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In Reply to: RE: Solved by FLAC + broadband posted by knewton on February 17, 2017 at 14:17:53
MQA is currently being used as a compressed audio format to stream Warner titles that were previously mastered for a hi-res PCM release e.g. DVD-A or download. Universal is said to follow.
For existing recordings, MQA have claimed that if the ADC is known, its identity or characteristics can be encoded, and then the decoder can utilize that information to select a reconstruction filter. I guess you would have to assume the same ADC is used for all the tracks of a multi-track recording. But I haven't seen them discuss the details of this process publicly or give examples, so I don't know what to think about its potential benefits, if any. Also, given how quickly Warner encoded their catalog, I think there is a very high probability that the encoding was generic.
Those are just practical issues though. My real complaint about MQA is theoretical. In their AES paper, Stuart and Craven proposed a Gaussian as an ideal system impulse response. In image processing, a Gaussian filter is also known as a blur filter or unsharpen filter. It is used to reduce sharpness or to artistically blur images. It will have the same result on a music waveform, smoothing over transients and blunting attack. How is that transient optimized? In the paper, Stuart & Craven note their ideal is equivalent to "only" 30m of air. Why is 30 meters ideal and not zero meters? From an engineer's POV, their idea just seems dumb.
Gaussian filters provides smooth rather than abrupt transitions, both in the frequency and the time domains. It provides a ripple free frequency response, while simultaneously exhibiting an aperiodic (non-ringing) impulse response. It's the aperiodic impulse response that MQA apparently values.
I'm not as familiar with image processing applications, and unsure this is directly comparable to pixel smoothing within an image. Possibly, image processing utilizes Gaussian filters for their natural Guassian shaped frequency domain properties, while MQA utilizes them for their natural Gaussian shaped time domain properties mentioned above.
The narrow channel bandwidth relative to the information bandwidth of CD necessitates the well known SINC function brickwall bandlimiting anti-alias and anti-image filters associated with it. It seems to me that MQA uses a much wider channel bandwidth, not for any musically significant ultrasonic content, but to enable utilization of relatively slow sloped aperiodic filters, such as the Gaussian, without incurring aliasing.
As for the audibility of MQA's time domain optimization, that's the key question for we audiophiles. MQA's strategy seems to be to offer performance benefits to music consumers so that they pull the format, while offering distribution management and cost savings to the music industry so that they push the format. If MQA delivers the promise of a compellingly better sounding digital listening experience combined with lower vendor cost, it has a chance of success. The key words are: compellingly better. Of course, fear of a new format, some of it rational, some of it irrational, is also playing it's usual and expected role.
Have you heard MQA? Just asking
So far, I have listened to a handful of Warner's MQA releases on Tidal using the Tidal app's software decoding. I picked ones that I already had on CD and DVD-A or high res download, and where I felt there was a pretty big difference between the CD and DVD-A. Selections were from:
Buena Vista Social Club
Fleetwood Mac Rumours
Yes Close to the Edge
Grateful Dead American Beauty
Phil Collins Face Value
Chris Thile Bach Sonatas
The MQA files sounded a lot like the DVD-A rips/hi-res downloads. I'm pretty sure they were made from the same master files as the DVD-A or download, and not the CD master. I would have to do more careful listening to say whether the MQA files are as good as the DVD-A rips, but they are at least close.
As a compression algorithm, I think MQA is pretty novel, but I don't think it is necessary given the near-ubiquity of high bandwidth broadband connections. Thanks to the demand for streaming HD video, we have the bandwidth we need to stream hi-res audio using a lossless, open standard like FLAC.
As a technology that aims to control the whole end-to-end chain from recording to playback, I think MQA is bad. Digital audio and computer-based audio are thriving because of open standards. MQA is a power grab that would change the landscape into something much more label friendly and not consumer friendly. And on top of that, their choice of system impulse response is just wrong.
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