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Integrate Alpine TME-M740BT Touch Screen Monitor with OEM HU?


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Someone sent this question via PM. As my answer got more involved, I figured I'd post it so everyone could see (and possibly contribute)...


How does the "mute" button on the steering wheel work for the HK cars? Does it mute at the amp, or the headunit?


I have an Alpine TME-M740BT screen. I want to mount it in the cubby.


It is blue-tooth ready, and the wiring info is on page 20 here:




There are left and right audio outputs, and an audio interrupt output.


My thought would be to run the audio interrupt lead to the "MUTE" wire on the headunit wiring harness (just 'T' it in). Then run the L/R audio leads to the HK amp (though I don't know where or how to tie them in).


That way I would be able to integrate the BT functionality of the screen into the audio system of the car. Then I simply find a convenient location to mount the microphone and voila!


What do you think? Reasonable?


Thanks for the help!

Unfortunately what you want to do is going to be a bit more complicated.


First, cars with the h/k system still have essentially the same OEM HU as non h/k systems. The HU feeds its four speaker outputs to the h/k amp, which in turn generates all 7 or 8 outputs for the car's various speakers and sub. See this post:

Since the h/k amp is looking for speaker-level inputs, you can't drive it with the Alpine's line-level audio outputs. Even if you did amplify that signal so you could send it into the h/k amp, you would need a non-trivial interface to be able to switch the h/k amp from the OEM HU to the Alpine without getting a "pop". You wouldn't be able to simply leave both connected to the h/k amp simultaneously.


The steering wheel's mute function is done in the OEM HU.


The steering wheel has a network of resistors. Each button sends a particular resistance to the HU. Here's Subaru's 2005 Legacy/Outback wiring diagram (on later years, the top button is "mute"):


(click for larger image)

Resistance value for each button:

  22 Ω  - Mute
 90 Ω  - Volume up
200 Ω  - Volume down
360 Ω  - Mode
690 Ω  - Seek up
1.15k Ω - Seek down

4.7k Ω  - Nothing pressed

Sure, you could build an external circuit that simulates pushing the Mute button. One problem is that the function in the HU is an on/off toggle. So if the HU was already muted, your circuit would take it off mute.


What would be better is a telephone mute input on the HU. When you connect that line to ground, the HU temporarily mutes whatever audio was playing, until you un-ground that line.


Many aftermarket HUs have that (and I'm guessing that's what the Alpine's mute line was intended for). Few Subaru OEM HUs have that (the only ones I'm aware of are outside the US). It's possible that in cases where the same OEM radio model is sold in the US market, the phone mute line exists inside, but they didn't bother to bring the wire outside.


Ideally you'd want to use a phone/Onstar input on the HU. Unfortunately, in Legacy/Outbacks only 2005-2006 HUs had that.

Now you might be able to connect the Alpine's line-out to the OEM HU's Navi audio input. That would pipe the BT audio to the HU's speakers.

Added: Pin-out for the i88 harness (has the Navi audio input) is

I don't know what happens to the audio source the HU was playing -- does the HU mute that or just reduce the volume? Does nav audio only get sent to the front speakers?

Added: I've been told the HU mutes the front speakers and routes the nav audio prompts to those. Also, if the HU was powered off, the nav system will still "wake up" the HU's audio section so you can hear the prompt.

The Alpine owner's manual says little about the "Audio Interrupt Out Lead". But I suspect you'll need a small circuit to convert it to what the OEM HU wants to see on the "Navi In" control line.


If you also want to be able to stream music from your BT device, you could probably "Y" the Alpine's audio outs into the HU's aux input too.

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Do all of the OEM HU's have a navi audio input?


Can anyone say what happens to the audio when NAV directions come out?


That sounds like it would be a fairly good solution.

[URL="http://legacygt.com/forums/showthread.php/proper-flip-key-interesti-159894.html"]Flip Key Development Thread[/URL] "Genius may have its limitations, but stupidity is not thus handicapped." - E. Hubbard
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