01-06-2022, 12:51 AM
(This post was last modified: 01-06-2022, 12:52 AM by phonicparty.)
I'm getting persistent noise in the audio, as soon as the system is powered on - hissing, crackling, whining, that kind of thing. It sounds like electrical interference from the Pi. This happens whether or not the amp is connected to the USB sound card via a 3.5mm cable. The noise is audible even over the sound of the engine when there is no audio playing, although when there is music playing it's often enough to cover up the noise.
I'm confident that it isn't alternator whine as it comes through even when the engine is off, and touching the touchscreen produces changes in the noise. The noise seems to cease briefly when the Pi's green activity LED flickers, which I can't explain.
My setup is this:
As far as I can see everything is grounded properly:
Things I have tried:
At this point I'm lost on this. I don't know why using a ground loop isolator means that there is no sound at all in the car. In any case, everything is grounded through the car's harness.
If anyone has any ideas on this then I'd be very grateful. I am open to the idea that the best solution is to buy a separate dedicated car amp that I can put under a seat or something, but that would be a very last resort due to the expense.
Thanks in advance!
I'm confident that it isn't alternator whine as it comes through even when the engine is off, and touching the touchscreen produces changes in the noise. The noise seems to cease briefly when the Pi's green activity LED flickers, which I can't explain.
My setup is this:
- I'm using a Raspberry Pi 4 powered through a CarPiHat.
- The CarPiHat takes power from the car's 16 pin harness.
- I'm using the recommended ugreen USB audio card to send sound to an Adafruit MAX9744 2 channel 20w amp.
- The audio output from the amp goes directly into the car's 16 pin harness.
- The amp is powered from the 5V pin of the Raspberry Pi. It's not the loudest, but it's enough for my purposes (I don't listen to a lot of music while driving - it's mostly for directions, etc).
- I have a Raspberry Pi Official 7" touchscreen powered from the Pi's other 5V pin.
As far as I can see everything is grounded properly:
- The two ground wires for the CarPiHat are connected to the ground pin on the car's harness.
- The Pi is (I assume) grounded through the CarPiHat.
- The amp is grounded through one of the ground pins on the Pi.
- The touchscreen is also grounded through one of the Pi's ground pins.
Things I have tried:
- A ground loop isolator between the ugreen USB sound card and the Adafruit amp. When I have the whole setup taken out of the car and connected to a speaker in my house the ground loop isolator significantly reduces but doesn't eliminate the noise. When I have everything hooked up in the car, there is no sound from the system at all when the ground loop isolator is used.
- Ferrite cores on the leads between the CarPiHat and the car's harness, between the Raspberry Pi and the USB sound card, and between the USB soundcard and the amp. None makes any difference to the noise.
- Powering the amp from the car's harness. I have tried this both to the 12V switched supply directly and through a 12V to 5V buck connected to the 12V switched supply. In both cases the amp was connected to the harness's ground pin). All versions of this gave a lot of alternator whine. This alternator whine goes away when a ground loop isolator is used, but again I get no sound at all through the speakers.
- Testing the system out of the car. The noise remains even when I have the whole setup taken out of the car and hooked up to a speaker in the house (with the CarPiHat powered from a PC power supply acting as a stand-in for the 12V SW, 12V C, and GND on the harness). It remains when the amp is powered directly from the PC power supply, rather than from the Pi. It remains whatever power source for the amp I use, regardless of whether it's 5V or 12V.
At this point I'm lost on this. I don't know why using a ground loop isolator means that there is no sound at all in the car. In any case, everything is grounded through the car's harness.
If anyone has any ideas on this then I'd be very grateful. I am open to the idea that the best solution is to buy a separate dedicated car amp that I can put under a seat or something, but that would be a very last resort due to the expense.
Thanks in advance!