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Feedback Loop on SID


c0nsumer

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Yeah I am only using 6582... you need to turn on bandpass and Ext In (i.e. filter external input) to get anything interesting... the 6581 filter is different so may not achieve self-oscillation or have too weak resonance, I don't really know much about that.

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I think a VCA would be nice to have in the feedback loop so you can make the amount of feedback voltage controllable and store this setting within a patch. However, stability might be an issue. A short delay might be interesting too by the way to create some weird flanging effects.

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  • 2 weeks later...

Yeah should work fine, you just want to keep it big so you're not affecting the audio output as it goes into whatever you use to amplify it.

What is the absolute minimum value one could use? Is 100K enough?

Also, if I want to create an insert point, I think the easiest way is to place a 2 pin SIL header on the feedback path. If no insert is used a jumper on the 2 pins makes it work the standard way. Any comments on that?

Would you by any chance know the output voltage of the SID audio signal? One of the tweaks I have in mind is a couple of diodes for clipping the signal, but I was also told that if the ouput voltage is on the high side, shunting diodes to ground could be dangerous.

Cheers,  Alex.

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  • 2 months later...

Hi guys.... I'm back again and I want to start joining in on the V2 beta testing... NOW!

I'm going to build Wilba's MB-6582 at some point, but until the components will be purchaseable from SmashTV, I'll be building a single-core version with two SID modules to get started.

Thus I've got a question for either Thorsten or Wilba (whoever get here first):

What type of pot will I need to implement the feedback feature PRECISELY? ... I'd be happy with a direct link to a german/uk electronics site with partnumber that will send to Denmark. I'm not that good at hunting down parts here in Denmark, and worry that I might get the wrong part or something  :-[

I'm just building a minimal V2 without LCD and control surface of any kind... I'll be developing a SoundDiver Adaption to tweak it (which I'll release here if Thorsten want it).

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I used a dual-gang 500k log/audio pot. Tolerances on potentiometers aren't great, so you will not get precision, however I can confirm that it's not that critical to performance... at least the pots I use are matched enough and used in a way that both SIDs in a pair sound the same, and reach self-oscillation at the same pot position.

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  • 5 months later...
  • 6 months later...

Sorry for bumping ;)

Just checked a bit how this loop thing works - a lot of different textures can be achived with a 8580, but with a 6581 (week 4183), seems there's no self-oscillation and little effect at all (I was able to get just a little gurgling effects on some sounds).

On the 8580, I thought it might be good (for guys like me who don't like huge high frequency self-oscillations) to limit the effect of the feedback loop (possibly with a 100 kohm resistor in series with the pot).

Just wanted to share my quick thoughts on the matter...  :)

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@DrBunsen - Now there's a wild idea! Haven't tried it, might give crazy results... I suppose you could get the benefits of the stronger 8580 filter plus the warmer basic sound of the 6581. Of course, in such a loop there might also be massive self-oscillations to watch out for... Hmm, maybe I'll try it in the near future!

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I would also love to have the feedback parameter value included in patches .. it really gives the sound a more analogue character vs. the traditional sid sound. While both are very important, they could be a vital part in a patch to make it dry and digital or wet and organic! (these statements are solely based on the soundclips on these pages + the clips on the original Alpha-mod-website  ;D) imagine a lead-patch containing of a warm feedback low-mid saw funtamental with dry detuned SIDish modulated pulsewaves on top.. like icing :)

But i wonder.. this mod will loop the sound from the sid back into it's own input, right? If the filters in the SID is 12dB, wouldn't the feeback-loop actually cause the filtering to reach a 24dB slope?? This is highly appreciated in synths ;) maybe even routing the audio through SIDs in serial, just like cascaded filters. If the filters are in bandpass-mode, they could be used to attenuate certain frequencies and maybe lend the sound a vocal character (kinda like a formant filter).. if lot of filters with high reso (at the same cutoff freq) is in serial, it will create very loud signals though, so caution is advised!! Waveforms with an obscure nature does best in this manner f.ex a noise-waveform will sound like a whispering, and a ringmodulated sound may have a robotic character. Cascaded bandpass-filters could also be used as EQ :)

My dream-scenario would be to have some gain-reduction function to compensate for the level when feedbacking with high reso settings. On synths like Novations Supernova II this parameter is called 'q-norm', but really is just a soft limiter (i guess.. i have no idea how!)

Anyway, these are commonly used techniques in modular synthesis, and they work well for me when i do VST-plugins f.ex..... they may not do good in a MIDIboxSID!

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  • 15 years later...


While looking at possibilities for my build (mbhp + arm2sid), I found this threat.

There is support for input already on the regular Armsid, Arm2sid should follow soon.

The original page/schemas from the 1st article is offline, I found a archived version here:

 

https://web.archive.org/web/20220331115422/http://www.bigmech.com/misc/c64mods/feedbackloop.html

 

 

 

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