Nattgris

Actually, Einstein first used L for energy... ;D

I wouldn't trust that formula much. The 6581 data sheet was written before the circuit was completed and it turned out that the circuit didn't follow the specs very well, especially not the filter. A quote from a transcription of the data sheet at http://stud1.tuwien.ac.at/~e9426444/ : 470 pF is what's inside the Commodore C64 (at least the later models), I guess that's where the value comes from. (I still don't get why the 8580 cap was chosen different from what's inside a C64 using the 8580.) stryd_one: Your last post is somewhat dangerous... I almost got stuck in an infinite loop. ;)

No, it's not a computer psu, it's a standalone psu that probably have been used for some form of office or lab equipment. I have no idea where or when I got it, I just found it in my junk box. If you want the details it's a "Mascot Type 6823".

If having a genuine C64 look is not on the top of your priority list I honestly don't see the point of reusing the C64 power supply. For my test setup i simply use a 5V/12V dual output switching power supply, directly connected to the 6581 without any other components. I regulate it to 9V with a 78L09 when using a 8580. Maybe it's not as easy to find for everyone, but I have an abundant reserve of power supplies of different kinds and I haven't paid for any of them. I think most people have lots of unused electronic equipment lying around, that's a good place to start looking for suitable supplies. If you're not used to electronics i suggest you find a supply with the correct voltages ready to go. Minimum fuss.

The C64 (at least some models) will still start even if the 9V is missing. If there really is no 9V AC from the PSU it's probably just the fuse that's blown. That could happen if you connect it to a faulty circuit. If you have the dark grey, slanted PSU it's really easy to change the fuse, the holder is located on the back side. There is two, probably only one of them is blown. Probably just as easy if you have the light grey but I don't have one of those so I'm not sure. Check the fuses with your multimeter in ohm mode (should read around zero ohm if it's working). Be sure to replace any broken fuse with one of the same type and rating. The 9V AC supply is simply a transformer and the fuse so there isn't much else that can be broken.

The 9V AC pins on the C64 PSU is completely separate from the 5V output. That is, they are not referenced to the ground pin as the 5V pin is. To measure the 9V AC you measure between the 9V pins, not between one 9V pin and ground. One tester to pin 6 and the other to pin 7 (not sure about the numbers). Of course you also have to set your multimeter to AC mode (sometimes marked ~) instead of DC mode, otherwise it will read 0V!

Has anyone verified that the filter works on the external input? Mine doesn't seem to. I silence all voices, and feed an externally generated waveform into the SIDs input. When I enable the FILEX bit the output amplitude drops to about half, but the cutoff doesn't affect the output at all. >:(

There is another thread on this forum where someone has compared the 6582 to the 8580 by ear and concluded that there is a difference in the filters. He posted two sound files to compare them too. I haven't listened to them, but i suspect if there is a difference it might very well be due to the same capacitors have been used for both SIDs. Well, by looking at the 6581 data sheet you learn that the recommended capacitor is 2200pF and that this would give a range of 30Hz to 12kHz. The 6582 data sheet is more or less identical, but the recommended capacitor is instead 6.8nF for a range of 30 Hz to 12 kHz. The 8580 data sheet ... well... is there one? What we DO know is that the capacitor chosen by Commodore for the machines using 8580 is 22nF. After my measurements on a 8580 I concluded that 22nF capacitors would give a range of 0 Hz to 13 kHz. Hmm, this is about the same as the other two with their recommended capacitors, respectively. This makes it seem likely that Commodore designed the different SIDs to be used with a filter range of 13 kHz and that the capacitors to use for the different SIDs are 2200pF for 6581, 6.8nF for 6582 and 22nF for 8580. However, as we know, the filter in 6581 is not up to spec, and it could differ a lot between revisions and even between chips of the same revision. For example the 6581 I have on my desk needs about 1nF to get a range of 12 kHz.

The maximum oscillator frequency at 1MHz (is anyone using anything other than that?) is 3929 Hz. Now it's still perfectly sane to have a filter that extends far above 4 kHz since the waveforms have high levels of harmonics. 3rd harmonic is at 12 kHz. We don't, yet. I will make a full test after I've played around with the 6581 (it works! ;D ) and switched back to the 8580. But considering the physics behind the filter I'd say it's linear. The caps are used to set some form of time constant in the filter. Changing this time constant would scale the time linearly and the cutoff would still be linear.

Remember that equation is taken from the 6582 data sheet. It does apparently not apply to the 8580. As hardly noone has ever seen one of those SIDs i don't think it's relevant. Interesting to note is that this equation together with the suggested 6.8 nF (all from the 6582 data sheet) gives about the same frequency range as the 8580 does with the recommended cap for that SID (22 nF).

