From: adrianw@cassius.ee.usyd.EDU.AU (Adrian Whichello) Newsgroups: comp.sys.apple2 Subject: Sound input to the 2gs [long] Date: 13 Jan 1997 22:53:35 GMT Organization: Department of Electrical Engineering, Sydney University Message-ID: <5beedf$o9r@metro.ucc.su.OZ.AU> Hi everyone, I've been mucking around with getting sounds into the 2gs lately, and thought I'd share the results, so far. Ages ago David Empsom wrote about using the internal ADC (to comp.sys.apple2): > It is the 7-pin molex connector in the lower right corner of the > motherboard, next to the Ensoniq. All stereo cards plug into this > connector. > > Most of the pins are used for sound output and associated demultiplexing. > Pin 1 is the sound input and pin 2 is analog ground. (Pin 1 is the one > closest to the back of the computer, right next to the legend "J25" on the > ROM 1 motherboard.) NB^^^^^^^^^^^^^^^^^^ THIS IS BACKWARDS!!! Pin 1 is closest the FRONT of the 2gs, ie close to the green power on light. > > The sound input expects a signal which has a maximum range of 2.5V peak to > peak. The input impedance is 3000 ohms. > > I'm not all that clear on audio impedances and voltages, so I'm not sure > if it is safe to connect anything directly to this pin. It may be > necessary to use a simple amplifier circuit to handle impedance matching. > > Can anyone elaborate on this? I found the following circuit on ground (in ftp://ground.isca.uiowa.edu/apple2/MiscInfo/Hardware/digitizer.circuit [quote] If you don't mind putting together a few parts, you can build your own adapter, though (explanation follows): GS Analog In ---------------+------------+----+----------||------ >> | | | 10-500 microfarad -+- -+- + capacitor To CD player, 5V Zener /_\ Signal /_\ = 1.5V Battery microphone, diode | Diode | - etc. GS | | | Ground -------------+------------+----+------------------ >> [end quote] but I changed it to this (a better ascii rendition of the circuit is available!): GS Analog 100 uF In ---------------+------------+----+---------+--------||------ >> | | < + -+- -+- +-----+ > 10K To CD player, 5V Zener /_\ Signal /_\ = 3V < < microphone, diode | Diode | - ><-+ etc. GS | | | < 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). Adrian. ps does anyone else hate the tabs that open the 2gs case as much as I do? I always hurt my fingers trying to push them in enough to get the lid off. -- Dr Adrian Whichello Phone: +61 2 9351 4824 Imaging Science and Engineering Laboratory Fax: +61 2 9351 3847 Sydney University Electrical Engineering Email: adrianw@ee.usyd.edu.au Australia WWW: http://www.ee.usyd.edu.au/~adrianw "I wish to God these calculations had been executed by steam!" - C. Babbage