Yet another crackle problem (Win7 64bit, ASIO4ALL)

I have been composing a rather small song sketch tonight and all of a sudden ran into heavy crackle problems I couldn't fix up to now. I have to admit I still am using ASIO4ALL until after my holidays where I'll finally buy an audio interface but right now, I'm stuck to some onboard chip. The thing is, I'm not even sure it's an ASIO related issue. For testing's sake I switched through all the different output device settings like DirectSound etc. and the issues stayed basically the same. I turned the buffer setting in ASIO4ALL all the way up, no notable difference either. I ran through the whole track while looking at the performance meter, the maximum usage was 30% CPU. The track consists of two instances of u-he ACE (which is known to be a performance killer, but my rig is strong, more on that later), an Albino 3 simply for sine sub bass (virtually no usage here I guess) and a Kontakt 4 instance. The main performance drop occurs when the melody plays (in the ACE) but as I said, no more than 30% total usage. Regarding my rig: The ACE synth is known to be a monster but so is my computer, actually: - Phenom X4 4x3.4GHz Quad Core - 4 GB DDR RAM - ATA133 HDD so neither the CPU nor the RAM nor the HDD should make any problems. I'm running Windows 7 64 Bit with Reaper x64. I even tried muting tracks (especially the melody) but it wouldn't change a thing. Before I started the arrangement, everything was fine, the 8 bar loop with all elements at the same time ran without problems. The key point seemed to be when I started recording MIDI input to automate the synth. When I figured out this might be the issue, I deleted all items with automation data but the problem still persists. So, what can I do? I tried virtually all settings in ASIO4ALL as well as Reaper itself. Is it really an ASIO problem and the audio interface would solve it or is there some other issue I don't know of? Edit: I hate to admit it but a minute after posting this post I figured out a partial solution. When the melody track is "record armed", the crackle is present. If I turn it off, no problem at all. This isn't a real solution though, seems more like a bug, or something. Any insight?

Does enabling Preferences->Audio->Buffering->"Allow live FX multiprocessing" help with this?

ACE doesn't have a x64 version so I guess you're using it with the bridge. As you said this is a plug-in that asks for tons of resources so running it bridged will give you problems for sure (and more at low latencies). Remember reaper's native bridge needs some time to communicate with the plug-in and receive its data/info so a 'live' scenario is not the best for using it.

The record enabled thing is kinda weird to me. I don't have ace I can't give it run and check (I'm running the same operating system/reaper) I'm guessing it couldn't be one of the usual suspects like uac/run as admin/wifi on/aero/ or simple things like maybe having to have the processor run "background services" instead of "programs". Just thought I'd check

The Suspect you might have a quad core.. but are you recording to a dedicated drive rather than useing the OS drive ?? whether this is the problem in your case i dont know. but ive seen it be an issue in various daw set ups in the past. only way to know is to test running to a dedicated 32 mb cache drive 7200 rpm. such drives are around 70 buks. keep the OS on its current drive. so thats 2 internal drives. one for win n one to record to. if the second drive test dont resolve the issue. it might be other aspects, on board sound or plug ins used or a reap setting etc. it can be various things. but irrespective its best to record to a dedicated drive so win has no chance of interrupting the daw process to do its thing on the one drive. a question. do these crackle probs occur even with a few traks ?? or just at higher trak counts ?? do they always occur irrespective of trak count ??

While I indeed am using the same drive the OS uses, the problem occurs when even replaying (not recording at all, just "record armed") a MIDI track that triggers a synth. The overall track count in this tune is like 5 tracks or so (some drums, a sub bass, the main synth, some side synth), nothing more. I actually haven't encountered the problem until I used the ACE synth that seems to be heavy on the CPU but still. Load never goes over 30% and buffering samples shouldn't be an issue as it's a synth!

Crackle 

What you describe sounds a little like a problem I recently had. I removed plug-ins one at a time to reduce the CPU load and had some success. Also plug-ins were reading 2 to 3 times more cpu utilization than usual. The project would peg at 135% or greater with only a few plug-ins active. However, no matter how light the load I would still get a occasional crackle on playback and the transport readout would flash red. Also note, with no plug-ins active, the problem disappeared... By accident, I lost connectivity with my ASIO driver and was forced to do a complete reload of reaper....after I completed the task, reaper loaded like a champ and cpu utilization was down to almost nothing....fixed in my system. hope this helps