MQA technical analysis
As we know, the Bluesound software contains an MQA decoder. All the MQA code resides in a shared library, libbluos_ssc.so, which is called by various other parts of the playback software. There are three main parts to the library: decode, render, and preserve.
The decode portion accepts audio data and outputs decoded data at twice the sample rate of the input. It also provides feedback regarding the MQA status of the data being processed: none, MQA (green), or MQA Studio (blue). Non-MQA input is also upsampled to double rate. I speculate that this corresponds to the MQA Core decoding done by the Tidal player.
The render stage takes the output of the decode stage and upsamples it further. Gain can also be applied here. Again, non-MQA input is accepted.
This function appears to transfer some MQA signal encoded in the LSB from one audio stream to another. The Bluesound player uses it together with software volume and tone controls between the decode and render stages.
Having deduced the programming interface of the library, I wrote some simple test programs to process audio files and save the output. Attached below is a snippet from the output of running the decoder on the 2L-048 sample (recorded at 96 kHz). I'd be thankful if someone who has an MQA DAC could play it and report whether the indicator LED comes on.