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Forums > CSDb Discussions > reduce the audio-noise
2007-02-09 19:06
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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2009-03-04 15:25
Soren

Registered: Dec 2001
Posts: 546
I did bend that one leg on my sid chip, which gave me quite an improvement... along with $d011,0 and $d020,0, this should be perfect enough for me, when recording sidtunes or sid sounds :-)
2009-03-04 16:40
Geir Tjelta

Registered: Sep 2008
Posts: 304
Quote: I did bend that one leg on my sid chip, which gave me quite an improvement... along with $d011,0 and $d020,0, this should be perfect enough for me, when recording sidtunes or sid sounds :-)

Hello Jeff.. OK, maybe I'll sacrifice one chip, though. :-)
2009-03-04 20:05
Soren

Registered: Dec 2001
Posts: 546
Just do it carefully, then I can't see what could go wrong. Do ofcourse make sure that the bended leg doesn't touch any of the other legs ;-)
2009-03-05 07:47
Devia

Registered: Oct 2004
Posts: 395
As I already wrote a couple of posts up (#53), lifting the EXTIN pin will add a low frequency noice.

2009-03-05 13:19
ready.

Registered: Feb 2003
Posts: 426
what about removing the capacitor connected to ext in? this will also leave the ext in floating. This might be useful for the C64 versions with soldered SID to the motherboard.
2009-03-05 14:22
Geir Tjelta

Registered: Sep 2008
Posts: 304
Quote: As I already wrote a couple of posts up (#53), lifting the EXTIN pin will add a low frequency noice.



That's what I was afraid of. Just removing/bending a pin will leave other issues, like grounding. I get rather good quality with the audio cable I've made for the mixer, so I don't get any problems with lots of noise. So the trick by filtering the ext.in works good aswell. Remember, that with tunes not using filters, you can turn ext. in off (lda #8, sta $d417).
2010-06-11 03:47
clonK

Registered: Aug 2008
Posts: 63
Anyone have any solid answer for getting rid of the noise (15550hz -> 15763hz) on an ASSY 250407 PAL?
This is a breadbin with a 6581.

Can I tie the audio-in to ground inside the machine without having to use a cap and resistor? People keep saying you need to put the cap and resistor in between, but then other people say the SID is already protected by a cap and resistor directly on the board, and then other people say that it is only needed if you have 2 SID's and that's only for the second SID! arggg.... I'm going round and round and round in circles!

The bigmech mod only is only for 8580 c64's.

I did have a video out cable where I grounded audio in, but I've since scrapped that (fell apart) and started using a new cable for LCA. I want to do the audio in->ground connection inside the 64 and I want to do it correctly.

Do I put a cap and resistor between audio in and ground for a breadbin, 6581, 1 SID noise reduction or not? Disabling the audio in for the moment will be fine.

I've trawled the Prophet64 forum and googled but still no solid answer is appearing.

Cheers
2010-06-11 08:01
JackAsser

Registered: Jun 2002
Posts: 1483
Quote: Anyone have any solid answer for getting rid of the noise (15550hz -> 15763hz) on an ASSY 250407 PAL?
This is a breadbin with a 6581.

Can I tie the audio-in to ground inside the machine without having to use a cap and resistor? People keep saying you need to put the cap and resistor in between, but then other people say the SID is already protected by a cap and resistor directly on the board, and then other people say that it is only needed if you have 2 SID's and that's only for the second SID! arggg.... I'm going round and round and round in circles!

The bigmech mod only is only for 8580 c64's.

I did have a video out cable where I grounded audio in, but I've since scrapped that (fell apart) and started using a new cable for LCA. I want to do the audio in->ground connection inside the 64 and I want to do it correctly.

Do I put a cap and resistor between audio in and ground for a breadbin, 6581, 1 SID noise reduction or not? Disabling the audio in for the moment will be fine.

I've trawled the Prophet64 forum and googled but still no solid answer is appearing.

Cheers


I can only say what I did and it worked for me. The noice you hear is the horizontal sync from the video signal. What I did was shielding the chroma and luma lines from the audio line as good as I could. I.e. by cutting with a scalpel to make better distance. Applying copper tape where needed etc. Then ofcourse only use Y/C output + audio and not the composite or RF output. IIRC I also think I copper taped the whole VIC-cage.
2010-06-11 21:09
clonK

Registered: Aug 2008
Posts: 63
Quote: I can only say what I did and it worked for me. The noice you hear is the horizontal sync from the video signal. What I did was shielding the chroma and luma lines from the audio line as good as I could. I.e. by cutting with a scalpel to make better distance. Applying copper tape where needed etc. Then ofcourse only use Y/C output + audio and not the composite or RF output. IIRC I also think I copper taped the whole VIC-cage.

I don't get what you mean by "cutting with a scalpel to make better distance".
I also have a cable I built that just carries Luma and chroma.
Now I want to tap the audio out from the pcb and possibly ground the audio in.
I'm having a hard time locating the right spots on the pcb too actually... guess I should try and find schematics.
The bigmech mods don't correspond to the 6581 breadbin motherboard.
Anyone else done this?
2011-12-18 23:35
Hermit

Registered: May 2008
Posts: 206
Hi

Once with one of my C64s I tried everything what is written here & there (grounding,shielding,whatsoever), but I didn't feel significant change in the SID noise level. Somehow it's still a mystery.

I even changed buffer-capacitors and injected some LC-filters to the PCB wires to SID +5V and +9V power pins, that didn't help a lot either. However the noise is clearly in connection with VIC screen signal, I have the assumption that the big noise is coming from or caused indirectly by the digital part of the chip,as no workaround on the analog part helped.
You know, it's a hybrid chip which generates analog signal out of digital counters. I think somewhere the digital signal is not clear enough, and it is felt when it's converted to analog signal...maybe I'm wrong but:
Recently I bought an oscilloscope and measured signals on C64 chips. I can tell you, in places the cock/data/address lines are so heavily used that the measured signals are far from perfect 'pulse/square' shape. They're yet enough to work well, but wire capacitance and bus-load deforms some signals significantly.
And the most surprising measure was on leg 6 of SID,which is the rude clock signal from CPU leg 39. Tapping on these legs with oscilloscope, the SID-noise changed SIGNIFICANTLY however there was no change in screen content. The noise got more even, with a bit more high frequencies IIRC. (The scope might have enough capacitance (<10pF) to alter the clock signal?!Or there's a ground loop in PCB / my scope wire works as antenna? my imagination goes too far :) Maybe the poor clock-signal from CPU to SID has to do something with the legendary noise and a clock-driver could make the situation better...maybe...

On the other hand I agree this contributes to the lively sound which we got used to, and I don't think I'd further disturb C64s in search of the hardware noise...

CHEER$
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