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Forums > C64 Coding > SID POP
2018-11-13 13:18
oziphantom

Registered: Oct 2014
Posts: 249
SID POP

Is there anything other than changing the volume that causes the SID to pop?
I'm playing a digi, and then after a while Goattracker kicks back in, and with it a pop. The Digi leaves the Volume register with $1f in it, GT then sets it to $0F.

The SID is configured for Digi as follows :
7271	.2e49	a9 00		lda #$00					lda #$00
7272	.2e4b	8d 05 d4	sta $d405	                sta SID+$05         ; voice 1 ad
7273	.2e4e	8d 0c d4	sta $d40c	                sta SID+$0C         ; voice 2 ad
7274	.2e51	8d 13 d4	sta $d413	                sta SID+$13         ; voice 3 ad
7275	.2e54	8d 15 d4	sta $d415	                sta SID+$15         ; filter lo
7276	.2e57	a9 f0		lda #$f0	                lda #$f0
7277	.2e59	8d 06 d4	sta $d406	                sta SID+$06         ; sr
7278	.2e5c	8d 0d d4	sta $d40d	                sta SID+$0D         ; sr
7279	.2e5f	8d 14 d4	sta $d414	                sta SID+$14         ; sr
7280	.2e62	a9 01		lda #$01	                lda #$01
7281	.2e64	8d 04 d4	sta $d404	                sta SID+$04         ; ctrl
7282	.2e67	8d 0b d4	sta $d40b	                sta SID+$0B         ; ctrl
7283	.2e6a	8d 12 d4	sta $d412	                sta SID+$12         ; ctrl
7284	.2e6d	a9 10		lda #$10	                lda #$10
7285	.2e6f	8d 16 d4	sta $d416	                sta SID+$16         ; filter hi
7286	.2e72	a9 f7		lda #$f7	                lda #%11110111
7287	.2e74	8d 17 d4	sta $d417	                sta SID+$17         ; filter voices+reso
 
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2018-11-14 08:43
Mixer

Registered: Apr 2008
Posts: 287
Sudden output level change causes pop. When the filter is turned on it reverses the output signal from positive to negative initially causing it to pop, and vice versa when turning filter off.

I do not exactly know what happens when filter type is changed, it causes a pop too, perhaps there is some charge in the capacitors or something.

So, in theory, the output level should be at zero when filter is turned on/off or type changed.

Try: lda #$0f sta d418 lda #$00 sta $d418 lda #$1f sta $d418. if it makes any difference. If yes, theory is good, if no, theory is bad.:)

Then there is using filter during digi-play, which is another case altogether.
2018-11-14 10:56
Groepaz

Registered: Dec 2001
Posts: 9021
Quote:
lda #$0f sta d418 lda #$00 sta $d418 lda #$1f sta $d418

that will also cause a pop, a loud one even :)

if you are really really REALLY desperate to have no audible pop at all, you must change the volume by one every 2 or 3 frames, ie ramp it up (or down) very slowly
2018-11-14 13:33
Mixer

Registered: Apr 2008
Posts: 287
Quote: Quote:
lda #$0f sta d418 lda #$00 sta $d418 lda #$1f sta $d418

that will also cause a pop, a loud one even :)

if you are really really REALLY desperate to have no audible pop at all, you must change the volume by one every 2 or 3 frames, ie ramp it up (or down) very slowly


I did not expect that the ramp must be that slow, but I guess that makes sense. Hide the snap in the kick drum.
2018-11-14 13:48
Groepaz

Registered: Dec 2001
Posts: 9021
it must be slow so you dont get a "sample"... ie change it once per frame -> you get a 50Hz buzz
2018-11-14 16:16
Oswald

Registered: Apr 2002
Posts: 4365
I thought this topic is about SID music in POP style :)
2018-11-14 16:21
tlr

Registered: Sep 2003
Posts: 1204
Quote: I thought this topic is about SID music in POP style :)

...when it really was in involuntary glitch-style! ;)
2018-11-14 16:55
Hein

Registered: Apr 2004
Posts: 836
You can play your digi without filter, if your tune doesn't use filter at all. But that's not mentioned, so just guessing.
2018-11-15 06:23
oziphantom

Registered: Oct 2014
Posts: 249
I turned the filter off at the end of the digi, the pop didn't move, so the filter is not the issue. I will need to trace out the SID writes after the digi stops, and the make a nice test program that sets the values and see if I can track which write causes the pop.. if its not some "just so happens to happen when the some internal state is dead on X" kind of deals.
2018-11-18 16:26
Frantic

Registered: Mar 2003
Posts: 1369
If someone investigates SID pops related to the filter in more detail, I'd be very interested to hear about it.
2018-11-18 21:55
Pex Mahoney Tufvesson

Registered: Sep 2003
Posts: 38
As Mixer said, the old SID chip pops, mostly due to internal "idle" voltages not being anything near zero. So for 6581, yes, it will pop. The main source for this is that the amplification factor of the filter is negative. So, turning the filter on or off ($d418 from #$1f -> #$0f) will result in a relatively large jump in the DC-offset.

On 8580 this is way more subtle. But it's there.

I did investigate this for sample-playing purposes, and the long version of that can be read here:

https://livet.se/mahoney/c64-files/Musik_RunStop_Technical_Deta..

This has been used in quite a number of demos since then. The first one was Musik Run/Stop
and one of the latest ones was C=Bit 18

There can of course be more sources for popping sounds from the SID than (ab-)using the filter/volume register. If you do find any, please tell! / Pex
---
Have a noise night!
https://livet.se/mahoney
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