MQA technical analysis

[Jay-dub]

As an experiment, I took the MQA version of 2L-048 and zapped the high 15 bits preserving only the sign while leaving the MQA control bitstream in bit 8 and the lower bits intact. The decoder still reports this as "blue-light" MQA and tries it's best to decode it....

It should also be noted that the decoder calculates a checksum over the input samples, and I don't know what effect a mismatch here has.

 

It was supposed to be silent. The purpose was to see if it actually detected the audio bits being tampered with while leaving the control bitstream alone. Apparently it does.

I'm confused by this. According to the earlier post, it appears that you've managed to find a way to replace the audio content in an MQA file and still have it light up the blue-light indicator; according to the later post you haven't. Was the earlier post in error?

 

It seems to me likely that an MQA stream would have something along the lines of a checksum of the audio data, encoded through a public-key encryption (encoding key closely held by MQA, decoding key built in to all client devices). That way, the decoder could use the checksum to verify that the data isn't corrupted (as a condition for turning on the authentication light), but a hacker wouldn't be able to create an unauthorized MQA encoder by reverse-engineering client software. At least, that would definitely be a sensible approach. Have you seen any evidence that the decoder is doing something at all similar to what I've described with the checksum?

[Jay-dub]

On second thought, I'm not sure that what I just described would work. A hacker could get around it by analyzing a collection of authentic MQA files, calculating the checksums occurring therein, and creating a database of checksums associated to known authentication codes. The unauthorized encoding software would then take an audio stream, modify it (hopefully with minimally audible degradation) until it matches one of the known checksums, and then use the associated code to create an apparently authentic MQA stream from it.

[Jay-dub]

User-supplied, no. The Bluesound applies bass/treble controls between decode and render. This involves sending the data another round through the MQA-provided software library to restore the control bitstream needed by the renderer.

 

Now this is interesting. Just to be clear, are you saying that the MQA library performs the EQ in the Bluesound player? How flexible are the EQ capabilities in the MQA library? I could imagine anywhere from a handful of boost/cut settings to a full three-parameter shelving filter, plus a parametric peaking filter that could be cascaded as many times as needed. If it's anything like the latter, then it sounds to me like you're very close to the point where you could write an MQA player that would perform fairly general room-correction DSP, incorporating the MQA library that you got off the Bluesound player.