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Tablet as a head unit
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Old 03-19-17, 07:07 AM   #1
fogged306
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Default Tablet as a head unit

So I'm in the process of swapping out my head unit for a tablet and I thought I'd share some progress details, pics, and info that I've gathered so far for anyone else thinking about doing the same. Anyone with experience doing so, please chime in with any knowledge you have.

I was a car audio installer for a long time some years back, so I do have a lot of 12 volt experience. I may assume you know things and leave info out unintentionally so if anyone has questions, feel free to ask.

I picked an Nvidia Shield tablet to use in the dash. In hindsight, I should've went with the nexus 7. It's a popular choice for this type of thing and there's a lot more info out there for doing this. Also the stock opening for the GR sti radio bezel is not tall enough to fit the entire 8" screens height. I chose to cut off the top of the screen where the status bar is when mounting the tablet. I may modify the bezel down the road, but for now, I'm leaving it as is. I went down that road when I put an appradio in my truck that had a din and a half opening and that was a lot of work cutting, using body filler, sanding, masking and painting to get it to look right.

For right now, I just cut away the top middle tab on the bezel, and there's 4 nubs left from the injection process that were in my way that I removed from the backside of the bezel as well. I also had to cut the top tabs off for the vents and grind down the high spots on the top of the vents as well. I just epoxied everything back into place. Sorry, I don't have any pictures of this as I didn't really plan on doing a write up when I was doing this. The tablet is epoxied to the dash bezel as well.

For power, I just pulled apart a standard USB cigarette lighter plug and soldered leads on to it and tapped the ignition and ground wires in the radio harness. For the connection to the tablet, a 90* micro-USB OTG plug would be the best option here because the tablet gets very close to the edge of the bezel and you need the smallest plug possible sticking out of the side of the tablet to clear the dash when you're installing this. Using a Nexus 7 would probably make life easier here as well.

Right now I'm using the headphone jack for audio but, as I expected, the sound quality isn't great. This shield tablet has a mini HDMI out, so I will be trying to either use that with a converter box and a DSP, or a USB DAC and then on to my amp. When you do this, you need to add an amplifier to your system as the output of the tablet needs a way to power the speakers. When I get this sorted out, I will update the post, but for now I'm just using a 1/8" headphone jack to RCA cable and running that to my amp.

One issue that I may run into is that when using USB OTG for something like a DAC or the steering wheel control module I ordered, the USB host (tablet) will not be able to charge at the same time. I purchased a Y cable on amazon that claims you can use OTG and charge at the same time for this particular tablet, so once that comes in I will try it and update the post with a link if it works. I will be running a small, powered USB hub to connect the steering wheel control module and the DAC, and whatever else I may need to connect. The reason I need to be able to charge and use OTG at the same time is that I have this tablet set up through an app called automate, to turn the screen on and keep it on the entire time it sees power from the charging power port, and to go into sleep mode as soon as the power is removed. In otherwords, because I have the power wired to the ignition wire in the radio harness, the tablet turns on when I turn the key on and stays on. Once I turn the key off, the tablet goes to sleep. I will add a screen shot of the thread in automate that I used to accomplish this later.

I also am going to use automate on my phone to make a program that will automatically turn on my wifi hotspot on my phone whenever it connects to the tablet in my car, and turn it off when they disconnect. So in theory, the tablet should automatically connect via bluetooth to my phone every time I turn my car on, and that action will cause my phone to turn on the hotspot and the tablet will automatically connect to that without me having to do anything.

So far the only thing I haven't put much thought into is trying to come up with a solution for handsfree bluetooth operation for phone calls. Sorting out how to wire in an external mic to the tablet through either USB or the headphone jack while keeping the audio output on might be a challenge.

The steering wheel control interface I will be using is this http://www.rcjoycon.com/carpc-joycon-exr/ I'm hoping this will make the interface much more smooth because the volume buttons on the tablet are inaccessible with it mounted and going through the settings menu to get to volume is a pain. I did find an app that will overlay specific buttons on the screen if needed and it does work, but steering wheel control will be that much more seamless. I will update on that as well when I get to that point.

Anyway, here's a couple pics of it after I mounted it and of it in the car. When I pull the unit back out to add the parts I've ordered, I will take some pics of the backside so you can see what is needed to be removed on the bezel and how it is wired/mounted.





