Comment 27 for bug 1810636

Ok, I'm pretty sure this is a pulseaudio issue now and I'm going to close.

I reverted back to linux-firmware 1.176, and I had also used undervolt [1] to help reduce thermal load on the Core i9 Dell XPS 15 which has suffered from thermal shutdown under high load. I disabled undervolt service from running and ensured everything was back to normal values - the stuttering improved, but was still present.

I then focused on the difference between my Mi8 and my Ubuntu 18.10 installation. The Mi8 is running a LinesageOS ROM that supports LDAC/Apt-X codecs. I noticed with the regular AD2P codec in Ubuntu that I was getting some noise included in the Bluetooth audio encoding, and it sounded much cleaner via the Mi8 connection. I then found pulseaudio-modules-bt repo on github [2] that includes [3] a Ubuntu 18.04|18.10 repo that enabled LDAC/Apt-X codecs, LDAC is supported by the Sony bluetooth speaker.

After installing and rebooting the machine, I confirmed that the codec was correctly initialised in the a2dp sync:

berg@bxps:~$ pactl list sinks | grep -e bluez -e a2dp_codec
 Name: bluez_sink.E0_37_BF_09_F9_8B.a2dp_sink
 Driver: module-bluez5-device.c
 Monitor Source: bluez_sink.E0_37_BF_09_F9_8B.a2dp_sink.monitor
  bluetooth.a2dp_codec = "LDAC"
  device.api = "bluez"
  bluez.path = "/org/bluez/hci0/dev_E0_37_BF_09_F9_8B"
  bluez.class = "0x240414"
  bluez.alias = "bshg"

The sound is now perfect, and copying files across the network results in no stuttering. I think this proves that the btcoex support is working correctly and there is no issue specifically with the Qualcomm firmware. I notice if I pair the bluetooth speaker and play back audio my network throughput is limited to about 14-15 MB/s, when I turn of the speaker it improves to 17-18 MB/s. So the module is correctly allowing bluetooth to operate with priority.

Resuming the undervolt [1] service, I notice no stuttering as well with the updated pulseaudio and pulseaudio-module-bt packages in [2].

I'm going to continue testing for a few days before closing this bug, but I'm pretty sure I've gotten to the bottom of this one. Thanks @kaihenfeng.

[1] https://github.com/georgewhewell/undervolt
[2] https://github.com/EHfive/pulseaudio-modules-bt
[3] https://github.com/EHfive/pulseaudio-modules-bt/wiki/Packages