MiSTer FPGA Forum

Posted: **Fri Apr 16, 2021 10:53 pm**

@sorgelig Thanks for all the SID updates!!! Nice!!!

Posted: **Sun Apr 18, 2021 10:17 pm**

Latest SID updates are just awesome! It sounds... well, like a REAL SID now!

Even Tim Follin's music is perfect now in Ghouls'n Ghosts tittle screen music, which makes all kind of crazy things with filters.

Posted: **Fri Apr 23, 2021 7:25 pm**

Looks like a few more today. I ran a bunch of demos yesterday, and things are sounding really good!

I still want to make an analog filter module that would plug into some GPIO pins. Then a SID Filter Plugin could be created that would route audio and control signals to the analog filter. The original SID filter is a State Variable filter, but there are other topologies that can be used that still provide multimode (LP, BP, HP) such as a cascaded four pole filter with pole mixing. This could all be done transparently to the SID. Different filter types would obviously sound a bit different, but as long as the control signals were properly scaled, it would work quite well. It's not important in the scheme of standard C64 enjoyment, but it would be a fun project, and fun to experiment with in the C64 environment, or for use with SID trackers, and other music creation tools.

Posted: **Fri Apr 23, 2021 8:04 pm**

Filter parameters aren't just 3 wires. It has filter cut-out frequency, resonance value. Plus some channels are filtered while others aren't. A lot of info should be passed to filter module. It's more easier to connect a whole SID chip than filtering module. Anyway it's not suitable for MiSTer core. Source code is available so you can create your own version with some external circuits and experiment with that.
All kinds of filters you can do by discrete external filters are possible to model in FPGA. It's just up to knowledge of filter math.

Posted: **Fri Apr 23, 2021 9:52 pm**

Love the work you’ve been doing on the C64/SID! It’s MORE than adequate for C64 use. The analog filter project idea is more because a group of friends and I are just C64, SID, and synthesizer nerds, designers, etc.

I’m aware of the SID side requirements. I don’t know much about what’s left available on the MiSTer to control it with though. That’s the part I’d need to learn more about. Luckily a relative of mine works a lot with FPGA (including DE-10) and the SID chip.

We’d need to find out what IO is unused on the MiSTer side.

12 bit - cutoff
4 bit - resonance
Mode selection
Channel routing to filter or direct out
Etc.

We’d have to have some logic on the filter board as well, to handle routing, CV scaling, etc.

Could do some kind of analog pass-through and mixer on the sub-board too. I’m good with all the analog stuff, but I’m definitely no programmer though I’ve dabbled with simple CPLDs before. That’s where my relative and friends would come in.

It’s hypothetical at the moment though as neither of us has enough free time at the moment.

Posted: **Thu Apr 29, 2021 9:38 am**

So, how hard/impossible would it be to create a SID Synth core with 12 channels output and midi support?

A bit like this dude did with a MegaDrive, just done in FPGA and with Midi control

https://youtu.be/V0kq0yCTpNE

Posted: **Thu Apr 29, 2021 12:37 pm**

Kjeldsen wrote: ↑Thu Apr 29, 2021 9:38 am So, how hard/impossible would it be to create a SID Synth core with 12 channels output and midi support?

A bit like this dude did with a MegaDrive, just done in FPGA and with Midi control

https://youtu.be/V0kq0yCTpNE

That would actually make a really cool synth core.

Posted: **Thu Apr 29, 2021 4:38 pm**

Kjeldsen wrote: ↑Thu Apr 29, 2021 9:38 am So, how hard/impossible would it be to create a SID Synth core with 12 channels output and midi support?

I was working on this a while back (haha, 2 years ago), just my thrift store Casio keyboard quit working so this is about as far as I got.

I was going to try and add the FM sound expander as another channel.

--> https://www.youtube.com/watch?v=mp_KNaRAHHM

Posted: **Fri Apr 30, 2021 5:28 pm**

Kjeldsen wrote: ↑Thu Apr 29, 2021 9:38 am So, how hard/impossible would it be to create a SID Synth core with 12 channels output and midi support?

Surely the holy grail would be a core capable of playing this: https://www.youtube.com/watch?v=nhz3vHYX0E0

Posted: **Fri Apr 30, 2021 10:30 pm**

robinsonb5 wrote: ↑Fri Apr 30, 2021 5:28 pm
Kjeldsen wrote: ↑Thu Apr 29, 2021 9:38 am So, how hard/impossible would it be to create a SID Synth core with 12 channels output and midi support?
Surely the holy grail would be a core capable of playing this: https://www.youtube.com/watch?v=nhz3vHYX0E0

That sounds fantastic!

Posted: **Sat May 01, 2021 1:28 pm**

bbond007 wrote: ↑Thu Apr 29, 2021 4:38 pm
Kjeldsen wrote: ↑Thu Apr 29, 2021 9:38 am So, how hard/impossible would it be to create a SID Synth core with 12 channels output and midi support?
I was working on this a while back (haha, 2 years ago), just my thrift store Casio keyboard quit working so this is about as far as I got.

I was going to try and add the FM sound expander as another channel.

--> https://www.youtube.com/watch?v=mp_KNaRAHHM

i wish i had talent and could cook something up, the MISTer is a Synth masterpiece waiting to happen

Posted: **Fri May 07, 2021 1:18 pm**

I did notice in the latest c64 core update that digitized sound doesnt sound as good as the previous release. E.g. Arkanoid intro music, digitized instruments sound somewhat dull compared to previous release. Having said that I did notice there is a filters setting now, am I supposed to be using one?

Posted: **Fri May 07, 2021 7:47 pm**

I've been messing around with the filters today and no matter what I select there is no noticeable difference. Are in game sounds supposed to sound different? Are there certain settings I am supposed to be using?

