# I2S DAC Volume Control

First of all, let me preface this with the fact that I'm not an EE -- I'm a hobbyist with some experience and a background in software development.

I'm trying to use a TI PCM5102A Audio DAC with a Sierra Wireless BC127 Bluetooth module to output line level audio into an in-car amplifier. Anyways, the I2S link is operational and I'm getting audio from the DSP in the BT module to the DAC, but the gain is through the roof -- so much so that I had to add a pair of 10k resistors just to be able to listen to it via a pair of earbuds. I also tried it directly on the amp, and it was heavily distorted. I have issued the appropriate commands to the BC127 to set the gain as low as it can go (VOLUME <A2DP Link ID> 1) and there was no change.

My questions are:

1. It feels like I'm missing something about how DSPs and DACs operate -- am I? Is there just an IC I need to add to tone down the audio?

2. Is there a more suitable DAC for what I'm trying to accomplish?

3. The BC127 has differential outputs that are suitable to drive a small speaker. Is there an IC I can use to convert the differential output to single ended line level?

Thanks!

• There are various I2S format disagreements which could appear as absurd gain for part of the range (and then hit clipping). Generally speaking though, this project doesn't make sense - if you're using a bluetooth module you can't control the details of, then you should be using one designed to output analog audio (the BC127 already has a DAC), not trying to pair it with your own DAC. – Chris Stratton Dec 17 '18 at 22:07
• @ChrisStratton - Thanks! I will play around with the I2S configuration on the BC127 side. I assumed that because it's data being transmitted, it would either "work" or output nothing. As far as the BC127's DAC -- It only outputs differential audio -- What could I use to convert that to single ended line level audio? – t3ddftw Dec 17 '18 at 22:13
• @ChrisStratton - You're right! The MSB of the byte was set to occur on the second clock cycle, rather than the first. With that sorted, the audio is now exceptional. I'd still love to hear your feedback with regards to my decision to use an external DAC -- As I stated, this is not my specialty :) – t3ddftw Dec 17 '18 at 22:29