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Dr. 8 Bit
Account closed
Registered: Sep 2006
Posts: 25 |
reduce the audio-noise
How can I reduce the audio-noise from c64?? some ideas?? a capcitor...mmm.. |
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MagerValp
Registered: Dec 2001
Posts: 1055 |
Noise reduction samples:
http://www.bigmech.com/misc/c64mods/noisereduction.html
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Devia
Registered: Oct 2004
Posts: 401 |
Yes, I've seen that.. which is why I'm a bit puzzled. But maybe there's some good explanation, like the AIN pin not beeing connected on the c64 I tested or some weird shit like that.. I'll have to do some more testing ;-) |
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Frantic
Registered: Mar 2003
Posts: 1627 |
@Devia: I'd be curious to hear about your findings. Let us know. |
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Devia
Registered: Oct 2004
Posts: 401 |
It's a load of crap!
Grounding pin 26 (EXT IN) on the SID is useless. Especially grounding it in the AV plug is down right stupid, since it's already pulled down via a 100k res.
Only the original c64 schematic from the Service Manual (251138) does not have EXTIN pulled down. And NOBODY actually has a machine buildt after that schematic.
Now, there is a 100nF cap serially connected on the EXTIN line and the pull down is like I said on the EXTERNAL side and not the SID side. This means that the EXTIN pin on the SID actually is not pulled down. It seems to be internally pulled up though (4.8V on my test subject).
Grounding the pin on the SID side of the cap via a lower resistance (I tested with 330 Ohm), theoretically should produce less noise. This is however inaudible on my test speaker with the high pitch sound simply getting all the attention.
If I lift the EXTIN pin, so that it is not connected, a low frequency audible noise gets added. This goes away if I ground the now not connected pin.
Although I might be able to produce reduced noise levels by giving EXTIN a more solid (lower resistance) ground, this is useless if I do not apply other methods for eliminating the characteristic high pitch sound.
I currently have no means of sampling anything, so actual audible and visual examples cannot be given.
I am very curious as to how the fuck AlphaA managed to produce such a convincing result!?
oh.. and I tested on 2 breadbins of different PCB layout and a C64c.
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Devia
Registered: Oct 2004
Posts: 401 |
@JackAsser: Go spank your boss and make him tell us what kind of trickery he played on your c64. What kind of filter did he add to what? And what did he do to the EXTIN pin?
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Frantic
Registered: Mar 2003
Posts: 1627 |
@Devia: In case the hight pitched noise (which I understood as originating in the h-sync-business rather than EXTIN) is what is most audible on your machine, perhaps the EXTIN-line simply doesn't pick up much noise in your room and hence; no audible difference? |
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Devia
Registered: Oct 2004
Posts: 401 |
Yes, well.. that was sort of what I was trying to say.. It may very well be that providing a low resistance ground on the EXTIN pin will reduce noise considerably, but if it's inaudible, it's not of much value.
I still need to test this while applying other means of reducing the high pitch sound. I would like to try out whatever JackAsser did along with lda#0;sta$d011;sta$d020 to get a more complete picture of which method actually does what. Doing this on old and new c64 and c128 while sampling and documenting is a bit of work, and I'd like to avoid having to set up everything several times to test out some other means, which then would be out of combination with other methods...
I want to build a test matrix first, so to say ,-) |
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MagerValp
Registered: Dec 2001
Posts: 1055 |
Mogwai used a 100 Ohm resistor on EXTIN, iirc, and on that machine it made an audible difference. Unfortunately I don't remember whos computer it was, or which model. Grounding the A/V plug is just referenced in that mod article, and he does a lot of other mods that affect the result.
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WVL
Registered: Mar 2002
Posts: 886 |
Actually, while I was fixing my c64 I thought I'd check out where what is in my old 1983 model c64 (artwork 251137 rev B).
This is a c64 with ceramic vic and most other chips also socketed, and I couldnt find any schematics or mods readily available for this exact motherboard.
On this model, pin 26 (ext-in) of the sid is connected to the + side of capacitor C12, a 10nF (? ;) ceramic disk capacitor (I wonder why they put + and - on the board?). The other side of the capacitor goes directly to the AV plug.
Pin 26 is at +4.75V from ground. So grounding it directly does not sound like a great idea ;)
I'm guessing the best way to avoid any noise coming in is to hook the - side of C12 to ground. Or maybe you can hook EXT-in directly to ground via a big enough resistor. But then I'm worrying how much current the SID can handle?
Let's say 1mA max, which gives R=V/I=4.7/1e-3=4700 Ohms.
Any comments?
edit - while I'm at it -> what's up with R38 that C= needed to put 2 resistors in paralllel to get a such specific value? |
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SIDWAVE
Account closed
Registered: Apr 2002
Posts: 2238 |
so there is still no working solution that is safe for the sid ? |
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