Using Wave Blaster Port instead of DIN or DB-15 Game MIDI connectors

[K-rnivoro]

I believe it's a much more elegant solucion for a MT32-Pi installed inside the computer (in a 5 1/4" bay). Almost any ISA Sound Card has that connector.

AFAIK, the WaveBlaster (https://en.wikipedia.org/wiki/Creative_Wave_Blaster) manages TTL levels for the MIDI-IN Pin 4. I'm not sure if the same circuit with the H11L1 opto-isolator from the Wiki would work connected to Pin 4 and GND of a WaveBlaster. Any thoughts?

[dwhinham]

A few people have already successfully done this, though it'd be nice if someone could contribute a schematic for the wiki; I personally haven't tried DIYing it.

You can ignore the H11L1 circuit for this purpose; that is for connecting to current-loop MIDI devices that use the DIN connector.

The Wave Blaster header gives you 5V TTL MIDI, which is fine for connecting to the Raspberry Pi GPIO, BUT you must step it down to 3.3V otherwise you risk permanently damaging the Pi. You can use a logic level shifter for this purpose (e.g. https://www.sparkfun.com/products/12009, but many more are available) or you could probably make a voltage divider using two resistors with the correct values.

For audio, I believe it's just line-level, so you could adapt the Pi headphone jack to the the L/R inputs on the Wave Blaster header, but the audio quality will be poor as we all know. A DAC module such as the one described in the wiki would give you better quality.

FYI, the WP-32 McCake will be available for purchase very soon (https://www.vogons.org/viewtopic.php?f=62&t=77676), which is a ready-made Wave Blaster card based around the Compute Module 4 that can run mt32-pi. I have been working with the creator on this device and it will be supported in the next version of mt32-pi (v0.10.0).

[K-rnivoro]

Wow!! Very cool project!!! A Compute Module 4 makes the project much more affordable! Did you have to use a step-down voltage converter (from 12V to 5V) to power the CM4 or the 5V rail of a PC is just enough?
Regarding your suggestion of using a level shifter instead of an opto-isolator: You're right, it seems easier but you wouldn't be isolating the GPIO of the PI. Are you managing it with a level-shifter in that awesome project?

[dwhinham]

It's not my project; it's created by Serdaco (aka dreamblaster on the VOGONS forum) - you should ask him for more precise information.

For powering it, my understanding is that a sound card may not be able to provide enough current for the CM4, so a floppy drive-style Molex connector is provided for powering it externally. But if there is enough current, it does have a step-down from 12V to 5V.

I don't think there is any need for isolation of the connection of the sound card to the Pi's GPIO UART pins, you can just tie them together via a level shifter, and I think this is what the McCake is doing. The isolation in a traditional MIDI circuit is for safety and protection of the devices because you have external cabling that could become damaged through wear and tear - in this situation it's a closed system so you can skip all of this. Though Serdaco could probably confirm this better as he is the designer.

[K-rnivoro]

First of all, thanks for you quick response.
Yeah, you're right. There is no need for isolation inside a PC Case, mostly if you are using the same power source for everything (the PC PS itself).
Regarding the current for the Pi (or CM4), I'm pretty sure the sound card cannot deliver enough juice and the Molex is mandatory, I was just asking if the +5VDC rail of the PC Power Supply has enough voltage and current, or a 12V to 5V step-down is also mandatory. Correct me if I'm wrong but the Raspberries (at least the "classic" ones) are very picky with the input voltage, anything less than 5VDC and you get the annoying lighting bolt in the screen.
I'm trying to identify a step-down converter in the Serdaco board, but I do not have the knowledge to get it.
Thanks.

[dwhinham]

I was just asking if the +5VDC rail of the PC Power Supply has enough voltage and current, or a 12V to 5V step-down is also mandatory.

Looking at my McCake, it uses both 12V and 5V. You can set jumpers so that it gets both of these either from the Wave Blaster header or the Molex. I'm not sure what components each of these rails are powering though and how they are regulated down to the 5V and 3.3V required by the CM4 and DAC chip, you'd have to ask Serdaco.

Correct me if I'm wrong but the Raspberries (at least the "classic" ones) are very picky with the input voltage, anything less than 5VDC and you get the annoying lighting bolt in the screen.

Yes - the threshold is around ~4.8V. With mt32-pi, there is an LCD error instead of the lightning bolt (obviously we don't use HDMI) and the CPU will throttle. You can enable a config.txt option to ignore the low voltage (avoid_warnings) and disable the throttling, and then the Pi will work fine as long as the voltage doesn't drop too much lower. We have to do this on MiSTer because of limitations of the DE-10 Nano's 5V rail.