Hi-Rez Highway

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Now I think we are getting somewhere

Posted by Charles Hansen on June 1, 2017 at 15:17:11:
>> When fully unfolded in hardware, by the same DAC, I see 44.1/24. <<

My guess is that the DAC is showing the sample rate of the (singly "folded") incoming file. It started out as an 88.2/24 file and was "folded" during MQA encoding by taking the audio data in the frequency range from 22.05kHz to 44.1kHz, discarding its lower bits, compressing with lossless techniques, and then storing it in the lower bits of the "baseband" audio (from DC to 22.05kHz). This process discards the lower bits in the baseband (which is what allows for the reduced file size of MQA for high-res streams).

The lower bits in the dual-rate data are also discarded for two reasons. One is that the now lowered dynamic range signal can be compressed into a smaller stream that requires discarding fewer LSBs in the baseband audio, and the second is that there is no reason to have a lower noise floor in the dual-rate audio compared with the baseband audio. (Noise-shaped dither does exactly the opposite, *raising* the noise floor at high frequencies.)

Inside the DAC, the lower bits in the baseband are separated out, uncompressed to re-form the restricted dynamic range version of the dual-rate data (the discarded bits can never be retrieved), and spliced back to the baseband audio to re-create an 88.2/17 file.

When you set the Tidal app to perform the MQA decoding, it sends an 88.2/17 audio stream (possibly with zero padding to create 24-bit words). The DAC then displays 88.2/24.

The bug is that when the two frequency ranges are spliced back together with the Tidal software (as seen in the YouTube video), there is audio data missing from the original 88.2/24 file in the frequency range between 21kHz and 23kHz. What we don't know yet is if this is also true when decoding is performed in hardware. If you have Audacity, Adobe Audition, or some similar audio tools, you may be able to answer that question.