Rendering of VST instruments

Hi all, I started using Reaper about half a year ago and so far I am quite enthusiastic about it. I invested quite some time in setting it up, made my own theme, programmed my own plugin to communicate with hardware controllers. Now it is time to start using it. I am working on a few projects already and the workflow is really good. However, I did some tests the other day and the results got me a bit worried. I hope it's some user error and a simple setting in the preferences will resolve it. What I did was create a track with one MIDI item of about 60 bars long. At the beginning of each bar I put one MIDI note, exactly on the first beat of the bar. Next I loaded a VST instrument on this track, in this case Stephen Slate Drum Sampler 4.0. The idea was to have that MIDI note trigger a percussive sound, with a clear attack. In this case I chose a HiHat sound which obeyed that requirement perfectly. Next I rendered this track using the 'Render tracks to mono stem tracks (and mute originals)' option. Then I zoomed in the stem track horizontally and vertically so I could see exactly where the rendered hits started. That's where I got worried because the hits don't line up where they should be. They move around in such a way that it looks as if they are always aligned with the beginning of the buffer. In fact, when I increased the ASIO buffer size to 1024 and rendered again I could clearly see that the position error increased with an equal amount. That means a hit could be as much as 1024 samples off from the position where it should be. At a sample rate of 44.1 KHz that's 23 milliseconds. Surely I have misconfigured something and without a doubt Reaper is capable of sample acurate rendering of VST instruments, right? I mean, lesser DAW's seem to do this without a problem. It's not rocket science. So can anybody point me in the right direction?

I'm interested in this because it sounds interesting. However, I must say, I've rendered VSTs probably hundreds and times and never noticed any audible issues whatsoever.

I realized there was a simple test I could do to see what was going on, namely to run Reaper in a debugger. It looks like Reaper is doing everything right. So now I'm suspecting SSD itself. Not a happy customer of mister Slate right now, although I doubt he ever sets foot in these quarters to read this. But I tested with a few other plugins as well, with the same results. Some big name plugins too. So I'm still in doubt.

I can also report, I haven't experienced any problem with time accuracy of rendered VST instruments. Although I use percussion virtual instruments very sparingly and usually just as raw rhythmic tracks without extra fiddling. I can definitely tell, that Rayzoon Jamstix, ReaSamplOmatic5000, free MT Power Drums and previously also Toontrack EZDrummer 1 are all fine. Same applies to melodic instruments, which will has to use same principle. No shifts regardless of buffer size. Are you sure that there isn't any built-in humanization (its calculated parameters can somehow interact with processing block size) isn't turned on in your Slate VSTi? (just thought, i've never use it) Michal

If you are only SEEING an apparent misalignment and not actually HEARING it, then I suggest to you that it is not happening -- that is, it's only the graphics and not an actual mis-render. DAWs are not all that accurate in the graphics department. They just came out of the Stone Age as far as GUIs are concerned. So ... I would ask: Anybody actual hearing the track(s) off by X samples or can make it show up on a scope zoomed in? 'Cause if nobody hears it, it ain't there. Had to ask, because all the posts I'm reading here are all about "I see this" and "I see that", nothing about, "Man, this sounds messed up!"

Assuming we're doing this with a VST that doesn't change the sample randomly, should the rendered audio null with the midi/vst output if you invert the polarity on one channel? I tried it with Addictive drums and SampleTank 2.5 (no round robin), and it did not null. However, when I rendered at different buffer sizes, then inverted the polarity of one of those tracks, AD did not null, but ST did. So, it seems to me that playback of the VSTi does not directly align with the rendered output, but the rendered output does not change with buffer size. I also played around with the "preserve PDC monitoring..." setting (something that has always confused me) but that seemed to have no effect on this experiment. I would be interested to know from some of the folks who believe the rendered output matches the midi file - if you invert the polarity of the midi track, does it null with the rendered track?

Does the sample file have X ms of nothing at the front? ... how long is the 'build up' to the hit peak? Guessing you want the peak to line up, right.

It sounds like the VSTi is not responding to the delta time property of vst midi events, IE where in the current buffer the event should occur. Reaper could be mangling the deltas too, but I think I would have noticed... But you never know, other hosts have screwed this up without anyone noticing for a long time. Anyway. The definitive test is to setup a usually reliable instrument with a short noiseburst or hihat, and have it play a line of 64:th notes. Set a large ASIO buffer. Then gradually increase the tempo. If the sound impulses are not evenly spaced you'll hear it easily. If all is well, they'll form a steady tone at high tempos. I'll do this when I get home.

OK, did some testing. Reaper seems to be generating delta times just fine, which indicates that the OPs VSTi doesn't handle them right.

Could someone please comment on the null test, as I mentioned in post 15 above?