c0nsumer Posted October 3, 2006 Report Share Posted October 3, 2006 Hey everyone, a friend pointed me to this link: Commodore C64 Modifications :: Adding A Feedback LoopThought you all might find it interesting... There's some good samples in there, too.-Steve Quote Link to comment Share on other sites More sharing options...
TK. Posted October 3, 2006 Report Share Posted October 3, 2006 Great link, I especially like the examples about the self oscillation :)Best Regards, Thorsten. Quote Link to comment Share on other sites More sharing options...
Wilba Posted October 4, 2006 Report Share Posted October 4, 2006 What a cool site!It was only the other day that I had a "what if" thought about routing SID output to input... and then someone's done it (and proved it's worth doing!).The mod he did to the SID2SID board to use the audio in of the second SID uses a 100k resistor to ground, which I assume is just copying the C64 schematic? TK's SID module uses a 1nF cap to ground. I'm wondering about the difference and whether the 100k resistor would reduce noise even more... he seems to get some really low noise! Quote Link to comment Share on other sites More sharing options...
stryd_one Posted October 4, 2006 Report Share Posted October 4, 2006 Nice site indeed. What a geek :) Hey TK, is there room in a SID patch to store the feedback parameter if it's done with a digipot? Quote Link to comment Share on other sites More sharing options...
c0nsumer Posted October 4, 2006 Author Report Share Posted October 4, 2006 That reminds me quite a bit of some of the most fun I've had musically. That is, taking some analog filters, feeding them back on themselves, throwing in some additional effects, and making simply interesting thumpy bouncy dancy sounds. Which were completely impossible to reproduce nor sync with anything. :DI need to get / build more analog stuff soon... Quote Link to comment Share on other sites More sharing options...
stryd_one Posted October 4, 2006 Report Share Posted October 4, 2006 completely impossible to reproduce nor sync with anything. :DIt's possible, grab your calculator ;) Quote Link to comment Share on other sites More sharing options...
c0nsumer Posted October 4, 2006 Author Report Share Posted October 4, 2006 It's possible, grab your calculator ;)Well, if I hung a bandpass filter and a freq counter off of it...Actually, they are Electrix boxes, so they sent their position via MIDI, but I don't think it was accurate enough to reproduce this. The devices are weird and you could tap the knob to change the sound, even though they didn't move enough to send a new position out via MIDI. Quote Link to comment Share on other sites More sharing options...
Wilba Posted October 7, 2006 Report Share Posted October 7, 2006 I can confirm that a pot in between a SID module's input and output does not blow up the SID ;D and does make some funky sounds. I've also observed that if you lower the SID's resonance then you can tweak this feedback knob to get better (and more!) resonance, to the point of funky self-oscillation squeal.What I plan to do now is hack a pot apart and break the resistor strip near the anticlockwise end... so that end is ground, the wiper goes to input, and the clockwise end goes to output. This will make a sort of switch, so when the knob is fully anticlockwise, the input is tied to ground (less noise), otherwise it will range from 1M to 0 ohms as you turn clockwise. Quote Link to comment Share on other sites More sharing options...
Mr modnaR Posted October 7, 2006 Report Share Posted October 7, 2006 don't they do pots that have a 'click-off' setting? like with radios, you turn it fully anticlockwise until it clicks to turn off the radio, but if it's on, the pot just does volume. Quote Link to comment Share on other sites More sharing options...
ris8_allo_zen0 Posted October 7, 2006 Report Share Posted October 7, 2006 Why not replacing the pot with a VCA with ratio 0 to 1 driven by an AOUT?(just an idea - don't tell me how to do 8) ) Quote Link to comment Share on other sites More sharing options...
stryd_one Posted October 7, 2006 Report Share Posted October 7, 2006 That's another way to implement midi control but the big question is whether the new MBSID has space in a patch to save the feedback levels... If not, then the level couldn't be saved in a patch....However, a DCA or VCA could be set to 0 feedback and switched to ground when a patch is loaded, and set by a CC with just a little extra code, and you could send the CC at the beginning of your song to set the feedback as required.... This is starting to sound like a daughterboard hey? Quote Link to comment Share on other sites More sharing options...
Wilba Posted October 7, 2006 Report Share Posted October 7, 2006 don't they do pots that have a 'click-off' setting? like with radios, you turn it fully anticlockwise until it clicks to turn off the radio, but if it's on, the pot just does volume.Yeah, but I haven't found one with a good range... it needs to be at least 500k, and dual gang...Also note that any kind of feedback, even feedback going in through 1 Mohm, introduces noise... there's a distinctly audible difference in noise between a grounded input and input connected to output through 1 Mohm.From my initial playing with this new knob, I don't anticipate the need to automate its setting, I doubt I'll be tweaking it during live playing or recording... but I've noticed controlling the SID filter resonance can tweak how much the feedback has an effect - it's like they get multiplied together or something, I'm no filter expert... but maybe tweaking the resonance parameter is enough for most people.I fully recommend spending a couple bucks on a 500k or 1M pot and giving it a go... Quote Link to comment Share on other sites More sharing options...
cheater Posted October 7, 2006 Report Share Posted October 7, 2006 What if you decoupled the feedback and kept the ground on the input? Would that perhaps lower noise? Quote Link to comment Share on other sites More sharing options...
stryd_one Posted October 7, 2006 Report Share Posted October 7, 2006 I doubt I'll be tweaking it during live playing or recording... Ahh but what about loading patches? That's the real reason to want to save it or control it by midi....it's like they get multiplied togetherThat'll be the log curve on the outputs, the resonance is a peak and gets amped more... Maybe... I think ;) Quote Link to comment Share on other sites More sharing options...
