dmmartindale@watcgl.UUCP (Dave Martindale) (08/08/85)
About 5 months ago, I went flying with a pilot friend who owns an intercom. I was very impressed with how much easier it was to talk to other passengers, even in the back seat, without shouting. I was sold. So I started to look around for an intercom of my own. Cost was a definite object; an intercom that can be had for $200 in the U.S. would cost me about $300 CDN once I imported it and paid taxes and such. And to buy it from a local FBO would raise the price to about $400 CDN. Below are some things I learned while shopping around. It isn't an evaluation as such, since I've never used any of these except two. But it is a summary of features that should help you decide what you want. I'll only talk about portable intercoms - I didn't look at built-in ones since I don't own an aircraft. The mikes of the pilot(s) are always plugged into the intercom in order to provide continuous audio to the intercom's circuitry. There are two fundamentally-different ways that intercoms connect the pilots' mikes to the mike circuit of the aircraft. In one type (Telex, Sigtronics) portable push-to-talk (PTT) switches for the pilot and copilot are also plugged into the intercom, and the intercom has a single mike cable that plugs into an aircraft mike jack. When either pilot presses the PTT switch, the intercom routes the appropriate mike's signal to its mike cord and closes a switch that keys the aircraft's transmitter. At the same time, it may mute conversations by other passengers so the pilot can hear the sidetone clearly. In the other type of intercom, there are no PTT jacks on the intercom. Instead, the intercom has two mike cables that carry signals from the pilot's and copilot's mikes continuously. Normally, you plug each of these cables into the jacks of portable PTT switches, plug both PTT switches into a "Y" connector, and then plug the Y connector into the aircraft mike jack. The Softcomm and RST intercoms include the Y connector; I don't know if the Oregon Avionics intercom does. The former method has the advantage that there are fewer connections to make when setting up the intercom, and thus fewer wires strewn around the cockpit and fewer connections to give problems. But you *have* to use portable PTT switches with it. The latter method is more flexible. If the aircraft is already equipped with yoke-mounted PTT switches, you can simply connect the mike cords coming from the intercom into the jacks intended to be used by the headset mikes and then use the yoke-mounted PTT switches. If only the left yoke has a PTT switch, you can use it and mount a portable one on only the right side (provided the aircraft has an AUX MIKE jack). Since I'm not particularly fond of portable PTT switches, I decided to look for this latter type of intercom. Another minor difference: Since the second type of intercom doesn't *know* when a PTT switch is depressed, it can't do anything special about handling the audio during that time. If a passenger is talking, the pilot will hear that along with his own voice. If there is sidetone from the radio, it will mix with his voice coming through the intercom function and the two copies will add or cancel depending on their phase. (This hasn't been a problem yet). None of the intercoms include PTT switches. Single-pole PTT switches, such as the Telex PT-200, can be used only with the first type of intercom. Double-pole PTT switches that break the mike circuit when the switch is not pressed (e.g. Telex PT-300 or David Clark C10-15) can be used with both types. Some intercoms have "aux" or "tape" inputs; these are designed to provide background music, and in all intercoms (as far as I can tell) this input is muted whenever a radio call is received or when someone is talking. A few also provide tape outputs so you can record conversations during flight (this might be useful for flight instruction). If you are at all handy electronically, a tape output could be added to any intercom - it would just be a resistor and a jack. Only two intercoms have the ability to mute voice activity when radio calls are received. Some intercoms have a "pilot-isolate" switch that disconnects the pilot from the intercom and connects him/her directly to the radio, while the passengers can continue talking. All of the 4-place intercoms *except* the SoftComm consist of two boxes. One contains all the controls plus the pilot and copilot headset jacks, and the second box (connected to the first by a cable) has the passenger headset jacks. Now, for the intercoms I know of: (All prices are in US$.) Telex TC-200. 2-place; upgradable later to 4- or 6-place by adding accessory boxes. Designed to be portable or built-in. List price $300 for 2-place, $140 for 4-place upgrade, $108 more to bring total stations to 6. The basic unit is advertised for $195 in Flying, so that is about as cheap as you are likely to find it. Available from local FBO's, etc. This is perhaps the best human-engineered of the intercoms. It has individual volume controls for each headset (a feature exclusive to it). It has a three-way mode switch - in one position it functions as a normal intercom, in another it gives a "pilot-isolate" mode. In the third position, it connects the pilot and copilot via the intercom and both have radio access while the passengers hear only music. There are LEDs to indicate which PTT switch is keyed. The panel is backlit for night use. If power to the intercom is interrupted, it automatically connects the pilot directly to the radio. None of these features (except a manual pilot-isolate switch) is found in any of the other intercoms. On the other hand, the panel lights probably draw enough current to make battery operation infeasible - it has to be plugged into a cigarette lighter. (though the terminal board can probably be wired to avoid powering the lights if you like). If you make the intercom into a 4-place, I don't think you can easily disconnect the rear-seat box, making it clumsier than necessary when you only have two people. All other two-box 4-place intercoms allow the disconnection of the rear-seat box. The squelch takes 2 seconds to turn off after someone has spoken. An auxiliary input is available on the terminal board at the back; you have to provide the cable to attach to it. A friend has one of these and likes it. Sigtronics SPO-20 2-place, SPO-40 4-place. $200 and $230 list. Mail-order prices in Flying are $155 and $180. Should be available from FBO. 6-way unit also available. In some sense a no-frills Telex. Only controls are on/off, volume, and squelch. No provision for tape input/output, battery power, or anything else. Designed so that pilot's headset is connected directly to the radio when power switch is turned off - provides some failsafe protection, but pilot must remember to turn the switch off if intercom power fails. I have no information on squelch delay. Sigtronics has been building intercoms for a long time, so this one is likely to work well. I've used a built-in version of this one that seemed fine. A friend of a friend likes his. SoftComm: This is fairly new on the market. 