#24 Reverb introduces noise

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opened 1 month ago by icebearbb · 9 comments

Hi dcoredump,

I just tried the FX settings and found that the reverb introduces quite a lot of noise. For one, it sounds like there is some aliasing going on (number format conversion anywhere? rounding?). Something else can be heard after the reverb dies down. That one sounds like a non-empty buffer is being repeated endlessly after the reverb calculation ends. Perhaps the reverb buffer needs to be zeroed after the reverb drops below some threshold.
I wanted to attach some sound demos, but they don’t let me upload any WAV, MP3 or ZIP here.
Have fun!

PS: The chorus sounds great though! ;) Trying the delay next...

Hi dcoredump, I just tried the FX settings and found that the reverb introduces quite a lot of noise. For one, it sounds like there is some aliasing going on (number format conversion anywhere? rounding?). Something else can be heard after the reverb dies down. That one sounds like a non-empty buffer is being repeated endlessly after the reverb calculation ends. Perhaps the reverb buffer needs to be zeroed after the reverb drops below some threshold. I wanted to attach some sound demos, but they don't let me upload any WAV, MP3 or ZIP here. Have fun! PS: The chorus sounds great though! ;) Trying the delay next...
dcoredump commented 1 month ago
Owner

Hi @icebearbb,

thanks for reporting. The reverb is a ready-build freeverb from the Teensy audio library, so it’s for me like a black-box. I will try to ask if the Teensy forum at PJRC can say something about problems with freeverb.

The chorus was a tricky thing... it took several hours to get there :-)

Regards, Holger

Hi @icebearbb, thanks for reporting. The reverb is a ready-build freeverb from the Teensy audio library, so it's for me like a black-box. I will try to ask if the Teensy forum at PJRC can say something about problems with freeverb. The chorus was a tricky thing... it took several hours to get there :-) Regards, Holger
icebearbb commented 1 month ago
Poster

Hey @dcoredump,

yeah, I thought so. I found someone else mentioning the noise back in 2017, https://forum.pjrc.com/threads/28186-Freeverb-for-teensy
They added a lowpass filter after the reverb, maybe like this:

Dexed ──┬───────────────────────────────────────────┬───>
        └────> freeverb ──> lowpass, e.g. 11Khz ────┘

In the meantime, I updated my Teensy libs to 1.52 but the noise is still there. I’ll see if I can reproduce the noise using some AudioLibrary examples. Then I’ll head over to PJRC and start nagging them about it ;)

Greetings, Lars

Hey @dcoredump, yeah, I thought so. I found someone else mentioning the noise back in 2017, https://forum.pjrc.com/threads/28186-Freeverb-for-teensy They added a lowpass filter after the reverb, maybe like this: ``` Dexed ──┬───────────────────────────────────────────┬───> └────> freeverb ──> lowpass, e.g. 11Khz ────┘ ``` In the meantime, I updated my Teensy libs to 1.52 but the noise is still there. I'll see if I can reproduce the noise using some AudioLibrary examples. Then I'll head over to PJRC and start nagging them about it ;) Greetings, Lars
dcoredump commented 1 month ago
Owner

I can try to add a LP filter, but htis will eat CPU cycles :( Hopefully not too much.

I can try to add a LP filter, but htis will eat CPU cycles :( Hopefully not too much.
icebearbb commented 1 month ago
Poster

I wrote an example to test freeverb. May it be that the amplitude of the signal generated by Dexed is very small? The test sketch (can’t upload it...) generates a triangle wave at an amplitude of 1.0 (however much that is) and quite literally fried my ears at the headphone settings I use for Dexed.
I was able to reproduce the freeverb noise behaviour at an amplitude setting of 0.05 ;)
Sending you the sketch for testing. Beware of the volume!

I wrote an example to test freeverb. May it be that the amplitude of the signal generated by Dexed is very small? The test sketch (can't upload it...) generates a triangle wave at an amplitude of 1.0 (however much that is) and quite literally fried my ears at the headphone settings I use for Dexed. I was able to reproduce the freeverb noise behaviour at an amplitude setting of 0.05 ;) Sending you the sketch for testing. Beware of the volume!
dcoredump commented 1 month ago
Owner

There are currently strange problems with the dual-engine version. Maybe Dexed was not very loud, but have in mind that an amplitude of 1.0 is a maximum - no headroom!!!

I found a way to get Dexed a little bit louder, but it seems that the engine has a big dynamic. Maybe the real DX has a kind of compression at the end of its sound engine...

Can you try the current code? The 2nd engine is disabled and it should be louder.

Regards, Holger

There are currently strange problems with the dual-engine version. Maybe Dexed was not very loud, but have in mind that an amplitude of 1.0 is a maximum - no headroom!!! I found a way to get Dexed a little bit louder, but it seems that the engine has a big dynamic. Maybe the real DX has a kind of compression at the end of its sound engine... Can you try the current code? The 2nd engine is disabled and it should be louder. Regards, Holger
dcoredump commented 1 month ago
Owner

I have added a 7.5kHz LowPass filter after the reverb(s). That sound better. Hint: You should set Reverb-Send to 100 and Reverb-Level to the value you want to get. If the send is low you may get more noise... the send is not really needed in single-engine version but for a dual engine you need to decide how much signal has to be routed to the reverb engine.

I have added a 7.5kHz LowPass filter after the reverb(s). That sound better. Hint: You should set Reverb-Send to 100 and Reverb-Level to the value you want to get. If the send is low you may get more noise... the send is not really needed in single-engine version but for a dual engine you need to decide how much signal has to be routed to the reverb engine.
icebearbb commented 1 month ago
Poster

Wow, that sounds a lot better now! Panning the engines and having the reverb go all around is quite a treat ;) Thank you for fixing this.

Wow, that sounds a lot better now! Panning the engines and having the reverb go all around is quite a treat ;) Thank you for fixing this.
dcoredump commented 1 month ago
Owner

So the issue can be closed?

So the issue can be closed?
icebearbb commented 1 month ago
Poster

Yes, thank you!

Yes, thank you!
dcoredump closed this issue 1 month ago
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