Apple ][ Card Audio Characteristics

[phipli]

Good afternoon folks!

Does anyone have...
1. An LC family Macintosh like the LC 475 or older...
2. An Apple II LC PDS card
3. An oscilloscope
4. Some spare time!

I'm curious to find out about the electrical characteristics of the audio output from the card. It comes out of the card on Pin A1, right in the corner. The following is an ethernet card, but shows the pin in question on the underside. If you're looking from above, on the solder side, it would be in the far back left of a Pizzabox LC.

If you could let me see an image of the waveform shape when the IIe card is playing audio, and give me an idea of what the max and minimum voltages are?

My expectation is that it is some kind of PWM or PCM sound, between 0 and 5v, but I'm curious to know, as I'd like to re-use that pin to inject audio into an LC for other purposes.

The LC Dev Note says the following :

Cheers!

 

[KennyPowers]

I have all of those things but #4 at the moment. However, the excerpt you posted from the LC Dev Note suggests to me that it works in the same rudimentary way that sound works on an Apple II. On an Apple II, each time you access memory location 0xC030, the position of the speaker is inverted. That's it. By controlling the frequency of your accesses to 0xC030, you can cause the speaker to emit different pitches. I suspect toggling pin A1 high and low is analogous to inverting the speaker position on a real Apple II. Maybe I'll find the time to set all that stuff up and see what it looks like on a scope, but my hunch is it'll toggle between 0v and 5v just like the speaker on an Apple II would move in and out.

 

[joevt]

Apple II sound is 1-bit but it's not just a square wave. There's same oscilloscope captures in this video:

 

[phipli]

I have all of those things but #4 at the moment.

Isn't it always the way - I suspect if I had more free time at the moment I'd probably use it to sleep!

but my hunch is it'll toggle between 0v and 5v just like the speaker on an Apple II would move in and out.

Yes, as per my original post. I have the option of potentially feeding in a similar 3.3v logic signal, but I don't own an Apple II card, so before I cause harm due to an oversight, it is always good to validate what the documents say

Apple II sound is 1-bit but it's not just a square wave. There's same oscilloscope captures in this video:

I mean, the same is true of any digital logic to varying degrees - it is an analogue world, and there is at least two decoupling caps in the signal path and resistors to rails (on the LC 475 specifically) acting as a filter / smoothing. Everything is trying to knock the corners off when you try to be digital in the analogue domain.
Interesting video - the individual is trying to synthesis unusual waveforms intentionally though. I'd be more interested in the characteristics of a "beep".

 

[NJRoadfan]

What does the circuit look like between the PDS slot pin and the audio output? Its likely it attenuates the 5V TTL signal down to line level before mixing it into the main audio output.

 

[phipli]

What does the circuit look like between the PDS slot pin and the audio output? Its likely it attenuates the 5V TTL signal down to line level before mixing it into the main audio output.

Pretty basic - it mirrors the signal coming out of the DFAC for the host's sound on the LC and LC II. Blue is straight out of the DFAC, red is from the LC-PDS. Note after the highlighting ends, they're common. Other than the decoupling and resistor not much going on, not exactly fancy mixing.

 

[NJRoadfan]

Other than R899, which appears to be a pull up resistor. That may be a show stopper from using this pin as-is.

 

[phipli]

Other than R899, which appears to be a pull up resistor. That may be a show stopper from using this pin as-is.

I appreciate the thoughts, but there are still several options, that isn't a show stopper. I could for example, remove R899, or, as I am most likely going to, I could feed in a signal that had similar characteristics to the Apple II card.

That is why I'm interested in seeing the actual characteristics of the output signal from the Apple II card, I don't have one to test.

 

[NJRoadfan]

The speaker jack on a real Apple II has +5v on one pin and the other pin is the sound output from TTL logic routed thru a bunch of passives. I don't know what the IIe card's output looks like.