Jurbo Posted September 18, 2008 Report Share Posted September 18, 2008 What happens to the sound when the filter capacitors differ in value? Does it cause two separate resonance peak frequencies? Does it cause a softer cutoff, less abrupt filtering? Does someone know the theory on this?I'm asking this because after some cap fiddling, I found a very nice sound on my 6581 with ceramic caps (probably not too closely matched), as opposed to somewhat closely matched (5% tolerance) plastic ones. The filter felt softer in character around the cutoff using the ceramics than the plastics which gave an almost on/off character to a sound fluctuating around the cutoff frequency. 6581 filter seems to be very sharp at the cutoff, I'd like less abruptcy.So, anyone with actual theoretical knowledge on this? Thanks! Quote Link to comment Share on other sites More sharing options...
vout Posted September 19, 2008 Report Share Posted September 19, 2008 Hi, I noticed this effect too - when I was experimenting with different values of filter capacitor I found that the cheap ceramic disc capacitors give a veiled, slightly grainy sound, compared to plastic film ones which have a more detailed, open sound. This is nothing to do with the capacitor value/filter cutoff, but seems to be the 'sonic signature' of the capacitor. I liked both types of sound, so I have included a multipole switch in my stereo sid that adds/removes the ceramic capacitors from the circuit. Not wishing to get into any audiophile voodoo here, but the sonic effects of different capacitor types does seem to be pretty well documented and accepted these days, there is lots of stuff online about it.Cheers, vout Quote Link to comment Share on other sites More sharing options...
stryd_one Posted September 20, 2008 Report Share Posted September 20, 2008 I figured this out chatting with W a while back.... Now I forget. Does anyone have the -internal- schematic of the SID? I can't find it.... Edit: Hey nils, the quote DB linkage is busted :( Quote Link to comment Share on other sites More sharing options...
Futureman Posted September 21, 2008 Report Share Posted September 21, 2008 I might be talking out my arse, but I remember hearing a bit about the different sounds of capacitors etc..(I used to work with audiophiles... ughhhhh....)Metal film caps are at the top of the list, sonically.. I think Polystyrene introduce the lowest THD (Total harmonic distortion) ..While Tantalums & Ceramics Introduce the Most.I have not built my Sid project yet.. but maybe some samples are in order? Who knows, even a rotary switch with a whole lot of different types of caps being switched in? Quote Link to comment Share on other sites More sharing options...
Futureman Posted September 21, 2008 Report Share Posted September 21, 2008 And for those that want to read up on it..http://www.reliablecapacitors.com/pickcap.htm Quote Link to comment Share on other sites More sharing options...
stryd_one Posted September 21, 2008 Report Share Posted September 21, 2008 I was more referring to the OP's question, about what happens when the matching between the two caps is not precise.... I seem to remember it effecting the linearity of the cutoff frequency, but I don't wanna go explaining it based on my memory ;D Quote Link to comment Share on other sites More sharing options...
nILS Posted September 21, 2008 Report Share Posted September 21, 2008 Edit: Hey nils, the quote DB linkage is busted :(Whoops. Fixed. Quote Link to comment Share on other sites More sharing options...
Jurbo Posted September 28, 2008 Author Report Share Posted September 28, 2008 Hmm, no definitive answer yet... I went googling around a bit, and couldn't find any documents that would specifically say what goes through each capacitor at any given filter configuration. Then I thought about it, and could it be something like this:- In lowpass or highpass mode, only one cap is utilised- When combining different modes, both caps come to play, for example one cap doing the lowpass filtering and the other doing highpass (notch filter)Could this be the case? If it goes something like this, then it would be logical that using closely matched caps the cutoff frequences of "both" filter channels (caps) would be the same, resulting in the loudest possible resonance etc. Also, you might be able to control the width of the bandpass frequency band with the difference of the two caps (closely matched caps make narrow band, different caps make wide band).In a nutshell, one cap for lowpass, other for highpass, then all the other modes from combining and subtracting the resulting waves with each other and the original signal.Does this make any sense or am I totally off here? Just interested in knowing the logic behind the fascinating SID filters... Quote Link to comment Share on other sites More sharing options...
stryd_one Posted September 29, 2008 Report Share Posted September 29, 2008 Find me that internal schematic and I'll tell you ;)(it's around somewhere, I just couldn't find it) Quote Link to comment Share on other sites More sharing options...
strophlex Posted September 29, 2008 Report Share Posted September 29, 2008 If you ask the school book, one cap wold be needed for a first order filter resulting in -6 dB/oct while two caps would be needed for a second order filter and -12 dB/oct like the SID specs. So it might be that both caps are used for the filter and that would definitely result in most resonance if they are matched well. However I have no idea about the SID filter construction, so I am just guessing... Quote Link to comment Share on other sites More sharing options...
stryd_one Posted September 29, 2008 Report Share Posted September 29, 2008 That's for RC filters, and I don't think that's what they are. I seem to remember they're hanging off opamps, but again... my memory....I've searched again for that internal schematic and damned if I can find it. I'm sure it's even been posted here before... Anyone at all? Quote Link to comment Share on other sites More sharing options...
Jurbo Posted September 29, 2008 Author Report Share Posted September 29, 2008 Does this link contain what we are after (scroll down about a quarter of the page)?http://www.bel.fi/~alankila/c64-sw/ Quote Link to comment Share on other sites More sharing options...
stryd_one Posted September 29, 2008 Report Share Posted September 29, 2008 It's not exactly the diagram I was after, but YES!! Thanks!!It's late, I'll do this tomorrow unless some synth geek does it before then :) Quote Link to comment Share on other sites More sharing options...
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