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Can’t control volume on my SammichSID


AB2010
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I’m having some issues with volume control using either SwinSIDs or 8580s in my sammichSID. When attempting to control the volume from the CNFG page by reducing the value from 127, nothing happens until I get to about 8 and then it just turns off. No gradual change in volume, just on or off.

 

When setting an LFO to modulate the volume in the mod maxtrix, I only get a rhythmic thumping in time with the rate of the LFO. The volume will cut out at the peaks of the waveform but there is no gradual change in volume. This is with any waveform.

 

I can can lower the volume of a patch by lowering the sustain level, but that isn’t a good solution for sound design.

 

Anywhere I should be looking for cold solder joints or bad connections? I figure it must be somewhere between the PIC and the SID, but there doesn’t seem to be any documentation on that. 

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Keep in mind that the SID only supports 4-bit volume control (ie, only 16 different settings are possible). So you’re never going to get a smooth change in volume, either from the volume printer itself or from modulating it with the LFO. The LFO/mod matrix can be a bit tricky to figure out as well, which may be compounding your issue.

I don’t think there’s any way a cold joint or other construction error could affect only the volume parameter.

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If you have a sammichSID, can you check something for me? If you change the volume from the CFG page from 127 down to 0 do you hear 16 steps in volume? I don’t hear any changes until I get below 8 and then it just goes silent? Maybe I have  a failed PIC?

I've looked at the datasheet for the SID and my understanding is that the PIC sends a message to the two shift register chips, and then that goes to the A and D pins on the SID. Maybe my PIC isn't sending out the right message to change volume?

Edited by AB2010
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I can give my MB-6582 a listen later just in case.

If you look up the SID datasheet, you can see the table of address pins and data pins. If you think it’s an issue with the communication between the PIC and SID, scope these pins as you change the parameter.

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I was able to see some pulses on the D pins, but honestly I don't know enough about the design to take this further. I will say that when turning the VOL parameter in the CFG page, I can here 15 or so pops ... and this would seem to represent the 16 volume steps that the SID has ... I just don't get a volume change with the pop. It could be that the PIC isn't sending the right value for each change pulse.

I swapped the 74HC595 chips and the problem remains, so I don't think the issue is with those chips.

Anybody care to chime in and let me know that turning the VOL parameter actually changes the volume of the output (16 steps)?

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  • 3 weeks later...

I was able to resolve this issue. It turns out SWINSIDs (at least the ones I have) do not have any control over the master volume register. That still doesn't explain why I couldn't control the master volume on my 8580s. I may have had an internal jumper set incorrectly.

 

After double checking everything and testing SWINSIDs vs 8580s, I have (very noisy) master volume control. 

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