Please see My Embedded Project History page for a history of the projects that led up to this vGuitar rig.
I've been a gigging guitar player for over 40 years. I've been a professional software engineer for the same amount of time. Over the years my "rig" has evolved from a few foot pedals, to multi-effect pedals and loopers, to most recently, an iPad based system that includes an FTP Triple Play guitar synthesizer with SampleTank, ToneStack, and Quantiloop pro running in an AudioBus framework, with IO provided by an iRigHD usb audio device for guitar sound input and final sound output. I use a variety of controllers including 4 analog volume pedals, a SoftStep II foot pedal, and a MPD218 that I put on the floor as a foot switch.
The basic problem with the iPad setup is latency. I have measured 38ms of latency from the time I pluck the guitar until the sound from the guitar makes it up through the iRig into the iPad, gets through the Audiobus framework, and is returned as a line output by the iRig. This is independent of the particular apps (effects) I am using. And it even appears to be independent from Audiobus itself. This appears to mostly be latency caused by conversion to USB, and the sound making it's way up the USB stack into the iPad sound architecture and back out to the iRig.
Another major issue is the complexity, and the resultant fragility, of the setup. There are no less than 21 connectors, including a 7 port USB hub, and a dozen or more cables, where a failure of any one of them can basically fail the entire system .... live and in real-time ... in the middle of a song while I am onstage.
Another factor is related to the fact that I am using a general purpose device (the iPad) for a very specific function, and it is not optimized for that. This manifests itself in many ways.
You have to accept the architecture IOS provides (Core Midi) and the way the apps that you purchase interact with it. In turn, each app is it's own closed, proprietary architecture.
For example, SampleTank cannot be made to respond to a specific MIDI device. It accepts any midi device, all glommed together by Core Midi, and the only way you can differentiate devices is by using the precious midi channel numbers, 1 through 16. Of course, in full blown mode, the FTP pickup uses no less than 14 of those 16 channels, due to it's own bogus setup (it uses channels 1-6 for the strings, and then duplicates them again on channels 11-16, while also using two or three channels for mysterious proprietary messages).
Don't get me started on the FTP itself! It outputs so many midi messages, including sysex, during a gig, that often I think the whole system just crashes because of overload. I have spent a LOT of time reverse engineering the FTP usb pickup. Part of my project will be to isolate, and filter it, so that it presents the correct set of midi events that I want to see, and not millions of other messages that clog up the system downstream.
Anyways, please understand that I absolutely LOVED the the iPad rig when I first got it. Mostly because for the first time I personally could add bass, organ, piano, violin, or spacey synth sounds, to my onstage repetoire. And Quantiloop was just that ... a quantum improvement over the $280 4th generation loop pedal that I was using. I explored dozens of loopers (Loopy, etc), and I'm sure some of them are nice for certain uses, but Quantiloop was the only one I found that made sense for use with a guitar rig in a live setting. Especially as I play with other people (a hand drummer) in my show.
But after a few months the drummer was complaining about the latency. He can hear the acoustic a few feet away before he hears the sound come out of the PA. Being sensitive to those kinds of things, that means there's a 38ms window in which the beat lay, and he really did not like that. We go beyond thinking in our music. It's a physical experiential thing, and his brain did not know which beat to follow. It's even worse when he had to play against a loop, which he generally hates to do anyways. We DO NOT use any pre-recorded loops or backing tracks ... ALL the sound that you hear on loops at our gigs is generated at the gig, responsive to the current audience, their mood, dynamics, and tempo. It's just the way we like it.
Anyways, that resulted in me measuring the latency, not only of the iPad but of the old foot pedal rig, and the nascent seeds of this project.
FWIW, your standard G3X foot pedal has a small, measurable 2ms delay, but that is so small that most people just consider them to be "real-time" devices ... but they are not. The loop pedal actually has a straight analog audio through, and so has zero measurable latency. Still 2ms is WAY BETTER than 38ms !!!