I'm trying to sketch out how to set up the audio in my car. I'll be replacing the stock head unit with a Nexus 7 and a JBL MS-8 (don't worry, they can be had for a lot less than that listing price) as DSP to handle the sound staging. The stock head unit has 4 channel out, which goes to 4 mids and 2 tweeters. I'm thinking of something like this:
- Digital from Nexus 7 to USB DAC
- Analog from USB DAC to MS-8
- Analog from MS-8 to stock amp
There are two problems though:
- I need to mix in a signal from a Bluetooth receiver for handsfree. This will likely have to come in as analog somewhere between the DAC and the MS-8.
- I need to be able to control the volume of the end signal, before it enters the MS-8.
Point 1 comes from being unable to use the tablet to receive calls. I can do some nice things with integration, such as displaying caller ID and pausing media, but there's no way to get the call audio itself from phone to tablet. The easiest way around this is to get a Bluetooth handsfree unit off the shelf and mix its audio output into the overall output somewhere. Since I don't know of a way to get this into the tablet as a monitor feed (there seem to be ways to record it, but not play it back live), this probably needs to happen somewhere in the analog portion of the signal. NOTE: Unfortunately I can't use the aux input on the MS-8 since it requires manually switching between inputs and doesn't mix them.
Point 2 comes from the MS-8 being designed as a configure-and-forget device, meaning I can't use it to control the end volume. I could use the Android volume controls but I doubt they work with an external DAC (they don't via Bluetooth) and it wouldn't control the handsfree volume. I'm willing to solder in an analog volume control knob (rheostat I assume) somewhere into the equation but I'm a computer/network engineer, not an audio engineer. I'm not sure how best to do this without introducing a ton of noise, especially with multi-channel audio.
Nothing's been bought yet, so I'm open to suggestions at any stage in the chain. A lot of this depends on the DAC, and I'm leaning towards an O2 but I'd love to hear there's some way to mix in the handsfree audio digitally and then get a DAC with built-in volume control.