Round Trip Latency, Big difference between Audio and Midi/VSTi in C13

I am reaching you today because of an issue with my RTL maybe only you can understand and explain.

I have a 7820x with asus MB, 16go ram, ssd etc… running windows 10 with cubase 13.0.50 and a focusrite 18i8 gen 2

I wanted to test my RTL, as I was feeling something strange. So i plugged a jack cable from output 2 to an input of the sound card t

for testing i use da bass plugged in directly and a light VST instrument (fabfilter one, but i tried other on with same result)
I recorded :

  • a midi track with a single midi note input triggering the VSTi instrument piano sound patch
  • a bass note with my guitar bass fender
    and of course:
  • the both generated sound (bass and VSTi) on a 3rd track coming from the jack cable (output 2 to input 2 of the sound card)

as you understand i can measure the delay in sample/ms between the directly recorded bass note and midi input note in difference with the generated sound coming outside the focusrite output 2 jack back to it, in order to measure RTL on both audio input and midi input with VSTi.

But the big problem came from the delay i measured between the midi note input and the waves sound generated by the VSTi.

Firstly from my understanding the result should by half of the audio bass, as sound triggered by midi note and generated by a VSTi should be affected only by the output latency indicated, right ? as there no ADC involved

I still understand the result might be more than the Output latency indicated only, which are not always accurate, and also because of many other parameters related to the DAW, the VSTi (well not really as i tested other one with same result) etc…

Straight to the point:

@ 32 buffer (2.4+2.4ms indicated by cubase) when the audio bass had 4.9ms RTL (very close to 2.4+2.4=4.8ms), the midi VSTi RTL result is… 7.9ms !

when i was expecting 2.4ms or slightly more. First it is more than the IL+OL, in fact it is nearly twice!.. I cannot understand why, i checked my DPC latency few month ago and it was kinda ok, i will try again.

about the result at other buffer size is kinda similar, with the midi vsti RTL systematically more than audio bass, when , again, it should be less than the audio the a full round trip of ADC + DAC of the audio input recorded.

only from 128 (6.44+6.44ms indicated by cubase) it started to be little less than audio with 12.97ms for audio and 11.56ms for VSTi

at 256 (12.1+12.1) audio bass is at 24.2ms (very accurate) and VSTi is still at at 19.4ms

i know focusrite are being very bad with their driver, but stil,l i wonder if it not a cubase problem. I will try today with reaper.

SO finally my question: Have you ever seen this strange result where VSTi-midi note sound having more latency than a full ADC+DAC RTL audio input recorded ?

massive thanks to read until now, hopefully you will have time to answer

note : WHY didnt i used RTL : i tested and have the exact same result than one reported by cubase (as you will see above) and aslo as my porblem is with midi/VSTi sound generation, RTL do only with audio.

Also i just checked latency Mon and DPClat, and no issue

Finally here the results that i measured and aso the one i just did completed under Reaper

just made an excel file to summarise and noted wit the last line that my computer or cubase is adding mostly 5-6ms to midi vsti sound generation… very annoying for me who need very low latency to be confortable with my playing keys and mesh drums kit

About reaper At 32 buffer reaper adds only 1.2ms to output latency for midi/vsti input when cubase adds… 5.5ms that is huge

Thanks again for reading, hopefuly you will have some clue