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Author Topic: Metrum Acoustics Quad Measurements (NOS Mini DAC)  (Read 12357 times)

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ultrabike

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Re: Metrum Acoustics Quad Measurements (NOS Mini DAC)
« Reply #10 on: December 06, 2013, 08:04:37 PM »

Thanks Hans!!!

For spectrum stuff ARTA does a pretty good job IMO. For time domain stuff the free version TrueRTA seems pretty decent.

Interesting results with the Metrum DAC. :money:


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xnor

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Re: Metrum Acoustics Quad Measurements (NOS Mini DAC)
« Reply #11 on: December 07, 2013, 02:22:22 AM »

Thanks.

I don't understand that after centuries with better tech people are still buying this.
It just breaks the very basis, the sampling theorem, and therefore fails to reproduce the signal unless it has a very low frequency (relative to the sampling rate).


If someone prefers the looks of a narrow spike as impulse in the time domain then that someone should seriously consider reading a book about digital audio.
« Last Edit: December 07, 2013, 02:28:38 AM by xnor »
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OJneg

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Re: Metrum Acoustics Quad Measurements (NOS Mini DAC)
« Reply #12 on: December 07, 2013, 02:53:21 AM »

Mind giving a brief explanation xnor?
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xnor

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Re: Metrum Acoustics Quad Measurements (NOS Mini DAC)
« Reply #13 on: December 07, 2013, 03:35:34 AM »

Digital audio is bandlimited. In case of CD audio to below 22.05 kHz.
An infinitely thin spike (the perfect impulse) contains all frequencies from 0 to ∞ Hz. When there is an impulse on a CD it is bandlimited to below 22.05 kHz, but non-oversampling DACs don't do bandlimited interpolation so they don't reproduce what's on the CD. Instead they usually follow the samples in a stair-step like fashion. So the narrow spike is actually a lot of extraneous spectral content aka images.
At high frequencies relative to the sampling rate the attempt of reproduction (can't call it reconstruction) breaks down.

Briefly: they break the sampling theorem, they produce images which can lead to nasty IMD (with intermodulation products in the audible range)
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Re: Metrum Acoustics Quad Measurements (NOS Mini DAC)
« Reply #14 on: December 07, 2013, 03:48:37 AM »

I don't understand how that is an issue exclusive to NOS DACs. Doesn't the LPF take care of the images regardless?
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Re: Metrum Acoustics Quad Measurements (NOS Mini DAC)
« Reply #15 on: December 07, 2013, 06:36:49 AM »

Well, this certainly does not exhibit what I would call pretty IMD according to measurements. That said, I'm not sure what to listen for to detect IMD. So far with listening I haven't noticed anything particularly out of the ordinary, but I also can't say I've listened long enough or compared with other gear directly to say that definitively. I would consider loaning it out to some of the more experienced folks around here for more critical listening tests if interested.

If I'm remembering correctly, many of the inherent issues of a NOS DAC can be mitigated by setting the sampling rate high and either using "HD" music or going the software upsampling/oversampling route (I still forget the technical differences between the two terms, TBH). These measurements show little difference in terms of IMD, but I do believe I remember seeing it greatly help clean up the form of sine waves up to 20KHz. And, as mentioned, going the software upsampling/oversampling route gives you greater control in picking out the filtering methods.

I also read that many NOS DACs are sensitive to feedback and reflections. One fellow posted measurements showing how impedance mismatches across the chain can cause reflections:

http://www.head-case.org/forums/topic/9905-metrum-acoustics-octave-a-nos-digital-filter-less-dac/?p=496655

Not sure how that might affect the measurements I did or how it would affect music playback/what we hear. I believe he replaced the DAC's coax jack with a BNC one. I've heard it's easy and recommended to do.

There is without a doubt inherent compromises with NOS DACs and a good bit of disagreement as to whether those compromises can be worth it. I think these measurements show that perhaps NOS DACs can be designed in such a way to minimize the amount of compromises taken, though I'm wondering if the best strategy will also pair this with a strong software back-end with good upsampling methods. That's the idea behind XXHighEnd and the Phasure NOS DAC, at least.