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Old 03-19-17, 07:11 AM   #2
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Default Re: Tablet as a head unit

Wow I like that! But how do you use backup camera and navigation? How would your audio sound like?
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Old 03-19-17, 07:37 AM   #3
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Default Re: Tablet as a head unit

There is a way to get a back up cam to work through that steering wheel control unit I linked, I believe, but I won't be digging into that at all myself because I don't use them. The nav works just fine when you're connected to your phone via wifi. I use google maps for navigation or you could download other nav apps to use as well. You could also get a tablet with cell service and bypass using your phone as the hotspot. Using Nav was the main reason I did this. For a $200 tablet, and I have nav in my dash now instead of using my phone, you really can't beat it.

For audio, using the headphone jack is fine, but for someone like me who is a bit of an audiophile, it's not cutting it. I knew it wouldn't so I had planned on using a DAC or something similar. I also already had an amp in my car so that is something you have to factor in for cost if you are looking to only replace your headunit only when doing something like this. There are compact amps you can use for just this purpose all over places like crutchfield if you want the cheapest and simplest route. Something like thisAlpine KTP-445A Power Pack Compact upgrade amplifier for your Alpine receiver — 45 watts RMS x 4 at Crutchfield.com
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Old 03-19-17, 07:54 AM   #4
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Default Re: Tablet as a head unit

Thanks for the info and I'm wondering is there a way to link a tablet to the oem head unit for the 2017 sti via iPhone or mini tablet? I know mirror link is for android but would it connect to use for my oem head unit? I like the oem head unit but is there anyway you can teach me to wire the amp to it. Thanks in advance
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Old 03-19-17, 08:34 AM   #5
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Default Re: Tablet as a head unit

For linking a tablet/phone, I couldn't tell you on your car. I'd be googling that same as you if I was to find an answer for you.

As far as wiring an amp to a factory head unit, there's a few options, but that all depends on the factory head unit. I've been out of the audio game for about 5-6 years now and it's an ever evolving field, a lot could have changed since. So I have no clue if yours is a standard amplified all in one unit or if it's a factory externally amped unit or what. A line output converter is generally the cheapest and easiest method for adding an amp to a factory system. But you might want to talk to an audio shop that's more up to the times than I am for your car.
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Old 04-25-17, 03:40 AM   #6
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Default Re: Tablet as a head unit

Just an update to this if anyone has run across this thread google searching like I did many times trying to get this set-up to work the way I wanted.

I have everything working the way it should now. The steering wheel controls work, and the unit gets power and can run in OTG mode at the same time with some experimentation, some soldering, a relay, and a DEI 555t. The tablet and the joycon are both finicky about the timing in which they receive power in order for them to work the way I want. I actually ended up making my own OTG host cable by splicing in a female USB plug to a standard USB cable, and soldering a wire on the OTG activation wire and the ground wire separately. Then I had to wire up a 555t to the keysense wire so that it connected these wires together when I put the key on the ignition for 1 second and turned back off. Then a relay that did the same thing when the ignition was turned on. The goal was that the setup needed to see the OTG sense wire kick on for a second, then when you turn the ignition, it sees power, and then the relay clicks the OTG back on. If it didn't see everything in that sequence, then either you would have OTG and no charging, or the joycon wouldn't function.

So cliffnotes to those who have some experience with this type of thing and want to tackle this. Shield Tablet with OTG charging and joycon steering wheel input: The OTG sense wire needs to connect for 1-2 seconds, then disconnect. Then both the Tablet and the joycon need to see 5v power, then the OTG sense needs to be connected again after power has been applied for the duration you want to use them. Now you have a tablet that is charging while you drive the car and steering wheel controls that work at the same time.

FYI, this setup works very well. The tablet only loses about 1-2% charge overnight and usually is back up to 100% by the time I make it to work. I also changed out my amp for an old, but much better quality Alpine amp I had kicking around, and the sound quality is fantastic now. Plenty good enough that I will be leaving it connected to the headphone output on the tablet.

The only other real issue I have now is that I still have no bluetooth handsfree, and the bluetooth internet connection I use from my phone doesn't automatically connect when I get in range of the unit. Automate doesn't have the ability to make this an automatic function, so I will try tasker next. For right now I just have to click on the bluetooth connect button every time I get in the car. Not terrible, but I'm aiming for seamless, so I will fix this at some point. As for the bluetooth hands free, it looks like I will end up having to use a parrot ck3000 and then an app like tablet talk.

Anyways, I couldn't be happier with this set-up. I'm also the type that never listens to the radio, so all my downloaded music and pandora are all I need in my car. This is one reason why this is a great set-up for me. for a total cost of under $300, I have an 8" screen headunit that is much more capable and far better screen resolution than even the best of navigation systems that cost over 1k.
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