I'm a musician with a home studio. I've been wanting to incorporate my iOS devices (iPad 3, iPad mini retina, and iPhone 4s) into my music making for years, but only recently have synchronization and MIDI technologies gotten to where they needed to be. Now I can sync my devices to my mac running Ableton Live, but this is where the trouble began.
In order to actually hear my devices, I'm running them through an analog line mixer alongside the output of my Mac. The trouble is that the amount of noise coming out of the headphone jack of the iOS devices is outrageous. It's not just normal line noise however, I'm hearing all kinds of access and data transfer going on. It comes & goes in bursts, some lasting only a couple seconds, some lasting much longer. Also there is a horrible, continuous high frequency whine, and little blips & "glitchy" sounds, much like if you've ever had your cell phone pick up noise from a computer, etc. Bottom line it it's all horrendous to listen to, and definitely not acceptable for any sort of music making.
But here's the interesting bit. If I disconnect the device from the Mac - all is quiet. Not just quiet, but SILENT, except for the audio I'm actually playing on the device. It's pure heaven to listen to... absolutely pristine. But connect the Mac again and... back into the bowels of audio hell.
This is true on all 3 of my iOS devices, with completely different cables. It's also true if I connect to my USB hub. However, if I provide power to the iOS device via an external source (like a Lenmar portable power pack), then I do NOT get any noise. It seems like some sort of grounding issue or something that only happens when physically connected to the Mac in some way.
Is there anything I can do to remedy this? As it stands now, the analog outputs are simply unusable for music making if I want the devices to be powered at the same time (and with how much power some of these apps draw, power is really a practical requirement). I've experimented with the MusicIO app, which allows for digital audio over USB. While it does actually work, I'm having audio glitch issues with it which are unacceptable (and completely different from the noise described prior). I also have an iConnectAudio4+ which works for digital audio, but only on a single iOS device at a time so really also doesn't get me what I'm after. Anything I can do to "isolate from ground" or whatever else might be causing this horrible noise?
EDIT: Some additional interesting info. The noise is not actually in the analog path of the iOS device audio. It's being introduced somehow into the path of my entire audio system by the analog out of the iOS devices, but is not in that signal path. To illustrate this, I can turn OFF the mixer that the iOS devices are going into, so that no signal at all is passing through. Yet I still hear the noise on the main outs of my system. If I unplug the analog out of the iOS devices - even though they are not currently able to pass sound anyway - the noise stops completely.
EDIT 2: I'm pretty certain now that this is a ground loop issue inside the iOS devices themselves. I can essentially break the loop by disconnecting either the Mac, or the audio output. If both are connected - even if the audio output of the iOS device is muted entirely - then I get all sorts of access noise coming out of my main system audio outs. But since this appears to be inside the iOS devices, I can't really do anything to "lift" or otherwise isolate the ground. I'm really stumped on what I can do about this other than add some sort of pro-level 3rd party docking solution to get balanced audio out (or even digital). But I really don't want to do this.