POKEY Filters

[rdefabri]

I'm sure this has been covered ad nauseum, so apologies if it's in another thread.  I'm not the most successful at finding stuff, and some of the older threads aren't relevant, particularly in light of the work by folks like @VinsCool and @makary.

 

I am curious about POKEY's high pass (and I think there is some sort of low pass) "filters".  IIRC, these are used in sort of an "on/off" fashion, rather than in increments, which leads to my question.

 

Is there a way to create filter sweep type effects, or is that not possible given the on/off nature of the filters?  Would make for some cool effects in music if they could be used in a sweeping fashion.

 

Just curious...thx!

[thorfdbg]

This is just misinformation - this is not a "high-pass filter" in the sense that it filters in the frequency domain. It is just an AND-gate between two channels, i.e. the output is only high if the output of the original channel and the channel which is included in the high-pass are both 1. There is neither a "low-pass filter".

[rdefabri]

22 minutes ago, thorfdbg said:

This is just misinformation - this is not a "high-pass filter" in the sense that it filters in the frequency domain. It is just an AND-gate between two channels, i.e. the output is only high if the output of the original channel and the channel which is included in the high-pass are both 1. There is neither a "low-pass filter".

Yes, I am aware, hence "filter" in quotes.  I found a document suggesting a "low pass filter", although I question the accuracy since I know there isn't anything like that.  I'll try to dig it up.

 

Regardless, my question still stands.  Is there some way to make the filter operate in a sweeping fashion?  Again, I'm guessing not, but there have been new capabilities uncovered leveraging software, so I'm curious if anything like this has been attempted.

[Mathy]

Hello rdefabri

 

Looks like something went wrong when you entered @VinsCool and @makary.

 

Sincerely

 

Mathy

 

[rdefabri]

24 minutes ago, Mathy said:

Hello rdefabri

 

Looks like something went wrong when you entered @VinsCool and @makary.

 

Sincerely

 

Mathy

 

Thanks!

[rdefabri]

Ok this is something like what I am looking for.  In the opening of this song, there is kind of a sweeping sound.  It’s not quite a filter sweep but sounds similar.

 

@VinsCool - did you help create this?  What was done to create that sort of sweep effect?  Really cool stuff!

[_The Doctor__]

what song? you didn't attach, link, or post it.

[rdefabri]

12 hours ago, _The Doctor__ said:

what song? you didn't attach, link, or post it.

Crud - I was posting that from my phone and must have messed that up...let me find it

[rdefabri]

This isn't the one I tried to post, but this has a similar "sweep".  If you listen to the bass line, you can hear the effect:

 

 

[Rybags]

Timestamp?

 

There seems to be all sorts in that - filters, 1.79 MHz mode on single voice, 15 KHz mode, possibly even 2-tone.

[rdefabri]

2 hours ago, Rybags said:

Timestamp?

 

There seems to be all sorts in that - filters, 1.79 MHz mode on single voice, 15 KHz mode, possibly even 2-tone.

Right at the beginning you can hear it - from 0:00 - 0:06.  The bass line has a sweeping filter sound...there's a full sound that starts to thin out as the filter (seems) to be applied.

 

It sounds like something is happening there incrementally as opposed to on / off.

[Rybags]

Not sure that's filter.

You can get cancelling of output when channels are in certain states on notes of similar frequency.

 

It looks to be in 15 KHz mode at that point (the video seems to have an overlay of the Altirra audio graphing maybe?) - though you can mix and match master frequencies applied to voices to an extent.

[Steril707]

Yep; i think that's just frequencies cancelling each other out.

[rdefabri]

25 minutes ago, Rybags said:

Not sure that's filter.

You can get cancelling of output when channels are in certain states on notes of similar frequency.

 

It looks to be in 15 KHz mode at that point (the video seems to have an overlay of the Altirra audio graphing maybe?) - though you can mix and match master frequencies applied to voices to an extent.

As I suspected, it's NOT filtering, but it sure acts similarly.  Perhaps it's accidental, but it is a (somewhat) desirable effect that mimics what a true filter sweep might sound like. 

 

So does that answer the question - can you apply the POKEY filter in a dynamic fashion to create sweeping effects?  Or is it simply an on / off effect?

Edited September 19, 2023 by rdefabri

[VinsCool]

The filter was applied at strategic timing, with different frequencies offset to produce that "sweeping effect", which was effectively resetting the channel phases to a known initial position, in order to cancel each others with slight phase offset to allow the filter to create a pulse width modulation effect.

 

Here it was reset with a clock 64khz to improve the pulse width precision, the mode switch was getting masked every beat with the kick drums.

