Reverb Pan Bug

[RubenRoeh]

Hey guys,

it would seem, I've run into a bit of an issue with the verbs. I've felt for a while that my sound wasn't coming out of the FM3 properly centered and today, to confirm my suspicion, I inserted a sine wave generator before my chain and started turning blocks on and off. Turns out, the reverb block was to blame. I tested this with the Sun Plate type, which I use a lot, and when all relevant parameters are set for neutral pan, the right channel is 4db louder than the left. Weirdly, this goes away the moment I switch from Ultra High or High Quality to Normal. I repeated the test with the Studio type and there the results were exactly reversed - centered on High/Ultra, lopsided on Normal. I turned the modulation off for all testing, because the moment that entered the equation, it became very hard to read. Wild fluctuations in level (probably due to the channels interacting with each other) with uneven mean levels in both channels. I used both the internal metering and an external from my RME interface, both tell the same story. The difference is also plenty easy to hear. I've checked all parameters, loaded a fresh backup, and made sure the unit was up to date, nothing fixed the lopsided stereo image. I'll attach the preset in question, just to provide the whole picture.


Sun Plate Ultra and Normal



Studio Ultra and Normal



Thank you kindly, Ruben

 

[GlennO]

That's not too surprising with a sine wave. The stereo effect in a reverb is achieved by using different all pass filters left and right, so you should expect to see somewhat different frequency responses in the two channels. Try adjusting the frequency of the sine wave and you'll see the imbalance change and even reverse itself. If you use noise instead of a sine wave it should be more balanced.

 

[mr_fender]

Keep in mind also the reverb algorithms do not preserve the stereo image. Each input channel for the block is not processed separately. They use a summed mono signal to feed the reverb algorithms which then creates a panned stereo image for the various wet delay voices of the reverb tails. Only the dry portion of the reverb mix is true stereo through.

 

[fractalz]

Stereo verbs have different resonance frequencies L & R.

 

[unix-guy]

Interesting though all the points are, it does seem a little odd that the behavior would change when changing the Quality setting.

It would be interesting to try the same test on FM9 and Axe Fx III and see if it's any different.

 

[GlennO]

Interesting though all the points are, it does seem a little odd that the behavior would change when changing the Quality setting.

It would only be surprising if it didn't change when using a sine wave source, due to the different filter network for those settings.

It would be interesting to try the same test on FM9 and Axe Fx III and see if it's any different.

You'll see the same thing with almost any stereo reverb when using a sine wave.

 

[RubenRoeh]

That's not too surprising with a sine wave. The stereo effect in a reverb is achieved by using different all pass filters left and right, so you should expect to see somewhat different frequency responses in the two channels. Try adjusting the frequency of the sine wave and you'll see the imbalance change and even reverse itself. If you use noise instead of a sine wave it should be more balanced.

You're right of course. Sine might not have been the best way to test this. I first noticed the lopsided image while playing though. This is just me trying to figure out where I went wrong. I used White and Pink Noise respectively and depending on the settings, the difference is at the very least much less severe.

Keep in mind also the reverb algorithms do not preserve the stereo image. Each input channel for the block is not processed separately. They use a summed mono signal to feed the reverb algorithms which then creates a panned stereo image for the various wet delay voices of the reverb tails. Only the dry portion of the reverb mix is true stereo through.

View attachment 121802

This I did not know! That's actually somewhat disappointing as some of the delays I route partially into the verb are very much stereo.

Thanks for the input, guys. If nothing else, I've learned something. Still have the feeling my wet chain is off center. Gonna keep probing my preset.