I actually don't propose anything, I'm just telling you what I found out and you can do whatever you want with the information. As I said, I'm not a MIDIbox user myself, so I don't think I'm in a position to propose changes to it. :) Let me see if I can explain... I discovered that the 8580 cutoff frequency is proportional to the 11-bit value in the cutoff register and inversely proportional to filter capacitance. That's it. In mathematical terms you could write CUTOFF = K * VALUE / C. The proportionality coefficient K was determined to be about K=143 for the 8580. What this means is that we have 2048 levels of cutoff to play with. These levels are distributed evenly across the audio spectrum, starting from 0 Hz. If we increase the register by one the cutoff frequency will increase with a fix amount. With the 6.8nF caps everyone seems to be using this amount is about 21 Hz. 21 Hz is a tiny amount when the cutoff frequency is high. Noone can hear the difference between 10000 Hz and 10021 Hz. In the low frequency range however, 21 Hz is very large. Consider the difference between 100 Hz and 121 Hz. It's impossible to select a cutoff between these two. No wonder a sweep of the cutoff sounds steppy. In the same time, the maximum cutoff frequency available is 21 Hz * 2047 = 43 kHz. This is far above what anyone can hear and a complete waste of levels. I don't think there's a practical use for filter frequencies above 10 to 15 kHz. By redistributing our 2048 levels to this frequency band (by change of capacitors) the levels will get spaced much tighter, providing greater resolution which is badly needed in the low frequency region. For example, using 22 nF capacitors as in the real C64 gives us a maximum cutoff of CUTOFF = 143 * 2047 / 22 = 13.3 kHz with a resolution of 13300 / 2047 = 6.5 Hz. That is more than three times as many steps in the useful frequency band, giving us a much smoother sweep. Again, this is completely a hardware issue, and nothing Thorsten could program can remedy the lack of levels. It's simply because the 8580 maps the values linearly to cutoff frequency. Initial testing of a 6581 suggests that its filter has an exponential mapping, giving much finer control over low frequencies at the "expense" of coarser control at high frequencies. Now to address your questions directly, Jaicen... At the lowest setting (zero) the output will certainly be muted (assuming a LP filter). At the lowest practical setting (one), the passband is still too low to hear the output (unless you have sub-sonic hearing. You might still feel it though, if you have mighty speakers. :) ) Again, assuming a LP filter, it will be more or less an allpass. At least if the maximum cutoff is chosen high enough to include most of the harmonics the SID can produce. Not really. More the other way around. With the current cap value the filter extends all the way up to 43 kHz which means only part of the range is usable (say 0% to 40%). With a bigger cap (say 22 nF) the filter only reaches 13 kHz (still high enough for most purposes) making the entire range of the filter (0% to 100%) usable.

I would like to do the same test with a couple of 6581 SIDs to find out how much they differ from eachother. Unfortunately I only have one and I'm not sure it's ok. I will post here when I find out. If the 6581 behaves in the same way as the 8580 (if it has a linear mapping between value and cutoff, and if cutoff is proportional to capacitor value) you could derive a formula for the cutoff similar to the one i gave above for 8580. If the constant is more or less the same for different 6581 SIDs (as we know there could be huge differences between chips) you could make a general formula like CUTOFF = K / C * VALUE where K depends on the type of SID used and C is the capacitor used. This means it's possible to calculate MIDIbox SID's mapping between CC value and SID register value at compile time to get the same frequency characteristic from the filter no matter what SID and capacitor you use. However, the response will of course be either steppy or limited in range if you choose a capacitor unsuitable for your SID. My main point is that by making the cutoff frequency you get from a certain CC value the same in all implementations of MIDIbox SID, it becomes easier for people to share patches that sound the same for everyone, while providing the freedom for each user to choose the tradeoff between low frequency filter resolution and filter range depending on his needs.

By the way, if you're interested in how I did the measurements I set the SID to output a low frequency square wave and turned up the resonance (which is working really well on the 8580 :) ) to max. Then I could easily measure the frequency of the ringing following each step of the square waveform, using a digital oscilloscope.

Hi! Sorry for posting in such an old thread, but it's really the most relevant. This is also my first post on this forum so bare with me! :) I started a project a while ago to control a SID chip from the computer, this was before I heard about MIDIbox. I'm not really a musician, more of an electronics guy, so it was more just for the fun of it rather than to actually produce something useful. While browsing the net for information I discovered MIDIbox SID, and I decided to design my circuit something like this. However, I couldn't find the selected value for the filter capacitor (6.8 nF) used anywhere else. All C64/C128 schematics (and my real C64) use 22 nF with SID 8580. I also found this thread where there were complaints about steppiness in the cutoff frequency and suspected what was the cause. Now my circuit is up and running and I have run some tests on it to try out the filter characteristics. I measured the cutoff frequency for a set of register values throughout the 11-bit range (1 to 2047). The following results were obtained with C=6.8nF. [table] [tr][td]register value[/td][td]cutoff frequency (Hz)[/td][/tr] [tr][td]2047[/td][td]46300[/td][/tr] [tr][td]1024[/td][td]22600[/td][/tr] [tr][td]512[/td][td]11030[/td][/tr] [tr][td]256[/td][td]5300[/td][/tr] [tr][td]128[/td][td]2700[/td][/tr] [tr][td]64[/td][td]1330[/td][/tr] [tr][td]32[/td][td]673[/td][/tr] [tr][td]16[/td][td]325[/td][/tr] [tr][td]8[/td][td]165[/td][/tr] [tr][td]4[/td][td]82[/td][/tr] [tr][td]2[/td][td]41[/td][/tr] [tr][td]1[/td][td]21[/td][/tr] [/table] This shows that the value to frequency mapping of the 8580 filter (at least my 8580) is indeed (very) linear! It also shows that the resolution with the proposed filter caps is really bad. The cutoff can only be adjusted in steps of about 21 Hz, which is really hearable in the low frequency range. Also, the cutoff range extends all the way up to more than twice the maximum audible frequency, wasting precious bits of resolution. I haven't had time to redo the entire test with other capacitor values, but i tried doubling the caps to 13.6 nF and the cutoff for the lowest value sank from 21 to 10.4 Hz. It's seems likely there's linear dependency on the capacitance as well. In that case the relationship between register value, capacitance and cutoff frequency would be approximately CUTOFF = 143 / C * VALUE, where CUTOFF is in Hz and C in nF. The capacitor used in the Real Thing (and therefore by Prophet64) is 22 nF which would give a resolution of 6.5 Hz and a range of 13.3 kHz. Seems far more reasonable to me. Ooops, this got rather long, sorry.