 

Further in the video I was experimenting with various combinations in order to create stable pulse width modulation, which was pretty convincing if you consider that it's difficult to make things sound stable in the first place.

[rdefabri]

5 hours ago, VinsCool said:

The filter was applied at strategic timing, with different frequencies offset to produce that "sweeping effect", which was effectively resetting the channel phases to a known initial position, in order to cancel each others with slight phase offset to allow the filter to create a pulse width modulation effect.

 

Here it was reset with a clock 64khz to improve the pulse width precision, the mode switch was getting masked every beat with the kick drums.

 

Further in the video I was experimenting with various combinations in order to create stable pulse width modulation, which was pretty convincing if you consider that it's difficult to make things sound stable in the first place.

That is pretty cool and rather effective.  Not quite a true filter sweep, but it accomplishes a similar effect IMHO.

[makary]

Doing this demo soundtrack years ago I wanted to see to what extend the pulse width / 'filter' / sweeps can be controlled. I got some results but I think @VinsCool has found more interesting ideas and results regarding these 'filter' mysteries.

 

 

[rdefabri]

On 10/1/2023 at 12:15 PM, makary said:

Doing this demo soundtrack years ago I wanted to see to what extend the pulse width / 'filter' / sweeps can be controlled. I got some results but I think @VinsCool has found more interesting ideas and results regarding these 'filter' mysteries.

 

 

What version of RMT are you using?  Do you do any post processing or are these played on actual hardware?  Great stuff - keep up the good work!

[+MrFish]

On 10/1/2023 at 12:15 PM, makary said:

Doing this demo soundtrack years ago I wanted to see to what extend the pulse width / 'filter' / sweeps can be controlled. I got some results but I think @VinsCool has found more interesting ideas and results regarding these 'filter' mysteries.

I really like all the musical work you've done on the POKEY.

 

[makary]

11 hours ago, rdefabri said:

What version of RMT are you using?  Do you do any post processing or are these played on actual hardware?  Great stuff - keep up the good work!

I made a fork of RMT with alternative tuning dedicated to 15kHz. And yes, you can run the tunes on real hardware. No postprocessing.

 

Thanks @MrFish

 

This so-called POKEY 'filter' works like this I believe:

 

 

Each edge from voice 1 inverts the signal. Each edge from voice 2 sends it low. As you can imagine trying to control this and use musically is pain in the neck  

[VinsCool]

On 10/1/2023 at 12:15 PM, makary said:

Doing this demo soundtrack years ago I wanted to see to what extend the pulse width / 'filter' / sweeps can be controlled. I got some results but I think @VinsCool has found more interesting ideas and results regarding these 'filter' mysteries.

 

 

 

This is the song that actually made me understand what was going on with the filter a few years ago, actually!

I have been doing now is more or less exactly the same tricks but with additional tweaks learned some time later.

 

This is the very first song I could say for sure used "absolute" control over the pulses, and this is the exact same tricks I have used in the much later Sketch 86 module.

 

On 10/3/2023 at 1:30 AM, makary said:

This so-called POKEY 'filter' works like this I believe:

 

 

Each edge from voice 1 inverts the signal. Each edge from voice 2 sends it low. As you can imagine trying to control this and use musically is pain in the neck  

 

This is pretty much the same conclusion I have come to, I still have not fully understood how this stuff works but experimenting with the "Filterchord" sounds really made it even more fun.

The generated waveforms are a lot more stable when the intervals are Just right, the more consonant the better.

Here's an example of the Filter usage where it produced a "Voice Choir" tone using a Major 10th interval, combined to the regular PWM filter played with the "absolute" manipulations from earlier.

 

I really could go on and on with more examples with the experimentation, but I think we can agree that there is a lot more going on with the filter when you find your ways around using it to produce impossible sounds  

It's so much fun, but also so much work to get things to sounds good, haha!

[makary]

I think this one shows some interesting usage of 'filters'. The filtered instruments use PWM but mixed with changes in volume envelope. The result is some kind of phaser(?) or flanger(?) effect.

 

BTW: melodic parts are based on 'Moment of Stop' by Dhor.

 

 

WOBBLE.xex

[olix]

... a really very impressive sound!

[Rybags]

Pretty good.  But I do suspect it is a filtered voice pair in conjunction with a low frequency note on the other voice/s.

[makary]

Yes, it is. But there's more to it. The effect becomes present only when specific frequencies and specific loops are set in the filtering channel/instrument. I hope I'll be more precise about it later on. I've just started messing with these settings. So it's more trial and error kind of work at this stage