2- and 4-place available by mail order from manufacturer only for $130 and (I think) $145 or $150. The 4-place version has all 4 sets of jacks in one box, forcing you to locate it somewhere that all headsets can reach it, plus where its cords can reach the plane's mike and headset jacks, plus where the pilot can reach its controls. Or, you can build extension cables for the two rear headsets. I think this is silly, so I eliminated this one from consideration (I wanted a 4-place). On the other hand, if you really like the features and price of this one, you could probably buy the 2-place and build the rear-seat box itself - the additional circuitry for 2 more mikes is probably just 4 more resistors. Normally powered by internal 9V battery. Can be powered by aircraft lighter socket via aux power jack. Has tape input and output jacks. Tape and power jacks look like standard mini phone jacks - I don't know if I'd trust them to stay attached if it got bumpy. This intercom will mute voice when a radio call is received. No mention of failsafe facility. Squelch turnoff is 500 ms. I have heard of no actual experience with this one. Oregon Avionics makes what amounts to a line of intercoms - you can select the features you want. Prices below, after I explain the features. There are 2- and 4-place versions available. This intercom has muting of voice when radio calls are received; a switch enables or disables this feature. (The 2-place version seems to lack the voice muting feature.) A pilot-isolate switch is available. An aux input is available, but there is no tape output. The aux input is always muted by the radio; the brochure doesn't say whether voice mutes the aux input but I would assume it does. The normal power source is a lighter plug, but a battery-power option is available. The standard version of the intercom allows only the pilot to transmit; an option adds the second mike cable so the copilot can transmit. Prices: The basic 2-place intercom is $130, the basic 4-place is $156. Add $39 to get the aux audio input and the pilot-isolate switch. Add $15 to get copilot transmit capability. Add $10 for battery power. (I assume that adding the battery power option doesn't prevent you from using the lighter plug, but I don't know.) I believe that you can have any combination of these options you want. Rotating the volume control to "off" connects the pilot's headset to the radio, as a manual failsafe feature. The brochure claims that the squelch action is very fast. Furthermore, it claims that the audio is filtered to remove engine and wind noise. The aux input (if present) is a mini phone jack. I have no experience with or comments about these intercoms, but if I was choosing one now this would probably be my choice. I may be in Oregon in a while, so I may try to drop into the manufacturer to see what they are like. RST (Radio Systems Technology). Available from manufacturer only. 2-place is $86 kit, $127 assembled. 4-place is $96 kit, $147 assembled. I bought the 4-place kit. So I can be much more critical (perhaps unfairly so) about this intercom than any of the others. Keep that in mind when reading the following. In kit form, this is the cheapest intercom around. And it does work, with some annoyances. It is powered by an external 9V battery or by a lighter plug; switching between them is automatic. It has tape both tape input and output. Despite what the catalog says and the picture in it shows, it does *not* have a pilot-isolate switch. All of the external cables enter the intercom via a connector, so if you want to use the intercom without interfacing it to the radios, you can dispense with the snarl of wires. It's small - the Oregon Avionics is the same size, everything else is larger. Negative things: The squelch turnoff delay is 5 seconds - annoyingly long (easily fixed by a capacitor change). The circuitry that squelches the tape input when a radio call is received is poor - it doesn't unsquelch cleanly. There is no filtering of the audio. The squelch is a bit difficult to adjust. The external wires that interface to the aircraft are stranded but are really wire designed for internal use within a piece of equipment. They tend not to coil neatly, and are not designed for rough service. The tape in and tape out connections are made with cables that are part of the intercom harness - thus they aren't going to come unplugged from the intercom in flight, but you have to do something about tying them up if you don't use them. So, it seems to have a few shortcuts in quality compared to what I know about the other intercoms. Still, it does work, and it's probably the best value for the money in kit form. If you are planning on building one of these, I can tell you of a few simple modifications that improve it (in my opinion). Now for the addresses of the manufacturers: All the manufacturers are willing to send out brochures if you write or phone; all the ones I tried could answer technical questions about features on the phone. Telex Communications, 9600 Aldrich Ave. So., Minneapolis, MN 55420 612-887-5510 Sigtronics Corp, 822 North Dodsworth Ave., Covina, CA 91724 818-915-1993 (SoftComm) Concept Industries Inc., 3641 San Gabriel River Parkway, Pico Rivera, CA 90660 800-255-2666 or 213-692-7761 Oregon Avionics Inc., 925 SE Clatsop, Portland, OR 97202 503-232-1870 Radio Systems Technology, 13281B Grass Valley Ave., Grass Valley, CA 95945-9897 800-824-5978 (orders) 916-272-2203 (technical questions) Dave Martindale, watmath!watcgl!onfcanim!dave