adamdea
You are not a sound quality evaluation device
With all the broo ha ha about hi rez, new formats and digital filters over the last 20 odd years, much of the fuss has boiled down it a single issue: does pre ringing matter?
[to skip the rant, just follow the link http://archimago.blogspot.co.uk/2018/01/audiophile-myth-260-detestable-digital.html ]
The main objection which could be levelled against 16/44 was that it requires filtering somewhere around the limits of human hearing. Orthodox theory has it that this should be accomplished by an infinitely steep, infinitely long linear phase filter equal to a mathematical sinc function. This filter "rings" before and after the signal for an infinitely long time, albeit at an infinitely narrow bandwidth. To put it another way, every sampling value affects the value of the reconstructed signal at all times (not just at that instant). [but that is not necessarily a bad thing, see below]
Recently it has been fashionable either to do something very unorthodox (a lazy filter a la MQA) or something heroically over specified (a la chord). The raison d' etre of the former is to avoid pre-ringing which is an inevitable property of the orthodox filter. It has long been hypothesised that this was what was wrong with 16/44 -that it involved adding something which was not there in the original signal pre-sampling, not just that it involved filtering out certain frequencies. The latter approach gets closer to mathematical orthodoxy, and therefore inevitably involves more and more "pre-ringing". Even John Atkinson, in one of his cute footnotes, has acknowledged that it might be thought odd to give rave reviews to both the solution and the amplification of the problem; not that that stopped him.
So is there a problem to cure? if not why are we bothering with solutions?
All along in every article I have ever read about the benefits of NOS dacs, minimum phase, hirez, etc etc, all we get to show the "problem" being "cured" is the impulse response which is the time domain representation of a filter. It represents what the filter would in theory do if faced with a dirac delta function -ie an infinitely short infinitely powerful impulse. It is not a real world signal- the filter and anyone close by would be evaporated by the force of the blast (don't forget that sound is an air-pressure wave). As regards a dac's filter, it is also an illegal signal (the sampling theorem is about band-limited signals).
You cannot therefore judge the effect of a real world signal on a filter in the time domain just by looking at the amount of ringing in its impulse response. Equally you cannot have an accurate reconstruction filter without some ringing in the impulse response. Why? because otherwise you can't fill in the gaps between the sampling instants. No ringing = wrong; and not just wrong in the frequency domain, but also wrong in the time domain.
So the ringing in the impulse response in fact maps to what the filter is doing right (reconstructing what happening between the sampling instants) as well as what it is or might be doing wrong (inventing a signal before or after the sampling instant which was not in fact there in the real signal). But which of these is it in fact doing when presented with a real musical signal? You would thinking that someone in the audio press would have tried to work this out. Unless I have missed something, you would be wrong.
As far as I can tell no one ever tries to show pre-ringing of an anti aliasing or anti imaging filter using a real musical signal. I once asked on Hydrogen audio and someone demonstrated using an acoustic impulse like a spark or a gunshot. Of course pre-ringing is a real problem and is known to those who devise perceptual codecs. But that problem was encountered with filters in the audible range where there is loads of energy in audio signals. Where is the evidence involving a 16/44 filter?
Over on Stereophile Jim Austin is now two parts into his allegedly in depth investigation into MQA and still uses impulse responses to judge filters by what night be called "visual inspection".
Why does no one ever try to show pre-ringing of a 16/44 filter using a real world signal? Let's assume that they are intelligent and inquiring. Well perhaps it's because you would struggle. You see pre-ringing in the impulse response because the assumed signal has infinite bandwidth (at equal amplitude no less) AND complete silence before and after. I and others have been going on about this for years-where is the real world example which shows what's wrong with an orthodox linear phase filter around 22kHz? None of the fancy filters or hi rez for that matter, could be justified unless/save to the extent that there were material pre-ringing to be removed. (of course that's before we even get onto the issue of audibility at the ringing frequency and masking).
Well at last someone has actually done a neat demonstration of pre -ringing (presence and absence) . It's the indomitable Archimago. I think it's a shame that this has not attracted much attention. Perhaps it was the wrong time of year.
http://archimago.blogspot.co.uk/2018/01/audiophile-myth-260-detestable-digital.html
If this is not a first then I would be delighted to see the antecedents.
