Thanks to all for the info. I did find the button on the tool bar and gave it a whirl. I can't say that the skies opened  and the earth tremored or anything, but it seemed to remove a little of the smear between tracks, everything just sounded a little clearer. I can see this as a good and bad thing, depending on circumstances, but in this case I left it on. The cpu meters climbed a little, but my computer didn't even break a sweat. I will say that I will always do an A/B comparison to see if it is beneficial. Thanks for pointing this feature out. New plan: continue recording @44.1 24 bit, but always check to see if upsampling brings anything of benefit to the table. One last question: does leaving this on during playback do anything beneficial when exporting? I export as. wav 16 bit 44.1khz

Anderton
This is the best part about SONAR's upsampling, you can get the quality of recording at 96 kHz with 44.1 kHz projects.

 
Hold on a second.  It's not as simple as that.  An A/D conversion at 44.1 gives you a maximum theoretical bandwidth of 22.05kHz.  Upsampling that signal to 96kHz after it has been captured at 44.1 doesn't add any high frequencies to the signal, or somehow make it sound "better".  
 
This is similar to taking an HD picture and showing it on a 4k television.  The 4k screen won't reveal 4k worth of resolution, because it simply was not there in the first place. Many 4k televisions have processing that synthesizes additional information which can make an HD picture look subjectively better on a 4k screen, but the additional information is mostly a result of sophisticated guesswork.  This is also the same with audio processing.  After upsampling a 44.1 signal to 96, there is processing that can be applied which can produce additional upper harmonics, and it may sound subjectively better, but that doesn't mean it is more accurate to the original source.

Rick O Shay

Anderton
This is the best part about SONAR's upsampling, you can get the quality of recording at 96 kHz with 44.1 kHz projects.

 
Hold on a second.  It's not as simple as that.  An A/D conversion at 44.1 gives you a maximum theoretical bandwidth of 22.05kHz.  Upsampling that signal to 96kHz after it has been captured at 44.1 doesn't add any high frequencies to the signal, or somehow make it sound "better".  
 
This is similar to taking an HD picture and showing it on a 4k television.  The 4k screen won't reveal 4k worth of resolution, because it simply was not there in the first place. Many 4k televisions have processing that synthesizes additional information which can make an HD picture look subjectively better on a 4k screen, but the additional information is mostly a result of sophisticated guesswork.  This is also the same with audio processing.  After upsampling a 44.1 signal to 96, there is processing that can be applied which can produce additional upper harmonics, and it may sound subjectively better, but that doesn't mean it is more accurate to the original source.

 
Sonar's Upsampling feature only works with software synths & effects plugins, because those upsample before rendering, if you enable them.
  
And thinking about it, I have a really stupid question: if I enable Upsampling On Playback on, for example, Z3ta+ 2, and then record using an aux track, the end result will have the quality of 88.2 kHz in a 44.1 kHz file?

Rick O Shay

Anderton
This is the best part about SONAR's upsampling, you can get the quality of recording at 96 kHz with 44.1 kHz projects.

 
Hold on a second.  It's not as simple as that.  An A/D conversion at 44.1 gives you a maximum theoretical bandwidth of 22.05kHz.  Upsampling that signal to 96kHz after it has been captured at 44.1 doesn't add any high frequencies to the signal, or somehow make it sound "better".

 
Actually this isn't about high frequencies, it's about removing foldover distortion. It is true that upsampling can give the appearance of better high frequency response, but that's a side effect of removing the distortion.
 

After upsampling a 44.1 signal to 96, there is processing that can be applied which can produce additional upper harmonics, and it may sound subjectively better, but that doesn't mean it is more accurate to the original source.

 
It is more accurate to the original source because it doesn't have distortion added by recording at 44.1 kHz. Remember that I was very specific in my claims...they most important part is in bold: "However, I've demonstrated in workshops at the New Music Seminar, AES/Mix Nashville, and at Gibson's New York showroom that recording at 96 kHz can make an audible, obvious difference under specific conditions that involve "in the box" plug-ins and virtual instruments."
 

So, this upsampling setting in Sonar... this has no effect on plugins that I've already internally set to playback realtime at project sample rate, and upsample on bounce or export? It only has effect on plugins that are capable of higher sample rate processing, but don't upsample automatically already at project sample rate or fast bounce? Is that the correct understanding of Sonar's implementation of the optional higher sample rate on fast bounce? Or does Sonar also allow for higher sample rates (upsampled) BETWEEN all plugins and busses when the process is engaged?
 
Also, don't know why more recording engineers don't notice this (or maybe they do), but if you look at those online SRC conversion tests... be sure to check out the tests for dBpoweramp conversion. It's relatively inexpensive software, and according to the tests, seems to give results at least similar to most much more expensive high end hardware and software SRC.

javahut
So, this upsampling setting in Sonar... this has no effect on plugins that I've already internally set to playback realtime at project sample rate, and upsample on bounce or export? It only has effect on plugins that are capable of higher sample rate processing, but don't upsample automatically already at project sample rate or fast bounce? Is that the correct understanding of Sonar's implementation of the optional higher sample rate on fast bounce? Or does Sonar also allow for higher sample rates (upsampled) BETWEEN all plugins and busses when the process is engaged?

 
I'm not quite sure what the question is, but the only limitation is that SONAR can't upsample past a certain point...for example if your plug-in oversamples internally at 192 kHz, SONAR is not going to bump it up to 384 kHz. 
 
 

Chregg
naturally all anti aliasing filters aren't gonna be created equal
 

https://www.youtube.com/watch?v=qdfWWg1FOyQ
 
Some appear to not be in place at all.

sorry to bring up an old thread, but where is the option to have Sonar upsample a VST softsynth to 96 Khz in a 44.1 project?  

I recently started recording at 96kHz after assuming for years, based on stuff I read, that I shouldn't be able to tell the difference. I'm shocked by how much better my acoustic guitar sounds.
 
Interestingly, though, the improvement is only noticeable with certain mics. I've got a C414 that sounds pretty much the same whether at 48, which is what I used to record at, or 96. The top end of my Joly-modded MK-012, on the other hand, extends significantly at 96. Way more air. I'm guessing this has to with the frequency response of the modded Oktava going higher than that of the AKG, but I don't know for sure.
 
Whatever the reason, I'm glad I finally tested it out myself, rather than just going by what I read. I was pleasantly surprised.

Also remember, 96kHz is much better for in-the-box digital processing.  VST plugins do better with a higher sampling rate.   yea some do upsample, but why bother (and some dont), just go for 96Khz, most modern computers can handle it now.