Noise Interference on Radio Mod

Poor radio! You cruel and heartless sod!  What a rotten thing to do to it. (I'll tell you why in a moment.)

Quick questions... Are you getting this noise on the bench or in the car with the engine running? What's it sound like all plugged up but playing a CD? What's it sound like when you use a mobile phone instead of the Pi? (No hum loops if you do that. See later.)

There is a lot possibly wrong with this that may be causing trouble. Your problem is working out which is causing trouble. Here are a few things to think about...

You used DGND as your earth. That is almost certainly Digital or Data Ground and will be associated with the RDS Data output from the tuner to the radio processor. I wouldn't use it for audio. I can't work out what BEGND stands for but it's probably a better bet for audio. Especially as it seems to be connected to the case. Ideally you need to trace back the earth that the outers of the AUX OUT phonos are connected to and use that. (Did I see that it has an AUX OUT or did I dream that?)

Earth Loops. You've very likely got an earth loop between the Raspberry Pi's power supply, the radio's supply and the audio earth between them. Earth loops tend to hum horribly. You my not need the earth on the audio lead at all (Because the earth is passed via the power supplies). But if you have noise on the Pi's power supply that'll then get into the audio anyway.

Here's why you are a terrible person... In effect you've wired the output of the tuner module to the output of the Pi. First of all, how do you shut up the tuner module when you are listening to the Pi? I suspect you are lucky and, not having an aerial, the tuner has muted as it has no signal. But that's not going to help if you want to use the radio in the car.
The other really horrid thing you've done is this. The amp has a high impedance input. The tuner has a low impedance output. (If you aren't sure what I mean by "impedance" replace it with the word "resistance" and you won't be far from the truth.) The tuner is expecting to drive into the amp's high impedance input and will be happy. You've tagged the low impedance output of the Pi across it. So the tuner is seeing the Pi's output and the Pi is seeing the tuner's output. Neither are likely to be happy about that. You really need to wire some sort of switch or relay in there to swap between the tuner and the Pi.

If you are getting noise in the car with the engine running... Well there is a lot of electrical noise in a car and radio manufacturers go to a lot of effort to filter it off the power supply lines. There'll be an L/C filter built into the radio and possibly a ferrite blob thingy on the wire just out of the radio. Your Pi was never expecting to wind up in a noisy car so it won't have the filtering in it.

Possibly a better bet at the moment would be to use one of those radio modulator boxes and use the radio to tune into the Pi. Although to be honest I used one of those years ago and it was horrid. Quiet bits disappeared into the noise of the radio and loud bits distorted. But then I'm a radio engineer by trade and I can't really can't listen to things that are even a little bit distorted.

Well that's a few things to think about!

If there are things in here that you don't understand then shout and I'll try and explain. I'm a bit tied up next week but I'll try and get back to you at some point.

Best of luck!
James

Hi mate,

My question about bench testing was more - is your problem engine noise or is it just noisy even with the engine off. But I'm getting the idea it's just noisy.

So, good news on the ground front. You are now on the correct one. My idea of picking up the outer of the line out phonos was just to find an audio earth somewhere. But having identified BEGND as the audio ground it is the right place to be.


Bummer! The question in my mind is "did it randomly fail or did it get killed by something".


Bummer again! Actually powering it off an external PSU is a good plan because it eliminates all ground loop problems for the moment. A massive raft of problems you can now ignore for a while.
So. All the vocals disappeared. I suspect you've accidentally got the 'S' signal due to some duff connection in the audio wiring. I'll explain.
Think of audio as Left and Right. (L & R from now.) If you add L and R you get Mono (M). No big shock there. But if you subtract them, L - R you get the difference between channels, otherwise known as the Stereo signal or S.
Those vocals are usually mixed right down the middle of the stereo image so they are prominent in the M signal but missing from the S. That's why I think you've got the S signal by accident.

This M and S thing is actually common in the audio profession. FM radio is actually sent as M and S, not L and R. If you convert L and R to M and S and amplify the S signal a little with respect to M, when you convert it back to L and R you've actually increased the stereo width.
Anyway, wiggle your wires and check your soldering and the problem should go away.



Here's where things get dangerous for your radio 'cos you'd have to modify it. How's your soldering skills?

I'll try and draw a picture later but the idea is that you have EITHER the tuner connected to the amp OR the PI connected to the amp. But not both. So you'd take the tuner's output (from the pins you've already soldered onto) out of the radio to one input of a switch or relay. The other input comes from your Pi. The output from the switch goes back into the radio and feeds the amp. But that means you need to cut the tracks on the circuit board currently connecting the tuner module to the amp and solder onto the ends of them. This is getting serious!

Although there may be a slightly safer way of doing that now I look at the photos... I'll get back to you.

If you use a relay you can drive it with an output from the Pi so that it automatically switches the Pi to the amp when you are using it and puts the tuner back on when you aren't.

James

PS, You are mad by the way. A sensible person would just buy a radio with a line in. I'm mad too so that's fine, but feel free to become sensible at any point.

Oh, don't some later Pis have a 4 segment jack with video on one segment too? If so it could be trying to dump video into the earth if you are using a 3 segment plug.

Sweetpea;

Thanks for all the info which I will check out and get back to you later in the week.

Well spotted ref the Pi 3.5mm audio out. It would appear that it is indeed a four pole output details here:

 

So you may well be right as I have soldered in a standard 3.5mm audio socket. I have a 4 pin 3.5mm audio plug with RCA L/R and composite Video so I'll try that with an RCA to 3.5mm audio jack converter.

Also...
There is no standard way of wiring those leads and / or the jacks. One of the chaps at work was playing with these and found audio coming out of the yellow phono and video out of one of the others... Depended on which lead he was using.

James

Sweatpea;

I do appreciate your time on this :-) you have been doing some great research.

As an update I spent a little time on this again doing some diagnostics.
Following an email to Mausberry I managed to get the 3A In Car supply working again (disconnected it from both 12v supplies and then reconnected it).
 
- When powered by the In Car supply (and even using a power supply from the mains) I am getting really bad ground loop noise (even with the engine off but ignition on (first click)
 
Although I have been advised that even though the Pi's 3.5mm audio socket also does videoI tried a couple of different audio leads with the Pi 1 x TRRS leads and 1 x TRS lead.

- With the TRRS lead the audio was particularly bad little or no audio.
- When I used the TRS I noticed that I could get vocals but one channel (the vocals) appears to be really quiet and the other quite clear (apart from the horrendous ground loop interference) 

Unless I can sort out the ground noise and quiet channel, I am not sure where to go with this? 


P.S. Just missed out on eBay for a Sony XPLOD Amplifier (damm those auto bids)

Sweatpea;

Today I decided to hack into the CD Audio Input and see how that would pan out. After some snooping around I found the L and R Channel and Audio Gnd then soldered in my 3.5mm Audio Socket.
I burnt a CD full of 5 minute blank audio tracks and played it.

I first tried a youtube video on my smart phone and the audio played out of the radio OK.
I then powered up the Raspberry Pi using the Mausberry in car 3A PSU and although there is a small amount of ground noise it is minimal compared to when I used the Radio module hack.
I played a video and the audio is OK.

I need a Ground Loop Isolator on the aux audio lead would this one do?

Audio from an external source is a little quiet and needs the volume up higher that when using a CD in the stereo. There is also an issue with the audio in Kodi (the media centre software that I am using) the audio is quieter than when playing using another media player.