Re: Adding a SID, VIC-20 & others

From: Levente Harsfalvi (
Date: 1998-01-03 13:29:52

>>> This is from memory, and may be completely wrong, but don't you
>>> have to set a bit in some register to enable EXT IN?
> Levente> No. Only, if you wanna filter the signal in EXT IN. Else, SID
> Levente> passes it completely to Audio out.
> Unfortunately the master volume does affect the output of the second
> SID. :/

Aaargh... :-( Yes, it does, of course. You're right. I missed it.

> To quote the PRG:
> EXT IN: "...The input impedance of this pin is on the order of 100
> kohms. Any signal applied directly to the pin should ride at a DC
> level of 6 volts and should not exceed 3 volts p-p. In order to
> prevent any interference caused by DC level differences, external
> signal should be AC-coupled to EXT IN by an elctrolytic capacitor in
> the 1-10 uF range.

> AUDIO OUT: "...The output level is set by the output volume control
> and reaches a maximum 2 volts p-p at a DC level of 6 volts. A source
> resistor from AUDIO OUT to ground is required for proper operation.
> The recommended resistance is 1 kohm for a standard output impedance."

Well it seems to confirm what I expected. Also, thank for quoting this (
helped me to figure out what happened to that 8580 I mentioned).

I don't know if you used any capacitors. But if not, all you should do is to
insert the appropriate capacitor between the two Sid's.

Second thing: as I look the C64 C sch diag, it's written there is no need
to use source resistor for 8580. It's indeed, needed for 6581. (You didn't
mention which Sid do you have).

AC coupling: you separate the signal's DC and AC parts. Or rather, you get
rid of the DC offset of the signal. Capacitors are ideal for this purpose, as
they act as 'open' for constant signals and 'short' for transitions.

Speaking of the Sid, the manual supposes your EXT IN input to have 6V dc
offset, or not to have offset at all (it causes problems, like my 8580 has
shown). So, again, I guess your SID's problem can be solved with that small

BTW I remember I've read how to add a second Sid to the computer. The doc was
on But that was on 'stereo Sid', so it doesn't mention these
problems with ext in. The doc even supposed not to connect these pins at all.
And else: why do you want to daisy-chain the Sid's? Do you want to filter
the first's sound with the second, or similar? (Else, I guess it would be
easier by simply adding the two output signal voltages at the end, and then
your Sid's won't affect each other, maybe, either with less noise. Or, maybe,
stereo sound is even better...).


 Yikes! Vic-20 rulez :-)

  BTW guys, I just have some of 3-4 megs of Vic-20 stuffs on my hdd. These were
I guess too collected by Ward and Paul.

  Having the docs and the covers is even better. ...Guys if you have some tons
of docs waiting for scanning (ie. it's worth to pass the whole beast to the
post), our department has some Hp Scanjet 3c's (600dpi color) so let me know
if I can help ya in scanning (or anything).

  Hmmm, btw this is the same idea as mine (except, mine was about Plus/4).
..Damn, I still haven't started the quest. Also, I guess the Plus/4 stuffs
should occupy a bit more space than these for Vic-20 :-)

[C-64 AEC signal]

  Marko, you may remember, some years ago we discussed how and why could the
Plus/4 crash when a routine fiddles with $ff06 (...same as $d011 on C64). I
supposed it was failing memory-refresh cycles, but you suspected that TED
(and VIC) pulls AEC low too early, and some of the faster ram chips may be
able to catch the wrong data flying on the bus. All this happens, of course,
if the program forces dma cycles by the videochip's registers.

  Well, I can confirm, you were right!

  I got a broken c64c which I fixed yesterday. One of the 41464 drams was
burnt, so I had to replace it. But I had no 41464 so I decided to take a
514256 with a socket and to do some 3d-net wiring :-). This was a 70 nsec dram.
After assembling the parts it worked on the first try. But as I tested it with
several demos, it crashed sometimes. Interestingly, it only happened when it
started to play with FLI, dma delay and similar techs. ...I kept on thinking,
then tomorrow I seemed to remember this discussion. ...After that, I took
a coil and inserted into the Aec line (between the VIC's pin and the board.
It seems, the delay I got was enough. No demos crashed since that (even if I
tried some of Crest's demos, which I guess contain more than enough d011

  BTW the coil is a 2.2uH one (it was from another broken c64 board). ...
Maybe, it's too, a good occassion to anyone experiencing such problems to find
the solution.


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