PROPHET64 & MSSIAH Userforum

Jaicen · 2006-08-22 20:00:32

Hey guys, I don't know if anyone else has discovered this, or wether it's unique to my SID.
I was playing with the TB, trying to see what I can get out of it with a 6581 (answer = LOTS), and I discovered that if I turn on the EXT input with nothing plugged into it, virtually all of the annoying background hum disappears! Seriously, it's like magic! The sound appeared to be unaffected.
I tried it with MONO, but it doesn't work the same, the noise disappears as long as you use the filter in envelope mode, not in manual sweep. At full cutoff there is some background noise too, but it's better than without the EXT being enabled. I'd now happily use the Bassline for recording on my old breadbin.
Now, is this something to do with the EXT input phasing the interference?? Or does it internally re-route the signal path in the SID?? Any ideas??

Last edited by Jaicen (2006-08-22 20:26:57)

KONEY · 2006-08-22 20:48:27

Probably there's a low-pass filter ebabled. With it most of the noise disappear but you also lose lot of upper harmonics. On bass sound this is not an issue.

Anyway you can do the same easily in your mixer by cutting off completely the hi-end freq.

Jaicen · 2006-08-23 00:47:54

Cutting the highs completely would sound terrible!
It's not a low-pass filter, though it has a similar effect. As I said before, I think it's some sort of phasing, where the noise is getting into the output, inverted and fed back to the SID through the external-input, cancelling it out. It's not cancelling any of the harmonics in the signal because they're not getting to the EXT-In. I can do you a sample tomorrow to demonstrate what I mean, it's made my day!

ron · 2006-08-23 11:43:13

Recently i discovered this background noise / EXT input relation, too.

I made an external audio break out box since using the old edition, running the SID out and EXT input signals through a 470 ohm resistor and a 1µF capacitor.

Code:

 
SID out -------[R]-----||--->Phonejack
SID in  -------||-----[R]---->Phonejack

Usually the EXT in is connected to a mixer subgroup so i never realized there's a huge background noise as soon as there's no well grounded device connected to the input.

What puzzled me was i copied that circuit to be installed internally but it didnt´t work like this - connecting an external device didn't eliminate the noise so i still need my old external box or install that one internally (got cube-shaped capacitors on the old box, the new one i was using radial - that's‚ the only difference so far ).

Last edited by ron (2006-08-23 11:48:54)

Jaicen · 2006-08-23 13:06:19

The more I thought about it, the more it made sense to me last night. I'm almost positive that it has something to do with the EXT-IN and SID-OUT picking up the same interference, but when the EXT-IN is fed to the filter, it is phase inverted, so the interference signal is cancelled. It suggests to me that the clock noise is getting into the signal path from poor isolation inside the SID, rather than from control lines around it.

linde · 2006-08-24 12:33:45

It sounds like you might be onto something there, Jaicen. I know from recording that the output from the filter isn't symmetrical. That might have some relevance.

Great discovery!

D Vibe · 2007-02-04 01:04:08

Are you talking about the "noise" of that the oscillators does never really close?

On my Sidstation (haven't got the P64 yet), I use an external gate (in a compressor) to get rid of that "problem"..

Or is this some sort of noise, as in white noise, that's always in the background?

Just curious..

/Daniel

Last edited by c-frog (2007-02-04 01:05:23)

jbuonacc · 2007-02-21 02:31:47

any update or more talk about this? very interesting...

though i did blow a SID the other night due to the EXT input.

D Vibe · 2007-02-23 17:38:55

jbuonacc wrote:
any update or more talk about this? very interesting...

though i did blow a SID the other night due to the EXT input.

Oops..

opkod · 2007-10-17 16:14:50

is the noise is like this one?

http://les.acharnes.free.fr/prophet64/VID0001.AVI

a huge noise appears when the gui window is open....

wsr · 2007-11-06 00:49:13

For monosynth, it helps me to turn off EXT there too, but you can remove some additional noise by pressing space (?) which will blank out the screen, also works in a similar fashion in bassline.

PROPHET64 & MSSIAH Userforum

#1 2006-08-22 20:00:32

Background Noise (Odd Discovery)

#2 2006-08-22 20:48:27

Re: Background Noise (Odd Discovery)

#3 2006-08-23 00:47:54

Re: Background Noise (Odd Discovery)

#4 2006-08-23 11:43:13

Re: Background Noise (Odd Discovery)

Code:

#5 2006-08-23 13:06:19

Re: Background Noise (Odd Discovery)

#6 2006-08-24 12:33:45

Re: Background Noise (Odd Discovery)

#7 2007-02-04 01:04:08

Re: Background Noise (Odd Discovery)

#8 2007-02-21 02:31:47

Re: Background Noise (Odd Discovery)

#9 2007-02-23 17:38:55

Re: Background Noise (Odd Discovery)

jbuonacc wrote:

#10 2007-10-17 16:14:50

Re: Background Noise (Odd Discovery)

#11 2007-11-06 00:49:13

Re: Background Noise (Odd Discovery)

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