It's unfortunate that the Metrum's DAC chips themselves have the tops scratched off. More can be said about other NOS DACs because the chips they use are known. From all my research, no one has yet definitively figured out which chip is in the Metrum DACs, though there are some good guesses and possibilities.
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Creative X-Fi HD USB SB1240 Measurements
« Reply #16 on: December 07, 2013, 07:04:58 AM »

By the way, I've attached all four measurements I did of the SB1240 to this post, since I did match all bit depths and sampling rates during tests. Just in case anyone wants to get more in-depth when looking at these.
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Re: Metrum Acoustics Quad Measurements (NOS Mini DAC)
« Reply #17 on: December 07, 2013, 08:48:33 AM »

I don't understand how that is an issue exclusive to NOS DACs. Doesn't the LPF take care of the images regardless?

Nope, only when you were to use a brickwall that is just above the 1/2 fs.
When a DAC can do multiple sample rates you either have to shift that filter or pass it though another filter.
No one does that and IF they did the squarewave and needle reproduction would not look as nice any more and that is what attracts buyers thinking that they have superior 'attack'. The squarewaves/needles would all be showing (severe) post ringing.

In Hans's case there is a brickwall filter in place though just above 12kHz and it is called TH-02  ::) that will at least get rid of the generated 'bit steps' as the driver simply cannot 'follow' those fast transients (ultrasonic sample frequency steps) but the (low level) generated harmonics of all signals below 5kHz will still be present (not really audible though as the distance is too big).
The higher the sample rate the higher the frequency of the steps.
At say 88kHz no headphones will be able to 'follow' those steps but some amplifiers may have issues with it.

When you use the DAC with higher (upsampled) sample rates you 'shift' part of the garbage to higher frequencies (its where DeltaSigma is based on) so the DAC will perform better when you upsample redbook to 176 or 352.
In an upsampling DAC that basically does the same but the algorithm is locked in its firmware.

Hans has a point where he mentions you can choose the upsample algorithm to be used with a NOS DAC and is clearly shown that Peter's algorithm does this better than the ones in the PC by looking at the numbers.

As a side note... Both the Metrum and XXHighEnd are designed by fellow countrymen (Dutch)  :-Z

On sound reproduction I am on Xnors camp...
All sounds on a CD are brickwalled and transients faster than a 22kHz sinewave can reach are NOT recorded but when looking at the bits (impulse and squarewave) do not have this 'limit' and reproduce a relatively slowly rising 20kHz sinewave as a 1MHz squarewave with all its harmonics.
BUT that sharp edge has NEVER been present in the original signal and if it has it was lost in the encoding so now ALL bit level changes (those in sinewaves as well) always produce very high harmonics that are NOT present in the original signal.
Yes these DAC's produce very nice squarewaves and needle pulses BUT these signals are NEVER present in ANY audiosignal and don't even sound nice. But it LOOKS really good and 'fast' on scope images.
Sound consists of sinewaves and they are NOT accurately reproduced by any NOS DAC instead they look like this:

    the picture is from Computer Audiophile and uploaded by PeterSt (the XXHE developper)

Pick your poison... a perfect looking squarewave or needle pulse as a sales argument (as they are never present in music, and also not in 'attack' of any instrument nor are these attacks encoded as Xnor explained briefly)  but terrible at sinewave reproduction and high levels of distortion / HF energy.... versus ... excellent sinewave reproduction (music consists of sinewaves) and less good looking squarewave/needle pulses that do not exist in music and have harmonics HIGH above the audible band and WON't be reproduced by the transducers anyway. Well maybe some good speakers but do you really want high amounts of supersonic, UNRELATED to the audiosignal, frequencies 'energy' in your tweeter nor in your amp.

What is clearly seen in the provided plots (I have an SB1240 myself as well b.t.w.) is the added harmonics all across teh audible band reaching -70dB and NOT decaying for higher harmonics.
This is just 1 simple tone imagine what amount of harmonics you would be getting when you play music with a plethora of frequencies between 60Hz and 10kHz mainly... they will ALL be producing harmonics at THESE level DISTANCES so a signal at -20dB will also have harmonics but 20dB below the other ones that are louder.
This basically means that when you do a S/N ratio test you will get impressive numbers but as soon as you play music the noise level rises to about -70dB !
Fortunately it only rises when music is present so in quiet passages there is still a >70dB S/N ratio but the S/N ratio embedded in the recording (mic noise) will always be present and almost at the same level so might 'mask' that added noise.
Distortion levels < 70dB are inaudible anyway (acc. to blind tests not acc. to sighted tests).

