dmmartindale@watcgl.UUCP (Dave Martindale) (09/25/85)
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