Mr modnaR Posted October 7, 2006 Report Share Posted October 7, 2006 @wilba, you can get pots with a push pull switch too. they're used in electric guitars. Quote Link to comment Share on other sites More sharing options...
Wilba Posted October 9, 2006 Report Share Posted October 9, 2006 yeah, probably not dual ganged for stereo though... I know what the ideal solution is, it's just a matter of finding these perfect pots... my DIY solution is cheap and involves no hunting around for ages, which frankly I am sick of doing, I still can't find 22nF polystyrene caps, or <2% tolerance silver mica caps, or ultra bright diffused 3mm green LEDs, or knobs like TK's MB-SID... ;) Quote Link to comment Share on other sites More sharing options...
stryd_one Posted October 10, 2006 Report Share Posted October 10, 2006 LMFAO ;D Quote Link to comment Share on other sites More sharing options...
Jaicen Posted October 10, 2006 Report Share Posted October 10, 2006 I take it you're joking about the 22nF caps and diffuse LED's! They really are ten-a-penny around here. You can get them both from Maplin if you're in the UK. Quote Link to comment Share on other sites More sharing options...
Wilba Posted October 11, 2006 Report Share Posted October 11, 2006 I can get your standard stuff, but not but not polystyrene caps or ultra bright diffused LEDs. Maplin does not have what I want. Quote Link to comment Share on other sites More sharing options...
Mr modnaR Posted October 11, 2006 Report Share Posted October 11, 2006 rswww.comstock number : 466-3481 for the LEDs. (10,000mcd enough?) £0.80 each(!)they also do +-1% mica caps. they are expensive though.www.farnell.com3039640 - 22nF 63V Polystyrene cap.9520430 - 22nF 63V Polystyrene cap with RoHS compliance. both £1.12hope this helps. Quote Link to comment Share on other sites More sharing options...
cheater Posted October 16, 2006 Report Share Posted October 16, 2006 Just a thought but...what if we put something in the feedback loop?some ideas:1. noise gate - well, obvious one...2. microdelay - i think this could sound interesting. think of comb filters. (easy circuit?)3. overdrive/compresoor - well... hehe...4. amplitude modulation by the external filter input - this could be another easy circuit. and very powerful i think. Quote Link to comment Share on other sites More sharing options...
stryd_one Posted October 16, 2006 Report Share Posted October 16, 2006 Yeh a sidechain IO would sure be nice, that way you can run it through whatever... Quote Link to comment Share on other sites More sharing options...
Wilba Posted April 26, 2007 Report Share Posted April 26, 2007 Update: I've had a lot of fun with this feedback loop in my MB-SID, and thought I'd share some knowledge.First up, you don't need to follow the wiring like shown in that first link... he's obviously wanting to switch between using an external input, grounding the input, and adding feedback.The easiest way to try out the feedback, but still ground your input most of the time, is to connect a pot with the SID Audio Input (J4:I on SID module) to the middle pin of the pot (the wiper), and the SID Audio Output (J3:O on SID module) to the clockwise pin, and ground (J4:Vs on SID module) to the counter-clockwise pin. This way, if the pot is fully counter-clockwise, the audio in is grounded (just like that jumper you put on J4) and if fully clockwise, it's maximum feedback. So when you're not using the feedback, it's just as good as grounding the audio input with a jumper.I use dual-gang 500K audio taper pots (for stereo SIDs), but you can use a single-gang, or linear taper, or even change from 500K, perhaps 1MOhm might be just as good.Then go and turn on Ext In on the filter ;D You also get different results by turning down the Resonance to 25-ish (don't just leave it at max!) and then turning up the feedback to almost max, then modulate cutoff with an LFO of rate 66 and then tweak depth and cutoff... :o Of course it can also add some distortion if you turn off Ext in on the filter. Quote Link to comment Share on other sites More sharing options...
Jaicen Posted April 26, 2007 Report Share Posted April 26, 2007 Yeah, the distortion occurs because the EXT in is always routed to the VCA. If you turn the filter off, the extra gain not dropped throught the filter will overdrive the VCA. Quote Link to comment Share on other sites More sharing options...
2Methyl-Bulbe-1ol Posted May 25, 2007 Report Share Posted May 25, 2007 I try this mod tonight.I have a 6581.With the feedback at 100% it is not sounding incredible, just a little "filtered layer" on my sound.Are you using 8580 / 6582 for a good result ? Quote Link to comment Share on other sites More sharing options...
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