09-24-15: Robertsong
Bombaywalla, I think the confusion was....
when I mentioned "aliasing", I actually meant the "pre-ringing" associated with steeper filter slopes from lower sampling rates (ie. 44.1khz). I thought this was same thing. I have heard hi-res tracks from several different companies and the vast majority sound subjectivly better to me, and I assume that this perceived difference has to do with "pre-ringing".
What I'm getting at is...
does a redbook only NOS dac use a different method to reduce this ringing, or does it just compensate by providing [i]overall[/i] better sound somehow?
Of course it's the overall sound that actually matters, I just want a better understanding of WHY.
Thanks!
Robersong, pre-ringing & aliasing are not the same thing. Atleast I dont think of them as the same thing. Pre-filtering is an (bad) artifact of using a linear digital filter after up/oversampling. This pre-filtering creates hi frequency distortion & is often associated with "digititis" in digital playback. When people say "linear phase digital filter" I believe they mean to say an IIR (infinite impulse response) digital filter is used. IIR filters have 2 advantages: they can achieve a high attenuation factor (steep filter roll-off skirt) in a few number of poles (compared to a FIR filter) & 2nd, the delay thru an IIR is much less compared to an FIR. hence, if somebody wanted to monitor playback in real-time (such as in a studio setting) an IIR would be better suited. But, they have the down-side of pre-ringing that creates harshness in the sonics.
Aliasing is the effect of high frequency content, that is not low-pass filtered, folding back into the audio band.
When you up/oversample, you need a low-pass filter to band limit the resulting signal between 0 & Fs/2 & you can use a linear phase filter which, while low pass filtering, will create its own distortion in the form of pre-ringing. Maybe that's why you were thinking that aliasing & pre-ringing are the same thing. Pre-ringing has a similar effect as aliasing but the cause is different.
Yes, today a lot of the DACs have moved over to minimum phase filters which are FIR (finite impulse response) filters. these are purely digital filters with no analog equivalent & they are characterized with equal phase delay for all frequencies in the audio band. FIR filters simply cause a fixed delay in the playback path - when you press "play" it takes a short while before you hear the audio. IN non-real-time apps like home-listening it does not matter. And, yes, minimum phase filters get rid of the pre-ringing & have only post-ringing (which is considered natural).
In a NOS DAC that is playing at 44.1KHz there is no digital filter. There is no need for a digital filter because one is not up/oversampling at all. Since one is not passing the digital bit stream thru any digital filter, there is no signal processing on the music signal & the music signal remains untouched as it goes into the DAC. That is why in a NOS DAC playing at 16/44.1 the digital bit stream looks perfect in the time-domain i.e. there is no pre-ringing & no post-ringing (which are created by digital filters). On the output side, there is a low-pass analog reconstruction filter to smooth out the analog output of the DAC & to remove any high freq content that might be amplified by wider band preamplifiers &/or power amps. This low-pass filter could take the form of a tubed buffer output stage where the natural freq band limitation of the tube acts like a LPF. Or, in the case of my Scott Nixon Saru DAC+, there is a sinc (which is a sinx/x) filter with a zero amplitude at 65KHz. This means that lower freq like 30K, 40K, 50K are also attenuated (but not zero amplitude). And, it also might mean that the very high frequencies of 20Khz might also be attenuated a little.
In A NOS DAC there has to be some low-pass filtering to avoid the high freq hash from getting into the preamp & power amp which often have higher power bandwidths of 60KHz & even 100KHz. If this hi freq hash from the NOS DAC was not LPFed, the power amp will amplify it & it will degrade the overall sonics quite a bit.
All R2R NOS DACs are not necessarily 16/44.1, as Kijanki pointed out. Many are but there are several that operate at higher sampling freq. For example, my Saru DAC+ can accept upto a 24/96 input (which is the case in my setup - the digital gets fed into a Monarchy 24/96 DIP & its output feeds my NOS DAC. I believe the DIP is outputting a 24/88.2 signal). If that particular NOS DAC can accept a higher sampling rate such as 88.2K, 176K, 192K then you can upsample the digital bitstream on your computer (which can do a much better job than the processor in a DAC) to that higher sampling rate & feed it into your NOS DAC at the higher data rate without any digital filter.
So, in a NOS DAC, by avoiding any digital filtering we completely eliminate pre-ringing hence manage to keep a perfect time-domain response of the DAC. The frequency domain response of the NOS DAC doesnt look that great because there is a lot of hi freq content but we can deal with it by intelligently choosing a reconstruction filter (like the 2 examples I gave above) & containing the high freq hash to manageable levels. That's why some well-implented NOS DACs sound really good & give many up/oversampling DACs are true run for their money.
Hope this helps....
