I am using the BC127 DISKIT001 discovery kit as a Bluetooth audio receiver. Unfortunately, there is a noticeable amount of noise in the analog audio output.
When the board enters A2DP_STREAM_START, the noise starts and there is slight popping-sound. When the board does A2DP_STREAM_SUSPEND, the noise stops. The noise sounds like white noise, and I could not hear any patterns.
I have measured the differential audio signal coming out of the BC127 chip on the “DIFF AUDIO” ( J9 ) header. Green (CH1) is connected to the right positive differential, yellow (CH2) is connected to the right negative differential and red (M) is the difference CH1 - CH2, which is amplified in the differential amplifier. The measurements were taken with no headphones connected. The board has Melody Audio 6.1.5 and the test was done with default (out of the box) configuration.
The top part of the picture shows the voltages for when the Bluetooth device is connected, but no music is playing (A2DP_STREAM_SUSPEND). The voltages after A2DP_STREAM_START are as shown in the middle part , the noise is about 7 times higher than on the first image. The connected Bluetooth device (audio source) is muted. The last part shows the voltages when the connected Bluetooth device plays music. The noise is still visible (and hearable) in the audio-signal when the music is playing. As one can see, the signal to noise ratio is not good. The volume on the Bluetooth device is about 10%, but any higher would be too loud.
(Sorry for combining everything, as a new user I was only allowed to upload one image.)
Changing the volume (and different gain settings for analog audio) on both the connected Bluetooth device and the board does not change the noise. Increasing the volume to almost max is way too loud for headphones, but it improves the SNR. However, even then the SNR is not satisfactory.
I have tried different USB power-sources (PC USB-ports, and different USB-chargers), with no luck. I do not believe it is a problem with the headphones, as I have tried with both Sony MDR-1000X and Bose QC20, and it does not change the noise. The headphones work as they should (no noise) when connected to any other audio source. Changing the Bluetooth device that sends the sound also does not change the noise.
I was expecting less (no noticeable) noise, as there are several cheaper options out there with less noise.
Has anyone experienced something similar?
Is there anything I can do to reduce the noise?