unlike the RST and
Sigtronics, this second box is not detachable from the first, so even
if you have only two people you have the second box underfoot.  (If
this is annoying enough, you could just cut the cable and install a
4-pin Molex connector pair.)

All of the above features are present on all OA intercoms.  In addition,
the "complex" intercoms have these additional features:

There is a three-position "function" switch.  In the "all" position,
the intercom operates like any other - everyone hears intercom audio
mixed with incoming radio transmissions.  In the "auto-mute" position,
incoming radio calls mute conversation.  This seems to be the same as
the function that SoftComm describes as "priority override", but it is
not switchable in the SoftComm.  In the "isolate" position, the pilot is
connected to the radio directly and disconnected from the intercom -
this is the same as the "radio" position on the Telex.  (The passengers
cannot hear the radio or pilot, and the pilot cannot hear the
passengers.)  This isolate function is provided by a relay, wired in
such a way that if the intercom is switched off or its power source is
disrupted the pilot is automatically connected to the radio.  This
"fail safe" switching is only provided by this intercom and the Telex.

The threshold of the squelch circuit that provides the auto-mute
function is adjustable by a trimmer pot inside the case.  As long as
the aircraft radios have squelch circuits that fully mute their audio
when nothing is being received, this shouldn't need adjusting anyway.

There is a music input in the form of a mini phone plug that can be
plugged into your Walkman to provide background music.  This input is
always muted by either conversation or a radio call.

Prices:  The "simple" intercom is $130 for 2-place, $160 for 4-place.
The "complex" version is $170 for 2-place and $200 for 4-place.

All of these prices are for intercoms that allow only the pilot to
transmit.  For $15 extra, you can get an option that adds another cable
carrying the signal from the copilot's mike, allowing him to transmit
too (if you have a second PTT switch).  However, the option does *not*
include a Y-adapter, so you will have to build your own if you will
ever want to allow two pilots to transmit in an aircraft that has only
one mike jack.  (It seems that many aircraft have two mike jacks, so
this may not bother you).

The intercom is normally powered by the aircraft's cigarette lighter
socket.  There is a battery power option for $10 that powers it from
two 9V batteries instead.  However, then you don't get the lighter
plug.  The installation isn't very clean either - the batteries are
external to the intercom, and must be held to it with tape or a rubber
band.  If I wanted to use the intercom in a plane without an electrical
system, I'd build my own power source from two 9-volt battery
connectors and a lighter socket rather than getting the battery-power
option.

Final comments:  I bought one of the 4-place "complex" intercoms with
copilot transmit capability, primarily because of the squelch circuit.
I regard the auto-mute as potentially useful, but I haven't used it
yet.  This intercom *is* more expensive than the discount price for all
of the others reviewed except the Telex - you'll have to decide for
yourself if the features are worth it.

Manufacturer's address:
	Oregon Avionics Inc., 925 SE Clatsop, Portland, OR 97202
	503-232-1870

P.S. Anyone want to buy an RST 4-place intercom?

	Dave Martindale, watmath!watcgl!onfcanim!dave


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