I’m hitting an odd noise/artifact issue while trying to build a medium-sized polyphonic patch using LFO and SVF modules. I’m not sure if this is a CPU or gain staging issue. I started with 8 voices, brought it down to 6, and 5 voices seems to be the first point at which it works cleanly.
The goal is a 6-8 voice poly using LFOs with the following architecture per voice:
- 2 osc, each +Saw Wave to its own SVF channel; pitch from MIDI
- 1 SVF per 2 voices, LP1/3 to odd (left) bus, SVF LP2/4 to even (right) bus
- ENV for osc amp, gate from MIDI
- 2 macro controls for filter cutoff and resonance
The practical implementation is shown below with pages 5-6 and 16 of the patch. Pages 1-2 and 3-4 are the same as 5-6, except their SVF outputs go to BUS inputs 1-2 and 3-4, respectively. Page 16 sends odd bus channels to the left output and even to the right in order to spread voices alternately across the stereo field (experimenting). There are no additional modules in the patch, only the 6 voice pages and output on page 16.
Audio paths are the bold lines with black representing paths ultimately fed to the left output and red to the right. Each pair of pages shares an SVF and BUS on the left and MAC on the right, but each individually numbered block has its own MIDI module.
I’ve set output gains in an effort not to overdrive any downstream module’s input, and those are the levels depicted in the outputs of certain parameters. Otherwise levels are default 1.000 on both input and output. The output stage never appears to clip, visually.
The specific issue is noise that sounds like bitrate reduction or digital artifacting/aliasing when the 6th voice is activated. I don’t hear the noise in a 5-voice version of the patch, but I do hear a mild amount on the 5th voice when the 6th voice is added, even if the 6th voice is not active. Once the 6th voice is played, the aliasing(?) takes over more.
Overall module count:
- LFO - 12
- ENV - 6
- MIDI - 6
- BUS - 4
- SVF - 3
- MAC - 3
- OUT - 1
Could this be a CPU limitation? Maybe a complication of using LFO’s AMP input as my volume “VCA”? Is there an easy way to troubleshoot this myself aside from reducing voices? I tried bumping all the 0.5 output levels down to 0.4 and 0.4’s to 0.35, but no change with 6 voices. I hesitate to go down further because the patch is losing a lot of amplitude at this point.
EDIT/Update: I found the CPU meter; I didn’t realize RtEn had to be enabled as well. CPU load is showing around 60-65% on the 6-voice patch. This is with the SSP at 48kHz audio clock rate.
EDIT 2: I attached the preset to a comment below and wanted to include it here for visibility.
028 - poly6 saw.zip (24.7 KB)