There are a few audio systems on the market today, and I chose the Starcom1 system below.

The package I chose was the basic, full-face system, with an additional full-face headset. It's relatively easy to fit the headsets into any helmet, allowing both the rider and passenger to talk to each other in full-duplex stereo sound. Plus, you can feed your music system into the unit and both listen to stereo music. I have the XM radio plugged into mine, with the ability to also plug in a portable CD or MP3 player which I store in the tank bag. The Starcom1 can be (and is designed to be) hardwired to the bike. It can also be powered by a 9v battery which lasts only a couple hours, so don't even bother.

I installed the system in the area under the seat, near the rear light housing. It's pretty much the only possible place to put it on the ST. I routed the helmet connection wires out from under the seat through a small notch I cut in the tupperware - no big deal, the seat covers it anyway. Just be sure to cushion the notch with a rubber grommet or UHMW tape or something to prevent a rough edge from cutting through the wires.

After that, I installed the holder clips on the left saddlebag and used an electric wire clip to hold the wires from flapping about when not in use. All of these are attached with double-sided tape, so they can be removed in the future and there are no holes to drill. Both cords easily reach both helmets without having to purchase any additional extension cords.

The rider-passenger communication is voice-activated, so while you're listening to the music, if either of you talk, the system mutes the music until you're done talking, then after 5 seconds, it turns the music back on. It's pretty neat actually. It also claims to work with a cell phone and a GPS audio feed, as well as a CB radio. I haven't used it with a CB, so I can't speak to that, but I have with a cell-phone. The phone must have the ability to auto-answer calls when it's plugged in to work, and it seems to work well. For some reason, though, I'm still getting engine interference with the GPS feed, and I haven't yet figured out how to fix it. Speaking of fixing things, there is something you need to understand about automotive electronics when using an audio system, and that is the concept of ground loop isolation. Ground loops are probably the biggest pains for audio and video engineers to deal with. They can occur in any system: home, car or bike, audio and video. I'll speak below about how they apply motorcycle audio (which is the same as a car actually). If you have video on your bike, what the hell kind of bike do you have?

Ground Loop Isolation 101
Like I said above, ground loops are a pain in the ass, and unless you know about them, you'll be scratching your head wondering why your brand new system sounds so horrible. To make matters worse, the manuals I've seen in these systems do a terrible job of explaining how you're suppose to troubleshoot such a problem. I'm guessing it's because it usually takes a while for the concept to sink in. I know it took me a few times before I went "ok, that makes sense." Hopefully, these pages will help you reach that point more quickly.

A ground loop is a condition where a connection to ground is made through multiple points in an audio system, in this case, the Starcom1 system and the XM radio system, both of which are powered by the bike's 12v system. Ideally, both systems have their own ground, and each unit's ground current should be routed to that unit's ground, and no other place. The problem is that through normal audio connection methods (i.e. metal wires) you are not only sending the audio signal, but are also sending a small amount of voltage, and with voltage, you are sending ground current. To make matters worse, this cross-voltage is very susceptible to the other components that are also connected to the bike's power system, such as, well, everything else electrical on the bike: lights, turn signals, fans, heaters, the alternator and that little thing called the engine. All of these items will pretty much obliterate the signal, making it sound like crap when it finally reaches the audio unit.

The only way to fix a ground loop problem is to isolate the grounding of every audio source feeding into the audio system. You can do this with a ground loop isolator which you can pick up at Radio Shack (catalog# 270-054) for about $16.

You will probably need to purchase an additional audio adaptor or two because the ground loop isolators only come with standard RCA type (red & white) plugs, while motorcycle audio units primarily use headphone-jack style connectors. Basically, hook this into the feed between the audio source and the audio system and the grounds will now be isolated from each other, as well as the rest of the bike. The sound should then come through without interference. If it still doesn't, the next thing I'd check is to make sure that you are using shielded audio cable. Whatever you use, make sure all connections are solid and are taped waterproof.

Only other thing to remember is to keep the GL isolator away from any strong magnetic fields. On the ST1300, I found that mounting it just behind the fuses on the left side of the bike seems to work best.

If you're using a portable, battery-powered audio source (e.g. CD or MP3 player), for reasons that I hope are obvious now, you shouldn't have to worry about ground loop interference.

Ground Loop Isolation 102
So what does a ground loop isolator actually do? Most use a form of induction, which in turn is a form of electro-magnetics, to transfer the signal, but refuse the current. An electromagnetic coil on one side of the isolator is powered by the signal from the audio source, creating small magnetic field fluctuations. These fluctuations are then picked up by the coil on the other side of the isolator, generating an identical signal into the wires flowing to the audio system. Because the wires are not physically touching each other through the isolation magnets, no current is passed, thus isolating the ground to flow only through the audio source ground, and not into the audio system. Think of it as an electronic filter, in that it's allowing the audio signal to flow freely, while at the same time, blocking any dirty current from mucking it up.

More accurate isolators use optics such as LEDs and photoreceptors to transfer the signal, because light energy is completely unaffected by electric interference. These can be much more expensive though.