UPDATE: I will be returning the device to look for a faster interface, but I would still very much appreciate knowing how to measure round trip latency.
I'm running guitar through a Line 6 UX2 into Pro Tools using an ASIO4all driver and the Pod Farm 2 plugin for amp modeling. No matter what I do, I just can't seem to record tracks that sound tight and in time. I also can't get the buffer size lower than 128 samples, although a lot of people seem to think that should be low enough for real time monitoring/recording.
Is there a good way to measure my total round trip latency (guitar through ASIO and back to my headphones)? Also, seeing as I can't get down to the desired 64 sample buffer, would it be worth returning my UX2 and looking for a faster interface?
In your case I'd say it's probably the plugin that introduces the biggest delay on the chain.
Hardware latency on the UX2 isn't specified by manufacturer (from what I read), btw note that Line 6 devices also offer ToneDirect™ Monitoring / USB Audio Steaming, which is a separate audio path with its own Buffer setting. This allows you to keep the above ASIO Buffer Size at a high “stable” value, yet still benefit from low latency monitoring of your input signal while recording
The buffer size alone isn't a big problem... @44,1kHz, a buffer size of 128 samples introduces a delay of (approx) 2,9ms, which basically is the same "delay" you get standing in front of a speaker at a distance of 1m.
There's one method to measure the other way: Protools, Audio Out and back In.
Just use a simple cable on one Output to reinject it on the Input. Send an audio track (some hi-hats might be nice to have a good nice transient) to the Audio Out, back to the Audio In and record it on a second track.
You can then measure the delay between both tracks because one will add the DA and AD conversion you would have the other way around when playing guitar. Not perfect measuring but still a way to get an idea.
128 samples might be high for some guitarist. That's one thing I quite love on the Eleven Rack... with the Low Latency Monitoring, I never had any problem at all.