Seems the movement towards the NOS DACs is purist in idea but is not necessarily the holy grail of design (if any of the ideas are or math is)? Or it simply has not been really well implimented yet due to its being relatively new (I beleve it is NEO in the sense its a further investigaion of an earlier digital processing idea and is being openned again to see what the designers may have overlooked the first time).
Anyway thanks jb for attempting to get us to understand some of this stuff, if you have the patience most of could probably benifit greatly from the most simplistic explaination with possible simple analogies:
There is confusion because there is no universally agreed to definition of the terms oversampling and upsampling. Here are selected highlights and a simplified overview of the evolution of CD playback.
The standards for CDs represent the absolute minimum for recording
sound, not music. The samples are too small and the sample rate is too low. According to sampling theory, the low sample rate required a very steep, ‘brick wall,’ anti-image filter, which contributed to the bad sound of the first generation DACs.
At the time, microprocessors were too slow to be able to manipulate the sample data in real time and DAC designers did what they could to increase the clock rate of the DAC to reduce the severe requirements of the anti-image filter. This brand new idea was called 2X over-sampling. It involved doubling the clock rate and inserting a null sample in between each recorded sample. Today, that technique is sometimes called decimation. The problem is decimation halves the energy content of the signal and adds noise. I believe preserving the energy content is essential to accurately reconstruct music.
The next attempt, called 4X oversampling, involved using 4 DACs that operate on successive quarters of the sample period. The outputs of the DACs are summed. The sample rate is quadrupled and the effective sample size is increased by two bits. However, instead of a vertical step between recorded samples, as there would be with no oversampling, there is a diagonal line between the two points. Instead of a square wave, the DAC outputs more of a triangular wave. In terms of energy content, a triangular wave does not approximate the energy in a sine wave as well as square wave does. Today we call it linear analog interpolation.
When microprocessors were fast enough to manipulate the sample data in real-time we got 8X oversampling and the digital filter. The technique, more properly called digital interpolation, attempts to recreate the shape of the signal in between each pair of samples from the original recording. To do so, a large number of samples preceding and following the sample period being interpolated are examined to determine the shape of the signal through the period in question. The accuracy of the interpolation depends on the accuracy of the samples and a similar shape of the signal leading up to and following the interpolated period. The effective sample size is increased by three bits.
So far, only the added samples are the result of calculations, the original samples are preserved, and the sample rate is increased by an integer power of two. The next generation of digital interpolation used non-integer sample-rate multipliers and was called UPsampling to differentiate it from all previous incarnations of OVERsampling. The only thing that is different is the non-integer multiplier and the fact that now every sample is the result of a calculation. The output is essentially a digitally synthesized version of the original recording.
Asynchronous sample rate conversion is a variation of upsampling. Instead of a fixed sample rate multiplier applied to the input sample clock, the sample rate multiplier is determined by continuously computing the difference between the input sample clock and another reference or output sample clock.
In my opinion, the evolution of CD playback has gone from bad to worse. Although upsampling improves the quality of steady-state sine waves, it doesn’t improve the quality of recorded music. That conjecture is proven by the renaissance of NOS DACs. If the newest generation of CD playback technology were as good as the promoters say it is, there would be no desire for music lovers to want to revisit the past.
Unfortunately, in their zeal to reject everything that was wrong with prior attempts to reproduce music, NOS proponents have thrown the baby out with the bath water. Yes, the brick-wall filter is bad and so is the digital filter, but a proper reconstruction filter, also called an anti-image filter, is an absolute requirement for any digital to analog converter. While 8X upsampling and extending the 16-bit sample to 24-bits with what is essentially noise doesn’t enhance the music, 24-bit DACs are superior to 16-bit DACs whether or not you use all the bits. Also, there’s nothing wrong with upping the sample rate provided you preserve the energy content of every sample.
