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G4 Cube internal audio

thenebular

thenebular

macrumors newbie
Original poster
Apr 9, 2020
19
4
So I commented on a thread about Adrian's Digital Basement Cube and the lack of internal audio. There I mentioned the DAC that is on the production board. DAC 3550. I also posted about that on reddit and on a Facebook group and was led to Hap at http://www.alittlebytedifferent.com who has a prototype Cube with an internal speaker. I contacted him and got some nice high res photos of the top of his Cube motherboard with the same DAC on it. Closer inspection didn't reveal any real changes from my board to his. Different colour caps and resistors, but no real changes in values that I could see. He also told me that the speaker was connected to the 2 prong connector by the ram labelled J2. So I got to continuity testing from J2 and it lead me to a chip at a spot called U18, this chip had 04BK LM 4861M written on it, which told me that it was an LM4861 audio amplifier. So the production board still has the amp on it as well. J2 was connected directly to the output pins (8 and 5). More investigation ensues. Knowing what each pin did, I was able to confirm that GND is still connected to ground, VCC is connected to a capacitor, but I haven't tested with it powered on, so I don't know if the cap is smoothing out 5v power, but I suspect it is. Pin 4 is -IN and is connected through a resistor to -analog out on the DAC. +IN is connected to pin 2 labelled bypass, but in the sample circuits on the datasheet it was as well. Which leaves pin 1 labelled shutdown. I'm still a beginner at electronic circuitry so I'm still wrapping my head around how the shutdown works, so it's possible they either added or removed resistors to shutdown the amp. I asked Hap for photos of the the amp on his board, but as it's on the underside he may not want to go that far in disassembling his rare cube.

Anyway to me it's looking more and more that the internal audio is there, but disabled through firmware rather than actually physically disabled. Or if it is physically disabled it's only though the addition or removal of very simple components.

If anyone else has insigts, ideas, or whatever let's discuss. My photos of the chips are attached, here are the datasheets:
20200404_161257.jpg20200428_220633.jpg20200428_221442.jpg
 
So further update the I²S data input pin on the DAC is connected directly to one of the pins on the Lucent chip. I will be checking out the other I²S and I²C pins. I wish I had a scope so I could check for activity on them as well.
 
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Well, the I²C pins are connected as well which leads me to believe that the DAC is controlled from there. I got the last firmware update for the cube, but I need to know more about the initial boot post process up to the chime. I'm suspecting it turns on the DAC for the chime, then shuts it down, but I don't know much about programming let alone low level programming. I can't even get datasheets on the two main chips on the board. I can for the flash memory chip, but I have a lot of learning to do before I can even attempt to dump it.

Decompiling and deconstructing the firmware update is also a huge endeavor, but at least it's run in OS 9 so I don't have the unix HAL to deal with as well.

Seriously help would be much appreciated doing this is basically a university level project and I don't have profs or TAs to go to.
 
There seems to be only one ML4861 amplifier, which provides mono output according to the data sheet. There would need to be two of these for stereo sound. I feel that whatever audio was onboard was nothing more than for basic output similar to what's found on a powermacs or iMac G4 internal speaker. It was never meant to provide a 3.5mm output.
 
That is correct. The prototype has no 3.5mm socket, just a speaker connected to J2. However if we're hardware hacking, the DAC has an internal amp that could be used to run a headphone jack. However that's later, first goal, startup chime on J2, then general audio on J2, by then we should know enough to get the DAC to do what we want.
 
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I've purchased a PowerMac G4 internal speaker and it fits in perfectly on J2. Recently been researching Open Firmware and wondering if this can be enabled via that?
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Page 1 of G5 Open Firmware device list:
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Page 2
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PowerMac speaker
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Connects perfectly to J2
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Lots or room for it to fit
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quite possibly. There seems to be something on gpio@50 about internal audio. I don't know much about openfirmware programming, but that's a good sign. I was thinking I'd have to go into the bootstrapping code that loads OF as I never noticed that before.
 
Interesting...there's a LoBo connector that I've used to run a fan before(usually something small squeezed into the GPU cage to get some air circulation) and I THOUGHT it was J2. Looks like I might have been completely wrong in using that if indeed the case!
 
You may be thinking of the odd little connector at the bottom/end of the VRM board that is often used to power a fan.
 
Well I'm resurrecting this thread. I went back and looked at the OF screenshots and it seems the gpio50 audio entry was on the powermac G5.

The only audio entries in OF on the cube literally say pseudo.

If anyone can point me in the direction on some good resources for Apple PPC hardware hacking, or if you know the names of the engineers on the Cube I would be grateful
 
You're better off just buying a small USB sound card and storing it inside the cube. Check my thread here where I fitted RCA jacks to my cube:

RCA jacks in G4 Cube (and sprayed case)

Hey guys, In my previous thread I heavily modded my G4 Cube and sprayed it copper. I wasn't keen on the copper so today re-sprayed it gunmetal grey. After spraying the case, since I had the internals of the Cube out I decided to route the USB sound card internally, remove the outside...
68kmla.org 68kmla.org
 
This isn't about just getting audio that isn't an external dongle, this is about enabling existing hardware on the motherboard.

It has a DAC, amp, and speaker connector. They are still connected and powered and there's no way that Apple would have spent the money to put them on all the production boards if they weren't used somehow, which means there must be a way to re-enable them.

Unfortunately the resources for interacting and programming at that low a level on the PPC platform are not easy to find.


Steve030 said:
You're better off just buying a small USB sound card and storing it inside the cube. Check my thread here where I fitted RCA jacks to my cube:

RCA jacks in G4 Cube (and sprayed case)

Hey guys, In my previous thread I heavily modded my G4 Cube and sprayed it copper. I wasn't keen on the copper so today re-sprayed it gunmetal grey. After spraying the case, since I had the internals of the Cube out I decided to route the USB sound card internally, remove the outside...
68kmla.org 68kmla.org
Click to expand...
 
I realise that, but it only has one amp for the mono 'bong'. So the hardware isn't even suitable for stereo sound. You say it is connected to the Lucent chip, so it could also be used for the internal modem.

But yeah don't want to be negative about it! Just giving you a rather nice solution if your end goal was to get a decent sound solution.
 
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