[MTCD  Android Head Units Q&A] aturika:
So back in August I purchased the octocore unit, with Oreo, from Phoenix Android Radios for my 2018 Civic Si. Since then it's been hell trying to figure out the distortion from my Front Left and Rear Right speakers and why fading to Front Left causes extra distorted audio that plays from both Front Left and Rear Right speakers. Same goes if fading audio to Rear Right - extra distortion and also comes from Front Left. Fading to the Front Right or Rear Left sounds normal.

Issue:

Audio sounds distorted - especially in the upper hz ranges (none specific - but not apparent under 500hz). When I use the pre-installed Amp application and fade the audio to FL (Front Left) I also hear audio from the RR (Rear Right) - it is extremely distorted to the point of disgusting and vocals cannot be made out. If I fade to the FR or RL the audio plays only out those channels and sound quality is what I'd expect. It's important to mention that the distortion isn't as extreme when playing on all speakers - but noticeable enough to be annoying especially since I'm positioned closest to the FL. I can only listen to podcasts... playing music is just painful. :confused:

Original Theory:

I must have crossed wires somewhere. I tapped/soldered (w/ shrink) directly into the harness of the radio and sent the high-level output directly to my speakers, bypassing the factory amp as Phoenix instructed me. As a sanity test - I tested each speaker individually to check for phase issues. When using an alternate audio source (cheap Bluetooth amp) to send signal, the audio sounded great across all channels. I even intentionally crossed the FL & RR negative wires (maybe I'm colorblind and can't read the labels I put on the wires) and it sounded like what you'd expect... I've wired speakers before and not new to this at all. But I've made mistakes before. I had two friends confirm I'm not crazy here. :victory:

New Theory:

Maybe the radio's built-in amp is trash. So I dished out $140 and bought a 4-channel amp and hooked up directly to RCA outputs. After doing this - not only did I lose the Android sound effects (and other things), but the main distortion was still there. Using the Amp app and fading to either FL or RR would play the exact same distortion from both channels at once. Again, as a sanity check, I plugged my tone generator into each RCA on my new 4-channel amp. Each channel worked as expected. This confirms the wiring after my amp is good. And even if I managed to screw up 4 RCA plugs - it would just swap channels, not cross them. All RCA input/outputs are isolated to prevent touching each other.

Current Theory:

I'm getting some kind of interference, maybe, but it's only affecting two channels? Or there's probably a software/hardware issue. I used FooBar2000 music player to play music and set it to convert the music to mono - the distortion was gone! Then I used the same app to down-sample the songs to mono then up-sample back to stereo (redundant - extra CPU, etc.. I know) but this HELPED the distortion significantly. I can actually listen to a song stored on the unit/usb as long as using FooBar2000 to play it and not Bluetooth or any other apps on Android. This leads me to suspect the pre-installed Amp app is sending wrong signal to each channel or it's from the built-in amplifier.

Testing - Getting Scientific:

I decided I would do a test to see what kind of voltage I'm getting from the low-power output of the RCA and voltage I'm getting from the radio's high-power speaker line at various volume levels using the Amp fader to "all" channels and comparing those results to ones when fading the test-tone to the FL speaker only. For this test - I only used the FL and RR speaker and used a constant 1K tone. I noticed when checking voltages while fading to FL or RR that the voltage wasn't constant. It would move up and down by .01 to .02 volts. Testing while faded to all channels shows a very steady voltage. Here's the results:

[IMG="Voltages - RCA & Line Level vs. Fading Front Left, Rear Rear, All Channels"]https://i.imgur.com/oALdTHb.png[/IMG]

If anyone can help me get to the bottom of this please! I've spent so many hours/days slaving away trying to make this work. I've re-wired everything 3 times because I kept thinking I must have made an issue somewhere as Phoenix Android Radios support team and engineers don't know what it issue is. Latest was telling me to check ground - which I grounded w/ short thick gauge copper cable (multi strand) direct to frame. Confirmed good ground w/ multimeter (was was original ground).

If someone would like a video to demonstrate this better please let me know and I'd be more than happy to record one right away!

Extra Details:

[IMG="Version Info"]https://imgur.com/Xex6Brul.jpg[/IMG]