Thanks

Posted: **Fri May 07, 2021 8:40 pm**

Aspie wrote: ↑Fri May 07, 2021 7:47 pm I've been messing around with the filters today and no matter what I select there is no noticeable difference. Are in game sounds supposed to sound different? Are there certain settings I am supposed to be using?

Thanks

Are you using a set of custom filters?

Posted: **Sat May 08, 2021 2:56 am**

Yes I am using the filters posted by yxkalle in the other thread on this forum

Posted: **Sat May 08, 2021 10:41 am**

Aspie wrote: ↑Fri May 07, 2021 7:47 pm I've been messing around with the filters today and no matter what I select there is no noticeable difference. Are in game sounds supposed to sound different? Are there certain settings I am supposed to be using?

Thanks

Try the intro music of Cybernoid II, the effect is only noticeable with certain instruments.

Posted: **Sat May 08, 2021 5:28 pm**

I tried Cybernoid II. It's a great bit of music but I don't hear any difference when I cycle through the filters.

Posted: **Sat May 08, 2021 9:14 pm**

Try Bionic Commando. Its main theme has crazy percussion which changes drastically when cycling through filters.

Posted: **Sun May 09, 2021 3:16 pm**

Yes on Bionic Commando I could definitely hear a difference

Posted: **Tue Aug 03, 2021 7:26 pm**

Terra Cresta title music is completely missing the melody part that starts here:

https://www.youtube.com/watch?v=cAoZjiNNvgc&t=117s

(Using 6581 with default filter left and 8580 right, but changing that does not help.)

Posted: **Sun Oct 10, 2021 8:32 pm**

A SID data sheet was published a few days ago. Perhaps it helps to improve accuracy: https://archive.org/details/mos-6581-si ... t/mode/2up

Posted: **Sun Jan 09, 2022 8:08 pm**

There's an inaccuracy in the data sheet above on page 10. Probably Commodore intended for this value, but ended up using a different value later in production:

The two capacitors that control the filter are not 2200pF, but rather 470pF in the SID 6581 (R2, R3, R4 and R4AR).

In fact it's possible to replace those ceramic caps with 2 x 330pF (which Commodore did during some repairs) for an even more open filter sound - makes it easier to get the sound of Martin Galway's 128D (which uses a 6581 - unlike later 128DCRs that use the 8580) and Rob Hubbard's C64 (check Lightforce at around 4:00 mins which is probably THE most demanding tune on the filer).

If anyone's interested I have a custom test PRG that slowly runs through every waveform with an LPF to test the filter offset and slope. Especially useful if you want to record the output and analyze it spectrally via e.g. Izotope RX.

Posted: **Wed Nov 16, 2022 7:26 pm**

Hi lagerfeldt, I'm "playing" with the SID filters presently and I would be interested in that test program you mentioned, plz!

Posted: **Fri Dec 30, 2022 7:42 pm**

Sure, hit me up at holger at onlinemastering dot dk and I'll send the .prg

Also see ultimatesid.dk where I've got some pictures of these filter sweeps using spectral analysis in RX Advanced, as well as a lot more info on the SID chip(s) and the filter.

Posted: **Mon Jan 02, 2023 6:21 am**

Scarzix filter C64 .prg attached

Will do filter sweeps, waveforms can be switched in the player.

Direct link to my FAQ on the SID (scroll up for more info, less technical):
https://ultimatesid.dk/#section-sidtechfaq

Posted: **Wed Jan 11, 2023 1:45 pm**

lastninjaju wrote: ↑Fri May 07, 2021 1:18 pm
I did notice in the latest c64 core update that digitized sound doesnt sound as good as the previous release. E.g. Arkanoid intro music, digitized instruments sound somewhat dull compared to previous release. Having said that I did notice there is a filters setting now, am I supposed to be using one?

Old/original sample playback methods use a DC bias offset exploit on the output to playback samples. This DC offset is prominent with the 6581 chip, much less with the revised 8580 chip.

So make sure you have the 6581 selected?

Modern alternative techniques exist that do not use DC bias exploit, but Arkanoid and almost all older games and demos do so they're 6581 SID only.

The filter settings shouldn't affect this type of sample playback.

Posted: **Sun Jan 15, 2023 3:33 pm**

lastninjaju wrote: ↑Fri May 07, 2021 1:18 pm
I did notice in the latest c64 core update that digitized sound doesnt sound as good as the previous release. E.g. Arkanoid intro music, digitized instruments sound somewhat dull compared to previous release. Having said that I did notice there is a filters setting now, am I supposed to be using one?

Confirmed.

Much lower and slightly distorted in an unusual way. Perhaps some odd interaction with the FET emu.

The lowered level seems like a bug/error/oversight. Interestingly sample playback doesn't change when switching to the 8580, so maybe that's a clue.

Posted: **Sun Jan 15, 2023 4:12 pm**

This is the best custom filter, although the "Default" setting is very close, but a bit too open and therefore buzzy. The default sounds very much like the standard ReSID filter shape.

Slightly warmer and natural in the highs:

Alankila6581R3_3984_1.flt.zip: (2.33 KiB) Downloaded 186 times

Unpack, upload e.g. into the Filter_Audio folder.

Load as a custom filter *.flt in the C64 core and select either of the Custom 1/2/3 settings (all are the same once a custom filter is loaded).

Selecting "Default" will switch to the default filter.

Set both channels identically for true SID mono sound. Remember to select the 6581 chip, not the 8580.

Save settings for the C64 core or the filter will be reset when the core is loaded next time.

Posted: **Tue May 16, 2023 5:29 pm**

Nice! I will check this out ASAP!

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