AptX lossless codec is able to transmit in a lossless way, but it allows up to 44.1kHz. If the audio is at 48kHz, the device automatically picks AptX Adaptive (a lossy codec). On the other hand, Roon ARC resamples everything to 48kHz assuming Android mixer will do it otherwise with worse quality. However, recent Android versions (14 at least, in developers options) allow the user to choose frequency rate for bluetooth. So in summary, in the current scenario if I want to use AptX lossless and I stream a cd quality file to Roon ARC, it is upsampled to 48kHz and then downsampled again to 44.1kHz.
When I try to do the same in UAPP using Direct Driver everything works as expected. The file is delivered untouched to BT (bitperfect).
I ask the developers to allow the use of ARC USB Driver for bluetooth too to bypass Android’s mixer and avoid resampling.