Sauraen

SID background noise

6 posts in this topic

Since the beginning of testing the SID circuits for my build, starting from just the standard SID module on a breadboard, whenever I release a note each voice of the SID keeps playing a very quiet organ-like sound at the pitch it was last played at. If I set ADSR all to zero, then I can effectively play this noise with the keyboard. The noise stops entirely if the voices are sent to the filter and then all filter modes are turned off, confirming that this is an internal thing. I don't remember offhand whether the noise also stops if I turn off all waveforms, and I haven't tried setting the frequency to zero after setting the gate bit to zero (because then Release wouldn't work).

I don't hear this noise in any of the MIDIbox SID demos. I'm using my own code to drive the SIDs (from the LPC17 core), so there might be some software optimization in the standard firmware that reduces this that I don't know about. Has anyone had any luck getting rid of this noise?

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If memory serves, that's just how it is. Most of the MidiBox demos are using 8580/6582 chips, so if you are using 6581, you should expect generally more noise though I rather think of it as more "character" :) It's possible there is a work-around for some of that, but I have noticed it on my MBSID (even with 6582 chips) so part of it is just the nature of the beast, it seems.

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This issue is known as the adsr bug, it is a problem with all SID chips, and if you've heard a demo that did not have this sound, a noise gate is probably applied to the recording. The only real solution is to use an external ADSR and VCA. It's actually considered to be a characteristic of the SID sound that most CHIPTUNE musician's have some sort of nostalgia for.

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Back in my old C64 days, I actually replaced the 6581 in my machine a few times. The faint held notes were always one of the symptoms of a defective SID.

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defective SID.

All my SIDs have the ADSR bug. I wouldn't classify them as defective though.

Bugs are what give ald analog gear its charm. Nowadays designers of synth VSTs are trying to mimic the analog circuit quirks and instability bacause their products are deemed too clean... You get this as an integrated feature with the SID. Learn to love it :console:

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Thank you, I thought it was something like that. I have been testing with 6581s (less to replace if I break something).

In our build there are external audio attenuators on each SID that we can hook up to an envelope in the modulation matrix, though that will change the volume of the whole SID.

As far as learning to love the "imperfections", there's also digitally controlled feedback loops to enhance the growly filters and digitally switchable filter capacitors (the idea from ) to have even finer control over the distortion. Setting the filter to band-pass and turning up the feedback and the resonance creates an oscillator, with the cutoff controlling its frequency; setting up feedback between two SIDs and changing the filter characteristics of the second makes the two resulting oscillators interfere, creating all kinds of crazy full-analog sounds. I just got the capacitor-switching working yesterday; we'll put up a video soon. :)

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