Is it possible to reproduce low-pass filter on PSG ?

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Door Metalion

Paragon (1353)

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07-03-2021, 11:51

Hello everyone,

One of the features of the SID on C64 is the low-pass filter, that gives a more "bassy" sound when needed.
Is it possible to reproduce that on the PSG ?
Maybe by combining certain frequencies ?

PS : I'm thinking about using that on a SFX sound.

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Van ARTRAG

Enlighted (6502)

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07-03-2021, 12:31

No, the sole feature available is envelope modulation, not useful for filtering
And in software generating low pass waves needs cycle accurate code

Van Grauw

Ascended (9762)

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07-03-2021, 13:31

Low pass filter is a feature quite unique to the SID.

Envelope frequency can be set so high to reach audible frequencies, generating triangle-ish and saw-ish waveforms, but without volume control (only 1/2 amplitude through another trick). Maybe it is useful for your sfx.

Van st1mpy

Paladin (744)

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07-03-2021, 14:31

I guess Darky can do something. But I was thinking of something similar, so I made an external psg cartridge.
It has 3 separate outputs for each psg channel, then I put that through filters, effectors and then a mixer, so you can tweak the sounds all you want. It really sounds amazing especially dragon quest and final fantasy.
(Then I wanted separate channels for everything, so I got beeper output, fm ym2413 for separate drums and melody, and starting to design fpga scc with 5 separate channels. Need a bigger mixer soon.)

Van Daemos

Paragon (1940)

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07-03-2021, 16:03

I can confirm that darky has filters build in. You can tweak the high mid and low.

Van Metalion

Paragon (1353)

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08-03-2021, 12:31

Grauw wrote:

Envelope frequency can be set so high to reach audible frequencies, generating triangle-ish and saw-ish waveforms, but without volume control (only 1/2 amplitude through another trick). Maybe it is useful for your sfx.

Grauw, can you develop on that ?
How would that work ?

Van Grauw

Ascended (9762)

afbeelding van Grauw

08-03-2021, 17:55

Disable the tone and noise generators in the mixer for the channel you want to play the envelope wave on, this will make it output 1 continuously which can be volume controlled. Then enable the envelope generator with a sawtooth or triangle pattern, and set a small period according to the formula specified in the application manual. It will modulate the output so fast that it becomes an audible tone.

Additional notes:

  • The envelope generator has less range in the mid to high notes due to its higher divider, however especially in the lower end it works well.
  • The waveform is not exactly a saw or triangle because the scale is logarithmic, but close enough.
  • If you enable tone output on the mixer you can get some more complex patterns. If you do so, using frequencies which are harmonically related (a multiple of each other) gives best results.
  • If you want to lower the volume a bit, enable the tone generator and set it to period 0 or 1, the inaudibly quick oscillation of the tone generator will halve the amplitude.

You can check this video for a demonstration (enable subtitles).

Van defdanny

Master (254)

afbeelding van defdanny

08-03-2021, 14:25

Hi Grauw, Hi all,
maybe it's totally off-track and I misunderstood the sound theoretics discussed here completely....
I recently made a conversion of on old ZX Spectrum PSG-tune with Arkos Tracker 2.
Track is called "Sax Solo", video on YouTube here
During conversion I learned a new way to create really "bassy" sound with the PSG. It is used as the lead voice in this track. In this case, are we talking about the same "technique" you described?

Van Grauw

Ascended (9762)

afbeelding van Grauw

08-03-2021, 14:29

Yep, same thing.

Van defdanny

Master (254)

afbeelding van defdanny

08-03-2021, 14:45

cool. Really like your "PSG jam video"!

Van NYYRIKKI

Enlighted (5742)

afbeelding van NYYRIKKI

08-03-2021, 14:46

Grauw wrote:

Low pass filter is a feature quite unique to the SID.

Envelope frequency can be set so high to reach audible frequencies, generating triangle-ish and saw-ish waveforms, but without volume control (only 1/2 amplitude through another trick). Maybe it is useful for your sfx.

I guess you can also double the amplitude by wasting another channel, but hard to imagine it being very useful.

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