Re: Adding a SID

From: MagerValp (MagerValp_at_Goth.Org)
Date: 1998-01-02 11:35:21

>>>>> "Levente" == Levente Harsfalvi <> writes:

MV> chain SIDs by connecting the AUDIO OUT straight to EXT IN
MV> (strangely enough the C128 PRG doesn't mention this...) but that
MV> didn't work for me...

>> 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. :/

Levente> But else, I see there are at least some DC couplers in the
Levente> C64, placed to the EXT IN and to the output lines. I won't
Levente> beg, but the problem should have been some kind of dc offset
Levente> problem.

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. As the direct audio path (FILTEX=0) has unity gain,
EXT IN can be used to mix outputs of many SID chips by daisy-chaining.
The number of chips that can be chained in this manner is determined
by the amount of noise and distortion allowable at the final output."

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."

After cranking up the volume a bit on my monitor i noticed that the
SIDs were indeed producing sound, but with an audible 50-Hz-ish hum.
As I haven't put a suitable capacitor on EXT IN this isn't all too
strange... What does AC-coupling mean? Sticking a capacitor between
EXT IN and Vdd?

Levente> (If you, guys are sure that SID's can be connected by AUDIO
Levente> OUT and EXT IN without any extra parts then I guess I was
Levente> wrong.

Well, the C64 Programmers Reference Guide does mention that this is
indeed possible, though it doesn't provide very many details.

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