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I have done what you are talking about & it can be done without a problem*, basically there will be no impedance mismatch as the determining factor at line level is going to be more concerned with voltage. Use good quality connections & avoid using a 10' cable bunched up to do a 18" job & you'll be fine. You would adjust your head units volume initially & then not use it again, using your pc volume control instead. this way you will always have the volume levels in sync. with each other. keep in mind that the amp input is fixed, and if you adjusted the head unit volume directly, the pc wouldn't know how low or high the input to it is, & if you set the pc to say 3/4 volume & then turn down the head, you'd wind up listening to something low & then have your interupt blast louder, if you know what I mean.
*now while you can do it that way & I have done it sucessfully In my last system that way, I would not recommend this as the BEST way for a few GOOD reasons:
1)the head unit is the most convienient way to controll the overall system volume, this way sacrifices that,
2) not to mention the "what if the computer is off,crashed,virused, or some other way temporarily bugged, it would be nice to not have it between your never fail head & amp
3)the analog to digital & back to analog that the head signal goes through adds to degraded quality & if you want maximum sound quality, that's not it.
there are a few others, but you get the idea.
If you absolutely must have your audio interupt, then you can choose to just do it that way & it will be pretty good, maybe that would be enough for you, & maybe that's what you want.
The better way to actually run a head unit with a pc is to use the pc as an aux input to the head. This will allow the head to control the volume of everything, including the pc. now while it is possible to still perform your interupts it would be more complicated. Some head units have a phone mute that you could activate easily enough with a few electronics,& then you could introduce your alert to either just the front speakers or all by mixing the signals at the input of the amps, but you would then have the same problem of the two volumes being independent.
I actually got my system to work pefectly & the volume always stays in the same relationship to each other buy using the navigation mute input along with the guide control input on my alpine unit, but these inputs are only available on a multimedia head unit with it's own screen, althought that's what I did I doub't you'd want to add another screen to your mix.
the final thought I'd like to share with you is that even after I got my whole thing working perfectly, it actually proved to be annoying to me to have my audio volume be interupted & I finally settled on a seperate speaker to feed me alerts that I could turn up enough to be authorative when I'm concerned about the alerts & turn it down to just a background wisper or even off when I'm not as concerned, all the while audible when necissary & not constanly mute/raise mute/raise, I'm much happier with it this way, but that's just me. If you used a small driver on say a pillar or somewhere close to your head, you can have it work where it is information for you, & not a total take over of the car each time as it would be the other way. I have this aux information subsystem that is comprised of my alerts, along with the cell phone & the onstar all coming out of a small (2x 2.5" drivers) aux speaker aimed at my ear, I've had it the other way & working fine with it muting the radio to about 25% & it proved more annoying than the way I have it now, & if I realized this from the begining I would have saved myself a lot of unecissary work.
I don't know if any of my input here makes any sense to you & your install, but I hope I helped even a little....
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