Ken Newton
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Ken - you might be right, and it is probably not worth arguing about The "triangular" concept is not, after all, mutually exclusive. I first heard it used by Stuart himself in conjunction with MQA as a description of the spectral content of sounds in nature and in music. This triangular distribution is as opposed to the rectangular spectral capabilities of standard PCM as we know it.
No argument, we're just exchanging our understanding of the issues so to zero in on what MQA is doing.
The following linked document explains what I'm talking about better than I've been. http://www.dspguide.com/CH15.PDF
Refer to figure 15-3 chart 'a' to see rectangular, triangular, and quasi-Gaussian filter kernels (which depict the filter impulse response). These particular kernels are from a progressive cascading of simple moving average filter functions. Chart 'c' of that same figure depicts the relative frequency response of each of those three kernels. The text following the figure, on pages 281 and 282, goes in to some detail.
I don't know for certain, but it appears to me that full MQA effectively functions very similar to a cascaded series of moving average filters, as shown in figure 15-3. What is clever is that an triangular impulse response anti-alias filter (non-ringing) at recording will inherently form an overall quasi-Gaussian system impulse response (non-ringing) when passed through through an triangular impulse response (non-ringing) interpolation filter at playback.
However, I notice from an MQA provided chart of the resulting overall system impulse response that it does show about one cycle of post ringing. This seems likely due to MQA's indicated use of 3rd order filters, rather than the 2nd order pure triangular kernel filters such as shown in figure 15-3. This is probably necessary, as while MQA is focused on transient optimized digital filters, they only have a maximum of two octaves of ultrasonic bandwidth in which for those filters to reach their stop bands, or otherwise allow aliasing.
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