NOS DAC sound more natural

[buonassi]

I found this, but I'm not qualified to decipher it - though it seems relevant to ZOH and phase?  

 

https://electronics.stackexchange.com/questions/17486/effect-of-zero-order-hold-on-gain-and-phase-margin-in-continuous-analysis

 

note the FR and phase graphs.

[Miska]

ZOH (aka SAH aka S/H) has nulls at multiples of sampling rate. If you upsample/oversample using ZOH, the frequency/phase response is still equal to the original rate because you effectively don't do anything, just increase clock rate. ZOH rolls off slowly, so if you run ZOH for 44.1 kHz sampled data, frequency response begins to roll off already at 0 Hz and becomes most notable 10 kHz onwards. This is why a NOS DAC running at 44.1 kHz rolls off early.

 

Typical oversampling DAC chips, when you give them 44.1 kHz input, run digital filters to 352.8 kHz rate, and then go from there onwards using ZOH (because they don't have enough DSP capacity to do better), typically by 16x. For this reason, you can see images of the input signal around multiples of 352.8 kHz in the output.

 

While for example I run digital filters in software all the way up to 22.5792 MHz, because I'm not short of DSP capacity.

 

For example if we look at 0 - 22.05 sweep at 44.1 kHz sampling rate through iFi micro iDSD with filter set to "bit-perfect", the digital filter is disabled and only ZOH is in use:

You can see images around multiples of 44.1 kHz and the roll-offs.

 

If we then set it's internal digital filter to "standard", it runs digital filter up to 352.8 kHz and then ZOH onwards from there:

Now you can see images around multiples of 352.8 kHz. This is already a bit easier for analog filter to remove. But 2nd order analog filter quite doesn't do it yet, so you can see images up to 2 MHz in the output spectrum.

 

For comparison we can look same source, but upsampled to DSD512 rate in software with digital filters doing up to 22.5792 MHz rate:

We can see there are no images left in the output spectrum...

 

[jamesg11]

So, if the idsd receives say upsampled 44.1 to 705 from a player - what then occurs re these digital filters?

[asdf1000]

5 minutes ago, jamesg11 said:

So, if the idsd receives say upsampled 44.1 to 705 from a player - what then occurs re these digital filters?

 

Similar (but not the same) to DSD512:

 

 

[jamesg11]

Righto, thanks.

[Jud]

By the way, speaking of phase response: A minimum phase filter doesn't get rid of ringing.  It shifts the ringing to after the impulse.  There's an equal amount of ringing energy in either the linear phase or minimum phase case, so with minimum phase there is double the post ringing and no pre-ringing.

[Ron Scubadiver]

I doubt that there is any particular design feature which somehow produces superior SQ all by itself.  A DAC has several important processing stages and all of them have to be right and work together.  Some very highly rated DAC's are of relatively conventional design being no more than very well done versions of their much less expensive cousins.

[Miska]

9 hours ago, jamesg11 said:

So, if the idsd receives say upsampled 44.1 to 705 from a player - what then occurs re these digital filters?

 

It gives you images around multiples of 705.6 kHz, so 2x better than it can do with it's internal digital filters. Thus also the first image level is also lower because by that frequency, the analog filter has rolled off 12 dB more.

 

 

[jamesg11]

Thanks - /& on those graphs above, it’s better to use the ‘standard’ filter ...?

[buonassi]

15 hours ago, Miska said:

If you upsample/oversample using ZOH, the frequency/phase response is still equal to the original rate because you effectively don't do anything, just increase clock rate.

 

First off, thanks for chiming in!

 

If I'm understanding this quote above, Miska, you're saying that there isn't a phase shift at any of the frequencies between 0 and 22.05khz after passing through the S/H stage?  The high frequencies aren't delayed as they would be in a minimum phase filter?  

 

Perhaps this is the case when ZOH is physically performed vs emulating ZOH using a very weak cubic interpolation filter in software.  As I mentioned in a previous post, @Archimago measured the phase response of what he called the "ultralax" filter in software.  He modeled it to have a very similar frequency and impulse responses of what we'd see in a NOS DAC.  The phase was shifted much more so than in a traditional minimum phase interpolation filter.

 

But I do see where this is an emulation, and not necessarily indicative of the actual ZOH process.  Thoughts?  Confirmations? 

[Miska]

On 5/24/2018 at 3:36 AM, buonassi said:

If I'm understanding this quote above, Miska, you're saying that there isn't a phase shift at any of the frequencies between 0 and 22.05khz after passing through the S/H stage?  The high frequencies aren't delayed as they would be in a minimum phase filter?  

 

Perhaps this is the case when ZOH is physically performed vs emulating ZOH using a very weak cubic interpolation filter in software.  As I mentioned in a previous post, @Archimago measured the phase response of what he called the "ultralax" filter in software.  He modeled it to have a very similar frequency and impulse responses of what we'd see in a NOS DAC.  The phase was shifted much more so than in a traditional minimum phase interpolation filter.

 

But I do see where this is an emulation, and not necessarily indicative of the actual ZOH process.  Thoughts?  Confirmations? 

 

ZOH is a "linear-phase filter", just like cubic interpolation and linear-phase FIR. Although it is all-pass, so it doesn't filter anything.

 

You cannot "emulate" ZOH with cubic interpolation because it is different process. ZOH is just a sample copy. Even linear interpolation while being a very bad filter, does something while ZOH does absolutely nothing. For example if you have three sample values 2, 4, 6 and want to interpolate it 2x, ZOH yields you 2, 2, 4, 4, 6, 6 while linear interpolation yields you 2, 3, 4, 5, 6, 7. Both are grossly wrong for audio, but you probably see the point...

 

ZOH process doesn't really do anything, it is practically same as not doing anything at all. It is the simplest digital domain phenomenon you can have, because you just copy same sample value as many times as you want. When you do digital domain analysis of that, you just see wider frequency range because sampling rate is higher, but you can calculate the same easily without doing the ZOH process (because it function is well known). When you do analog domain analysis with ZOH it doesn't change anything compared to running at lower sampling rate, because from analog perspective it doesn't change anything.

 

ZOH is same thing as making digital picture larger by copying each pixel value N times in both directions. So it is not interpolation, but just get bigger image where pixels are bigger blocks - so it begins to look like built from Lego blocks.

 

Cubic interpolation is a real interpolation (filter), although not very efficient one, if you do it with image you don't get as blocky output.