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From: TLC (all posts)
Date: 2000-12-28
Subject: Re: SID CARD

Hi!

Christian Johannesdotter wrote:
>
> Hi guyz,
> Merry X-mas!
> Levente: There are some games which play the digis via SID!
> Those 1551conversions I made a couple of years ago included also
> SID support... If there was a DIGI (like in the hi score table)
> I "converted"that routy aswell.
> Still, I wouldn't wanna miss some SID + DIGI tunes played on SID
> only. If played on TED you always have to adjust the Volume :(
> Plus... I still have that unreleased (of course :) ) SID+DIGI
> AND TED DIGI Tune... :)

:-(

> I don't know much about hardware but I think SOLDER included
> that thing on purpose... to make it play digis... If I remember
> correctly that was a common hardware fix for new SID c64s
> published in 64er etc.

Yes, it should be something like that (people used to fix digi problems on 8580s by pulling the SID extin line low (with an about 100k resistor?...), to put a constant negative voltage offset onto the SID mixer).

I've never hacked a real C64-C such way to play digis, so I don't know how it sounds 'originally'. ...I just know, the sound of my SID card
(...the sound of _both of my SID cards, since I also had an earlier version, before it was stolen) have always been suspicious just from the first point.

Anyway, do you (still) have a SID-card? If so, please do a test.
Download the SIDwinder package for Plus/4, load it, and load 'Radiation'
(one of Taki's demosongs). Play it. ...Notice the weird distorted sound when the bassdrum comes in (in the 16th tick and on). Well, this is just one (but very well representing) example. If you can, temporarily disconnect the mentioned pin from GND (or what's easier, lift the SID out of the socket, bend pin 26 (EXTIN) out carefully, and re-insert the chip so that pin26 gets outside the socket). Turn everything on, and re-check the above tune. ...I'm sure you'll notice the difference.

As it seems, after removing the resistor (or disconnecting the pin) even setting high output volume with the trimmer shows no strong distortion
(as I previously noticed). The final interfacing stage gets probably also nonlinear when fed by the above DC offset.

One could also try something: In the real 6581, the DC offset on the mixer is opposite (positive). Maybe, if the EXTIN was rather pulled to the up instead of GND (by a few tenth volts), it would cause less distortion than the current pull-down solution (I don't know, since I haven't tested, but it is so in the 'real thing'). Nevertheless, one should still notice some distortions, since the SID 'op-amps' are very non-linear for the outside-of-the-middle signal area.

I also wanna wish a Happy New Year to you (and everyone on this cool list)!

With best regards,

L.