Bluetooth Audio Exploration

I have one of these laying around that I plan to take apart and mess around with.

It’s a bluetooth 5.2 audio transmitter with Qualcom low-latency (I’m assuming Aptx LL, considering it’s marketed towards gaming, specifically with the Nintendo Switch). I’m making an assumption that it’s similar to some other usb-based bluetooth audio transmitters.

These don’t have any auxiliary port for any audio jacks so I’m taking a guess that these accept a digital audio input.

I’m not too well-versed in sound engineering so I have a few questions to get the lay of the land:

  1. What kind of audio signal is coming out of the Proffieboard’s Spkr -/+? Analog? Digital? From what I’ve seen so far throughout this forum, it sounds like it’s analog since there’s talks of wiring it to an audio jack from other threads.

  2. Is there an output pad for digital audio? If not, can one be programmed accordingly? If so, what kind of digital audio formats are available?

At the most basic generic level, I’m thinking of the following:

  • these don’t have their own battery and pull power from the Nintendo Switch. Therefore, it’s possible to setup a connection for battery power simply by connecting proffie’s Batt -/+ to the corresponding pads on a USB C female port. (I’m leaning towards proffie’s 3.3V pad).
  • for audio, simply find the right pads of the USB C female port to connect to for digital audio output from proffieboard.
  • plug in this little transmitter via USB C and test if it works.
    — If it does, next step is modifying its shape to fit within the saber, and we’ll figure out dealing with range and interference later.
    — If it does not, then rule out pads one-by-one, both on Proffie and on the USB C female port.

When I get the chance, I’m going to try to figure out what’s what on that transmitter and just “stick” things together to see what happens on a testing rig (I’ll have to figure that one out).

If anyone has any details I should be aware of, please share. Keep in mind: I’m not an audio engineer nor a programmer or have any experience with anything sound/circuitry-related.

I just know how to solder (with soldering iron and flux) and have a general idea of what’s happening logically.

  1. The speaker signal is a ~400kHz PWM signal which gets smoothed out in the speaker to an analog signal. It can often be hooked up directly to an audio input, but it only works if there is a filter somewhere. Also, the signal level has to be adjusted, so you need a few resistors to make it work.
  2. Proffieboards can generate I2S or S/PDIF output, which is digital audio, but probably not what you need to drive this particular gizmo.

So, USB-C has two (or more?) ways to hook up audio output: The first one is to use analog audio over two USB pins in “alternative mode”. The other one is to implement an audio class usb device and receive digital audio data over the USB connection. I don’ t know if this gizmo uses the analog mode or the digital mode though. The analog mode might be relatively easy to get to work, the digital one would be fairly difficult.

I see. Then my first step is to figure out if this little gadget reads in analog or in digital mode.

Here’s hoping I get a response from the company :slightly_smiling_face:.

The amazon page says the communication is digital:

Cool, but hard to use with Proffieboards.

Haha of all things, I did not think to look there :sweat_smile:. My brain was fried yesterday.

I guess this will be shelved for now. The only benefit to considering this path is the size of the module being very small to begin with compared to auxiliary-based BT transmitters. And then figure out the antenna part.

I’ll switch gears to focusing on those auxiliary-based modules.

You mentioned that the 400kHz PWM signal needs to go through a filter…

Would I need some kind of amplifier as well? Or would resistors be enough to help smooth out the PWM into an analog signal?

Quite the contrary, the output signal from the board is likely to be too strong, which would mean that you would need some resistors to tone it down a bit. Resistors do not smooth anything out though. Capacitors or coils can smooth things out. A lot of audio equipment already has some type of filter on inputs, so it might not be required though. Most people seems to just do without.