So, is that it? Does it explain how unorthodox filters (or hi rez) "sound" better on all kinds of material (to some people?)
[to skip the rant, just follow the link http://archimago.blogspot.co.uk/2018/01/audiophile-myth-260-detestable-digital.html ]
The main objection which could be levelled against 16/44 was that it requires filtering somewhere around the limits of human hearing. Orthodox theory has it that this should be accomplished by an infinitely steep, infinitely long linear phase filter equal to a mathematical sinc function. This filter "rings" before and after the signal for an infinitely long time, albeit at an infinitely narrow bandwidth. To put it another way, every sampling value affects the value of the reconstructed signal at all times (not just at that instant). [but that is not necessarily a bad thing, see below]
Recently it has been fashionable either to do something very unorthodox (a lazy filter a la MQA) or something heroically over specified (a la chord). The raison d' etre of the former is to avoid pre-ringing which is an inevitable property of the orthodox filter. It has long been hypothesised that this was what was wrong with 16/44 -that it involved adding something which was not there in the original signal pre-sampling, not just that it involved filtering out certain frequencies. The latter approach gets closer to mathematical orthodoxy, and therefore inevitably involves more and more "pre-ringing". Even John Atkinson, in one of his cute footnotes, has acknowledged that it might be thought odd to give rave reviews to both the solution and the amplification of the problem; not that that stopped him.
So is there a problem to cure? if not why are we bothering with solutions?
All along in every article I have ever read about the benefits of NOS dacs, minimum phase, hirez, etc etc, all we get to show the "problem" being "cured" is the impulse response which is the time domain representation of a filter. It represents what the filter would in theory do if faced with a dirac delta function -ie an infinitely short infinitely powerful impulse. It is not a real world signal- the filter and anyone close by would be evaporated by the force of the blast (don't forget that sound is an air-pressure wave). As regards a dac's filter, it is also an illegal signal (the sampling theorem is about band-limited signals).
You cannot therefore judge the effect of a real world signal on a filter in the time domain just by looking at the amount of ringing in its impulse response. Equally you cannot have an accurate reconstruction filter without some ringing in the impulse response. Why? because otherwise you can't fill in the gaps between the sampling instants. No ringing = wrong; and not just wrong in the frequency domain, but also wrong in the time domain.
So the ringing in the impulse response in fact maps to what the filter is doing right (reconstructing what happening between the sampling instants) as well as what it is or might be doing wrong (inventing a signal before or after the sampling instant which was not in fact there in the real signal). But which of these is it in fact doing when presented with a real musical signal? You would thinking that someone in the audio press would have tried to work this out. Unless I have missed something, you would be wrong.
As far as I can tell no one ever tries to show pre-ringing of an anti aliasing or anti imaging filter using a real musical signal. I once asked on Hydrogen audio and someone demonstrated using an acoustic impulse like a spark or a gunshot. Of course pre-ringing is a real problem and is known to those who devise perceptual codecs. But that problem was encountered with filters in the audible range where there is loads of energy in audio signals. Where is the evidence involving a 16/44 filter?
Over on Stereophile Jim Austin is now two parts into his allegedly in depth investigation into MQA and still uses impulse responses to judge filters by what night be called "visual inspection".
Why does no one ever try to show pre-ringing of a 16/44 filter using a real world signal? Let's assume that they are intelligent and inquiring. Well perhaps it's because you would struggle. You see pre-ringing in the impulse response because the assumed signal has infinite bandwidth (at equal amplitude no less) AND complete silence before and after. I and others have been going on about this for years-where is the real world example which shows what's wrong with an orthodox linear phase filter around 22kHz? None of the fancy filters or hi rez for that matter, could be justified unless/save to the extent that there were material pre-ringing to be removed. (of course that's before we even get onto the issue of audibility at the ringing frequency and masking).
Well at last someone has actually done a neat demonstration of pre -ringing (presence and absence) . It's the indomitable Archimago. I think it's a shame that this has not attracted much attention. Perhaps it was the wrong time of year.
http://archimago.blogspot.co.uk/2018/01/audiophile-myth-260-detestable-digital.html
If this is not a first then I would be delighted to see the antecedents.
So, is that it? Does it explain how unorthodox filters (or hi rez) "sound" better on all kinds of material (to some people?)