So even though the numbers appear quite bad I highly doubt it will be audible.
What is audible is the HF roll-off (certainly when playing redbook) but a lot of people do not find that objectionable (I do).

These DACs can still sound good simply because our hearing isn't as good as we think it is. It certainly isn't because of rise times.

Take a good look at the 'attack' in any music piece at 'step level' in something like Audacity and see what 'rise times' are actually present in that signal.
Good luck in hunting for steps that are faster than 1/2 fs.  You will find even fast transients in music take a few samples at least.

Just my thoughts...

I would be using that expensive DAC with upsampling only and aim for the highest possible bit rate / bit depth it allows.
XXHE seems to deliver a better upsampled 'image' than the default windhose drivers do...
« Last Edit: December 07, 2013, 10:02:39 AM by Solderdude »
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Re: Metrum Acoustics Quad Measurements (NOS Mini DAC)
« Reply #18 on: December 07, 2013, 09:36:41 AM »

Yup.

I just started reading the Metrum linky marketing info:

http://www.metrum-acoustics.nl/NOS_mini_DAC_English.html

It seems they do not use any sort of analog filtering to smooth out the Sample & Hold (S/H) out the DAC. The S/H deal applies a sinc filter to the output, but it's a bit crude. It will have some amount of ultrasonic stuff which may or may not cause IMD issues in the audible range (like Xnor pointed out).

For background they link to this page:

http://www.sakurasystems.com/articles/Kusunoki.html

IMO the paper has quite a bit of issues: 

1)Through out his discussion, it is not 22.7 s or 22.7 ms, it's 22.7 us: 1/44.1 kHz.

2) I fail to see the jitter issues he is alluding too. And his "maximum limit of the acceptable error (maximum limit of the jitter)" makes little sense to me. I think he somewhat feels 0.5 LSB error is equivalent to 173 ps for 16-bit @ 44.1 KHz and 1.35ps for 20-bit @ 8x that. IMO it is not. It depends on the signal. To me jitter performance is data, noise, clock and PLL BW dependent. Adding more bits or interpolating should not introduce significant changes in the audio signal bandwidth, clock or PLL BW. It may add quantization noise (may not depending), but don't think that would be an issue if things are properly implemented.

3) A FIR filter will require more processing bits depending on stages. But 32-bits seems plenty if the FIR is properly implemented. His opinion that a 16-bit NOS is more accurate than FIR processed 20-bit 8x OS is not necessarily accurate IMO. The ENOB of the whole conversion process might be more telling. This can be calculated from the signal SINAD and PAR. If THD+N and IMD don't look too hot, neither will ENOB... so there goes that.

4) K's (Kusunoki) discussion of FIR filtering operation is also somewhat flawed. The delay taps are 22.7 us apart for 44.1 kHz... and 2.8 us effectively apart for a classic 8x interpolation filter... And even if the filter length was several thousands of samples, the length of the filter alone does not say much about what the filter is doing to the signal...

5) There is not trade off in time domain performance vs. frequency domain performance. If one is effed-up, so will the other. Going to the pulse and square wave plots at the original Metrum link, I wish they had included the sine wave plots at 1 kHz from K... They would look like a Mayan pyramid in the time domain (see Solderdude's post above). The "obvious notches" as he calls the S/H operation might not be detected by human ear, if not for issues with IMD and stuff (as pointed by Xnor), which goes back to ENOB.

6) IMO his PLL discussion based on the xyz ps criterion is flawed... because it's based on the xyz ps criterion which I felt was flawed.

