Balanced Audio

[mcaldwelva]

I noticed that the balanced audio circuit went away when you switched from 3.5mm to bluetooth. Are the R/L +/- outputs from the RN52 directly wired to the CDC connector? Or are there some components in between? I'm curious because the VS1053 chip I'm working with supports differential output, so it seems like I might be able to use the new board. Thanks!

[Seth]

I got rid of the balancing audio chips because the rn52 already has "balanced" audio (afaik); so therefore I just wired the rn52 outputs directly to the cd connector. I don't think we'll need an amplifier; it seems like we can drive the volume loud enough right now without.

Sent from my iPad

On Nov 25, 2015, at 16:03, mcaldwelva [via BlueSaab Forum] <[hidden email]> wrote:

I noticed that the balanced audio circuit went away when you switched from 3.5mm to bluetooth. Are the R/L +/- outputs from the RN52 directly wired to the CDC connector? Or are there some components in between? I'm curious because the VS1053 chip I'm working with supports differential output, so it seems like I might be able to use the new board. Thanks!

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[mcaldwelva]

That's cool. I looked up the datasheet on the RN-52 and it does say dual-channel differential output.

I don't think I can do it with the VS1053 chip after all. Adafruit faked me out a bit with their wiring examples, but after going over its datasheet more carefully, I see that it can only do differential output for a single channel.

[Ziq]

If stereo is not so important, wouldn't it be possible to connect one output from your BT-module to both inputs? Otherwise, if you are not so much into making your own circuits you could probably find something good enough on e-bay, example.

I have jacked up my RN52 to the CDC connector in the trunk. I've put an adapter to i dSub9 so in between so that I exchange to board with a USB-CAN device.

Sound is playing but I have two issues with it.

1. Sound volume is very low, I need to put maximum volume on both my phone and head unit for an OK volume.

2. There is noise on the audio line. I've narrowed the source down to the CAN-bus, if i disconnect the cables from my CAN transceiver the noise disappears (playback continues until the head unit realized no CDC-status-messages have come for a while). If playback volume hadn't been so low I don't think this would be much of a problem, now it's unbearable. In the wiring from the CDC-connector to the head unit the CAN-wires seem to be next to the other wires so I suppose noise transferring between the cables should normally not be a problem. I'm suspecting either my adapter cable or the fact that I've mounted everything on a breadboard (=big capacitor..).

Am I alone with these problems?

[Karlis]

The reason sound level seems to be low is because when a new device is added to PDL (Paired Devices List) of RN52, it sets the volume to the default level of 11 (out of 15). The level then leeds to be adjusted via either sending "SS,0F" command or 4 x "AV+". After that, up until the moment the RN52 is restored to factory defaults or the particular device is removed from PDL, the level will stay to the level set. Initially I had Vol+ button on steering wheel set to send "AV+" commands for first 5 times it is pressed thus also increasing the volume on RN52.

As for the background noise - that might be a consequence of the first issue. At high levels, the IHU seems to be generating some noise...

Hope this helps.

[Ziq]

Thanks!
Haven't looked into interfacing the RN52 with serial yet, turns out my motivation just became bigger!

I'm confident I found the source of the noise, there is a cable break on left+.. I'm simply to sloppy making cables!

[Karlis]

You're welcome! :) You can use the RN52 library I wrote. Just change the pin assignments and you should be locked and loaded. The library takes care of interfacing with RN52 over UART.

[mcaldwelva]

Thanks! I'm working with the 3.5mm BlueSaab module which has that circuit built-in.  The audio quality is good with this setup, I just thought the new bluetooth module would make a nice upgrade.Earlier in this thread, se4587 said they are getting good output levels with the rn52 output directly connected to the CDC connector. The differential output should effectively eliminate interference. I'd be looking at the pin-out and the volume on the rn52 itself... is it possible to rig it up so that the audio isn't going through the breadboard?

[Seth]

Yes, you could just connect the rn52 outputs directly to the cd connector pins on the 3.5 board, bypassing the audio amp/balanced audio chips (drv chips). I would remove those as well just to be sure. 

Sent from my iPad

On Dec 3, 2015, at 16:32, mcaldwelva [via BlueSaab Forum] <[hidden email]> wrote:

Thanks! I'm working with the 3.5mm BlueSaab module which has that circuit built-in.  The audio quality is good with this setup, I just thought the new bluetooth module would make a nice upgrade.Earlier in this thread, se4587 said they are getting good output levels with the rn52 output directly connected to the CDC connector. The differential output should effectively eliminate interference. I'd be looking at the pin-out and the volume on the rn52 itself... is it possible to rig it up so that the audio isn't going through the breadboard?

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[Ziq]

Noise was gone after I fixed the broken wire!
Hopefully volume is better when I change the module volume from 11 to 15. That will probably give decent volume but I think the difference from radio/cd will still be big. (=anoying when you change source)

[Seth]

I've always noticed that I have to turn up the rn52 volume to max, as well as the audio source (iphone) in order to get decent volume out of the speakers. With the rn52 at max and iphone at max, the volume level difference between cd changer mode and radio mode seems to be a lot better (not quite equal, but better than blasting your ears out as it has done in the past with my old module :)

[aaronbguerrero]

I agree! With my iPhone at full and the RN52 at full, it's much better. It's still pretty loud when the radio comes on, but not nearly as bad.

[Ziq]

Have you guys considered some kind of amplifier after the RN52?
I haven't looked into the specs but something similar to LME49724  should probably do the trick, right?

[Sandwer]

I've long been convinced that the output levels of the CD changer are an order of magnitude lower than you'd expect - both saablin and my first version of this both suffered from low-ish output levels. I created a 2 transistor amp based on a mic preamp circuit from t'internet and it worked a treat.  

If you take the CD changer to pieces - in order to liberate the socket :) you'll notice there is a transformer on the board near where the audio is routed to the socket. I'm of the opinion (happy to be proved otherwise) that this is to both drop the output levels to match the head unit, and to create the 'balanced' output.

Rich

[Ziq]

Each output of the CDC is actually directly connected to one of the four transistors close to the connector (verified by beep-test). I haven't taken the time to try and find the inputs of the transistor via beeping but it would be interesting to see what the output from the CDC looks like. Mine stopped working entirely a year ago so I did what you did- harvested the connector from it. The transformer is probably used to generate a galvanic isolation and could very well be feeding the transistors as you say.

Could you show how you constructed your amplifier? I'm quite annoyed by the differing volumes so it would be neat if I could correct it.