I’m trying to understand how big of an impact the audio interface has on the overall latency. For example, I am using a 2nd gen focusrite 6i6 at a buffer size of 256 samples. Some of my sounds are generated by my nord stage 2 which means that the sound needs to pass through the input and output buffer. The latency is reasonable but definitely noticeable. Cantabile reports a buffer duration of 5.8 ms. So the “theoretical” latency would be about 12 ms. How much more latency does the audio interface usually add to that? Almost all higher quality audio interfaces claim they are “zero-latency” but what does that translate to in non-marketing terms? And how does my focusrite 6i6 compare to an apollo twin mk2 or a cheaper Behringer interface?
The “zero-latency” feature is a different beast - this means that you can listen to the signal you are recording directly, without it having to pass through your PC. This can be pretty useful when recording vocals, but completely useless for playing guitar through amp-simulations or for processing the sound of your nord through VST effects.
The key factor for latency is the buffer size, so this determines most of the latency you experience. The big difference between interfaces is the efficiency of their drivers, i.e. how small a buffer you can get away with before experiencing drop-outs and crackles. Some drivers also add their own latency to the buffer latency, so their real-life latency may actually be bigger than just the buffers.
If you want to know precisely what your “real” round-trip-latency is, you’ll need to use a tool like the RTL Utility - you’ll need to patch an output back into an input of your audio interface, and the tool will measure the actual delay.
The hardware inside the interface is less of a concern when it comes to latency - the time the AD or DA conversion takes is pretty much neglegible compared to the buffer latency.
Currently, the RME drivers (Babyface, Fireface, etc.) are still considered the most efficient ones in terms of low-latency performance. Zoom interfaces have also proven themselves in terms of stable low-latency performance. I don’t have any real-life experience with the Focusrite or the Apollo Twin, so can’t contribute here, but the Behringer U-Phoria HD range has surprised me and others with its low-latency performance, which is pretty decent, considering its price.
I own a Behringer UMC204HD and have had it for some time. I have no issues at 256 samples. The Midas preamps are also surprisingly good and its build is quite beefy.
It does have zero latency monitoring, which operates as Torsten describes above.