19.008 What is required to build a Sound Input board for my IIgs?
Description
This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
19.008 What is required to build a Sound Input board for my IIgs?
I found the following circuit on ground (as text) in the file digitizer.circuit
at ...
ftp://ground.ecn.uiowa.edu/apple2/MiscInfo/Hardware/
[quote]
If you don't mind putting together a few parts, you can build your own adapter,
though (explanation follows):
GS
Analog 10-500 uF
In ---------------+------------+----+------------||------ >>
| | |
-+- -+- + To CD player,
5V Zener /_\ Signal /_\ = 1.5V microphone,
diode | Diode | - Battery etc.
GS | | |
Ground -------------+------------+----+-------------------- >>
[end quote]
I changed it to this:
GS
Analog 100 uF
In ------------+-----------+----+----------+-------||----- >>
| | Z +
-+- -+- +------+ Z 10K To CD player,
5V Zener /_\ Signal /_\ = 3V Z Z microphone,
diode | Diode | - Bat Z<--+ etc.
GS | | | Z 10K pot
Ground ----------+-----------+----+------+------------------ >>
The Ensoniq is designed to handle 0 to 2.5V input, but audio sources usually
swing more or less equally +/- about zero. Therefore we need a level shifter,
to put the appropriate DC bias onto the input. The 3V battery and the 10K pot
are for this. To stop the low internal impedence of the battery effectively
shorting the sound source (which happens with the first circuit), I've included
another 10K resistor.
The easy way to set this up is to use a program like AudioZap and with the CD
etc. end input shorted, set the centreline of the oscilloscope display to be
halfway up the screen, so the input signal will swing equally either side of
this reference voltage. The zener diode is to clip the top of spikes to limit
them to about 5V and the signal diode is to clip any negative going signal to -
0.6V. This is protect the Ensoniq chip from overload.
The capacitor keeps the DC out of the source. Pin one on the GS connector is
closest to the front of the computer (ie with the expansion slots at the
back.
The Ensoniq has a fairly low input impedence (about 3-5K), but most portable
cassette or CD players should be able to handle this, since most headphones
have a much lower impedence than this (usually around 50 to 100 ohms, even as
low as eight for old ones). You can use a tape deck as an amplifier and
"impedence buffer" for a microphone.
If you can get the file http://cassius.ee.su.oz.au/~adrianw/gsaudioin.bsq
there's proof there that it all works (a raw sound file I made).
Related FAQs Resources: R027MOCKBD.DOC (DOC file)
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