Anyhow, while I don't agree with half of what K said, that does not mean that the Metrum DAC performs horribly. Hanns did a great job describing what he heard, and makes sense to me:

As to be expected, the NOS treble roll-off is evident in these measurements. At 88.2KHz or above, this becomes much less of an issue (no more than -1dB at 20KHz, with 96KHz sampling). I find it helps me listen more comfortably and develop less fatigue over time. I do prefer a slightly dark sound as-is. The roll-off really isn't that noticeable at 88.2KHz or above, but you can defi nitely hear it at 44.1 or 48KHz. Setting the hardware to a higher sampling rate and using software upsampling can bring some very noticeable benefits. At that point, you can get into the fun of picking different upsampling methods. I'm currently using XXHighEnd's Arc Prediction method, as that is supposed to have no pre or post ringing.

That said, I can say with the Vali or Leckerton UHA-6S Mk.II and modded Tascam TH-02 (XXHighEnd software), I'm not hearing anything that's off about the sound right away. In fact, during my Vali listening tests, I found the Vali with Tascam did a fantastic job resolving fine, quick details with drums and cymbals (think fast progressive metal, anything that is easily smeared over). It did well outside of drums and cymbals, of course, but I was most struck by that right away. I found it very easy to track and place almost all aspects of what I was hearing. And, yet, I did not notice anything sounding harsh or unnatural (outside of the usual harshness of my go-to bands and genres). Very nice. Also noticed good dynamics, plenty of layering and depth to the sound with nice space between instruments and vocalists. Again, very happy overall so far...had a lot of fun listening and digging into the fine details of the music.

Now, I figure that if the Vali with Tascam was sounding that good, the DAC behind it had to be at least just as capable in order to present that. It might just be a good combo that works well together. I can't say for sure, and my listening tests have been limited to just the Tascam so far with those 2 amps. Take all of this with a grain of salt, but so far I am pleased. I'm also not necessarily saying it's the bestest setup I've heard either, just to make that clear. I'll have to listen more.
« Last Edit: December 07, 2013, 09:55:46 AM by ultrabike »
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Re: Metrum Acoustics Quad Measurements (NOS Mini DAC)
« Reply #19 on: December 07, 2013, 10:24:39 AM »

I'm really enjoying the discussion and information, even if some of it goes over my head! Lots to learn. I went into the NOS DAC purchase fully aware of the inherent issues (at least, as far as I understood it all). I just wanted to try something different that wasn't Delta Sigma (even if worse in most regards), and the buzzwords thrown around about it sounding "not digital" helped rope me in, as I seem to get listening fatigue rather easily and wanted to try a different avenue entirely. I was also disappointed at the lack of NOS DAC measurements and wanted to do something about that. In the end, I figured it would be a fun project of sorts! I was legitimately just too curious to not try one. The Metrum was a good option, because it sat right in between the price of the crappy eBay NOS DACs and the ones costing well over $1K, plus it came used with the JKSPDIF. It was more just timing and luck than me specifically picking out this unit, though I did do at least some research before buying!

Here is a post I found on sine wave response with a varying sampling rate:

http://www.computeraudiophile.com/f6-dac-digital-analog-conversion/metrum-acoustics-nos-mini-dac-octave-10867/index3.html#post151984

Still not perfect, but noticeably better. When listening, I go beyond 96KHz. I was just limited to that in measurements because of the SB1240.

solderdude did bring up the point that the Tascam TH-02 is certainly a limiting factor in my tests. While they do sound surprisingly good at times, I wanted to be transparent about everything I was doing simply because of how limited and imperfect my tests and environment are. It's a bit hard to find time to listen to the HE-500 when everyone else is asleep (I work 3rd shift and keep the same sleep schedule even on weekends). I will continue to say to take my subjective impressions with a grain of salt.

So even though the numbers appear quite bad I highly doubt it will be audible. What is audible is the HF roll-off (certainly when playing redbook) but a lot of people do not find that objectionable (I do). These DACs can still sound good simply because our hearing isn't as good as we think it is. It certainly isn't because of rise times.

Yes, I generally prefer a slight roll-off, but I understand why most others might not. But this really is the main thing I'm wondering. How audible are these downsides? How far can software take a properly implemented NOS DAC? You can find a lot of information on how NOS DACs are good or bad, but not much objective analysis on how much of that is audible or matters. It's always very black and white...
« Last Edit: December 07, 2013, 10:31:53 AM by hans030390 »
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