Stereo Card and Audio Digitiser for the Apple IIgs.
By Dominic Cioccarelli
Copyright (c) 1991 Apple Users' Group, Sydney
Republished from Applecations, a publication of the Apple Users' Group, Sydney, Australia.
Many people do not realize that the Apple IIgs has stereo sound output and mono audio input as standard. To access these features, all that needs to be done is to decode and filter the respective signals. The purpose of this article is to describe two constructions projects: a stereo decoder, and an audio digitiser, both for the Apple IIgs only.
A moderate knowledge of electronics will be required before attempting this project. I accept no responsibility for the damage you may do to your computer.
Stereo Decoder
The IIgs has the ability to produce not one, or two, but eight separate audio channels. Unfortunately, there is no hardware available, nor software written, which will take advantage of this feature. The possibilities of octaphonic sound are amazing. Not only would you have left/right separation, but front/rear separation and up/down separation: complete surround sound. The effect in games would be staggering.
In this construction project, though, only the construction of a stereo card will be described, due to the fact that only stereo cards are supported by most software. The stereo decoder circuit can be seen in figure 1. The circuit can be either built on a Apple protoboard (see the article "Real World Interface" from the April Applecations for details on suppliers), or can be built externally to the computer. I opted for the second option due to the cost of protoboards. If you decide on the second option, then two cables will have to be constructed, as against one with the first option.

The cable which must be constructed for both options is for connecting the stereo decoder to J25 on the GS motherboard. J25 is situated in the front right hand corner of the motherboard (the corner near the power light). J25 is the sound I/O connector for the GS, and is a standard 7 way SIL plug. Unfortunately, the only SIL sockets commonly available are 12 way (Dick Smith Cat No. P-2742). This doesn't matter, as the spare 5 pins can be simply cut off.
The number of wires in the cable which will connect to this plug varies depending on whether you are making just the stereo decoder, or the stereo decoder and the audio digitiser. If you are making just the stereo card, then you will need a 3 core cable, otherwise, you will need a four core cable.
You should connect one of the wires to pin 2, one to pin 4 and one to pin 6. If you are making the audio digitiser, then you should also connect one wire to pin 1. The order of the pins can be seen in Diagram 2 which is a picture of J25 as seen from above.
The length of the cable will depend on whether you are mounting the stereo decoder outside or inside the computer.
If it is mounted inside, then the cable will only need to be about 15-20cm long, if it is outside, then the length of the cable will depend on how far the decoder will be situated away from the computer. One word of warning though, it should be kept pretty short to reduce signal loss.
If the decoder is being mounted externally, then power will also need to be supplied to it. This can be derived from a number of sources. The power lines needed for the circuit are analog ground (ANG) (which is derived from J25 through the cable described above), +5v, -5v, and digital ground.
Unfortunately, all the power lines can't be derived from any one external connector. The closest you are going to get is the daisy chain port on the last drive in your drive chain. This connector supplies +5v (pin 6), digital ground (pin 1) but not -5v. It does, though, supply -12v (pin 5), and this can be dropped down to 5.2v by the addition of a 6.8v zener. -5v can also be obtained on pin 7 of the RGB video port. If you constructed the 8 bit I/O card, then power can be obtained from this on pins 34 (-5v), 26 (digital ground), 25 (+5v) of the Apple Bus.
If the decoder is being built onto a protoboard, then the pins for the power lines of the apple bus are given above. The wires at the other end of the cable (with the 7 way SIL socket on one end) should be soldered to the board onto which the circuit is going to be constructed. The power wires can also be connected to the board as well.
The circuit (diagram 1) can now be constructed. All ICs should be socketed. The IC sockets should be mounted first, and then the passive components (resistors, capacitors). The method of construction depends on the individual. Examples are veroboard, matrix board, a custom designed PC board, etc.
The last thing to do is to construct the input plug/cable for the stereo decoder. This is derived from the headphone socket on the back of the GS. The advantages of deriving the sound from here are twofold: the sound from the headphone socket is pre filtered, and when the input plug is connected to headphone socket, the tiny internal speaker of the GS is automatically disconnected.
The connections for this plug are shown in diagram 3. The plug is a standard stereo 3.5mm Phono plug (Dick Smith Cat No. P-1140). The connections, a & b in diagram 3 correspond to a & b on the wiring diagram, diagram 1. Thin shielded cable should be used to connect the plug to the decoder.

The left and right channel outputs can be connected to the back of a Hi-Fi amplifier. I connected them to the tuner input, though they can be connected to the AUX input if the tuner input is not free. Whenever the stereo card is to be used, the input selector on the amplifier should be switched to tuner (or AUX). If the amplifier is situated in another room, or is not available, then a pre built amp module can be used to amplify the stereo outputs to drive small speakers. These are available from many electronics stores, including Jaycar.
Audio Digitiser
The audio digitiser is particularly useful, and easy to construct. The circuit can be seen in diagram 4. This circuit does not include a low pass filter, as recommended by Apple, because I have found that a filter can sometimes cause more interference than if it is left out. If you wish to add the filter, the circuit diagram for it appears in figure 5. The filter should be placed between the audio input and the input to the digitiser.

The cable which is used to connect the digitiser to the computer to the GS is the same as that described for the stereo decoder, except that only pins 1 and 2 are needed. If you are constructing both the stereo decoder and the audio digitiser, then you will already have all the appropriate wires connected, as stated above. To connect sound to the digitiser, a 3.5mm phono plug (DS cat No. P-1132) should be used. The shield wire (ground wire) from the plug should be connected to analog ground, and the other wire goes to the audio input as seen on the wiring diagram (Diagram 4).
If you are only constructing the audio digitiser, then you will not even need a circuit board to construct the circuit on. Instead, the components can simply be wired together in a small jiffy box (DS cat No. H-2855). The +5v for the circuit can be obtained from various connectors on the back of the GS. The connections for these can be found in the back of the GS owners manual. As with the stereo decoder, it is probably easiest to get the power from the daisy chain port on the last drive in your drive chain. Note that the analog ground (ANG) is obtained from pin 2 of J25.
The 22K variable resistor is used to adjust the level of the idle state of the digitiser. Note that this is NOT a volume control. Before the circuit is ever used, the variable resistor should be adjusted so that it is half way around in a clockwise direction. This means that it is half way between +5v and ANG. This is because if the variable resistor is completely towards the +5v for an extended period of time, then there is a slight chance of damaging the Ensoniq chip, though this will not happen if the above precaution is taken.
To use the digitiser, boot up audiozap. This is a shareware digitiser program, available from the Apple II BBS. When in audiozap, select oscilloscope from one of the menus. You should see some interference, but mainly a straight line. Adjust the 22K variable resistor until the straight line appears centered. To record sound, plug the 3.5mm input plug into a walkman and press play. This is fine for recording music. To record speech, connect the input plug to the monitor socket of a tape recorder, insert a blank tape into the recorder, and press record-play. Sound from the internal microphone of the recorder will now be amplified, and automatically passed on to the digitiser. This may not occur with all recorders. If it doesn't work, record the sound onto a blank tape, and then play it into the digitiser, as you did with music.
Audiozap is not the best program to record sound with. If the sound isn't very clear, then it is not the fault of the digitiser, but the program itself, try increasing the recording speed. The digitiser program included with hyperstudio works much better.
Good luck with the projects, and if you have any problems, I can be contacted on (02) 634-6864, weekdays after 4pm, and most times on weekends. If anyone wants to buy an audio digitiser, I will be selling them for $15. Contact me on the above phone number if you are interested.
List of parts
Stereo decoder:
Quantity Description
1 74LS01 IC
1 74LS08 IC
1 4066 IC
1 TL072 IC
4 39 ohm resistors
2 100 ohm resistors
2 10K ohm resistors
2 390pf capacitors
2 180pf capacitors
2 18pf capacitors
2 10pf capacitors
1 7 way SIL plug
1 3.5mm stereo phono plug
Audio digitiser:
Quantity Description
1 4.7uf capacitor
1 22K variable resistor
1 100 ohm resistor
1 7 way SIL plug
1 3.5mm mono phono plug

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