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Why NOS actually may make sense.
Why NOS actually may make sense.
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Old 4th November 2017, 04:05 AM   #1
Svitjod is offline Svitjod  Sweden
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Default Why NOS actually may make sense.

When I first heard of NOS-DAC's some years ago, I shaked my head and thought the whole idea was totally nuts. If you look at a 15khz sine wave from a NOS-DAC, it looks like some sort of disease. The wave is jagged and should sound sharp and terrible.
But since then I have pondered the subject and nowadays I'm a bit of a believer. The turnaround came when I began distinguishing between amplitude related and time related distortion.
For me, it makes sense that our hearing is particularly sensitive to time related distortion. Many things supports this. First of all, our hearing makes judgments of the various sounds from the order which they appear, with reflections and such things. Secondly, it seems that many audiophile “myths” supports this. Many think negative feedback in an amplifier degrades the sound. They prefer perhaps a single ended class A tube amp before some scientifically crafted class AB monster.
Well, the thing is that the former mainly produces amplitude related distortion and the latter mainly time related. The NFB amp will distort much less, but the feedback will inevitably introduce a “smearing” effect at high frequencies. Actually a bit like the oversampling DAC's.

Many think an oversampling DAC sort of “fills in” the missing information. That's wrong, you can't recreate lost information ( mostly , actually Naims new format-MQA- is said to do just that ). What the OS does is to create a “pendulum effect” that is supposed to smooth out the jagged shape of , let's say, a 15khz signal. In order to do so, the interpolator – a FIR filter – looks backwards and “recreates” the sine wave. You can say this algorithm has an averaging effect. And this is probably the culprit.
In order to make sine waves look good at the oscilloscope ( very good amplitude related distortion) it assumes that this very wave doesn't change it's amplitude abruptly. If so, we have that famous smearing effect. The OS DAC trades amplitude related distortion with time related.

Look at the attached pictures from my oscilloscope. It's a 15khz sine wave in both NOS mode and oversampled mode. It's from my PCM1704 DAC and the rounded shape of the NOS output comes from the fact that I'm using a 3rd order analog Butterworth filter. I think it rounds off things nicely – look at the diagram of the filter. It should spare the following electronics from too much noise.
OK, compare it with the same DAC but with the interpolation filter enabled. It “smears” out things and makes them look better!

On the other hand, NOS-DAC's produces a lot of overtones that may be both audible and also disturbing to the following circuits. But - this distortion isn't really time related, is it! But those artifacts will probably be heard, and that's why it's always a matter of taste if you like NOS or not.
My own personal impressions are that a NOS DAC sounds both more round and soft and the treble is like flaky cotton. Summed together, a NOS-DAC sounds a bit like a juicy fruit cake.
The oversampling version sounds more “correct” and also a bit thin and sharp. There is the usual problem with “s” sounds and strings.
In the future, when the internet will have much more capacity, all audio will probably be transferred in 196/24 format, and then all these concerns will be only a memory. With such a high fs all DAC's may be NOS.

The sigma delta DAC's are also considered to be slightly inferior to good multibit ones. Also here we have a lack of time related accuracy. The S/D DAC relies on an averaging effect and can never present those minute rapid changes of the signal that a multibit DAC can. Mathematically, S/D DAC's are more or less flawless. But when the signal changes rapidly it loses precision.

But if we take the PCM1794 DAC as an example, it should sound very good. It has a 6 (or 7 in mono mode ) bit hardware ladder that is combined with a 512Fs ( 8 times oversampled ) S/D algorithm, we actually have 65535 discrete levels ( 512fs * 2^(7 bits) ). And this is the same as a 16 bit multibit DAC.

Finally, I think the bad reputation of OS DAC's comes from the aggressive reconstruction interpolator. But on the other hand, those totally unfiltered NOS DAC's out there are an extreme at the opposite direction.
Attached Images
File Type: jpg IMG_0018.jpg (519.4 KB, 784 views)
File Type: jpg IMG_0019.jpg (518.9 KB, 781 views)
File Type: png Analog.png (16.3 KB, 772 views)
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Old 4th November 2017, 04:19 AM   #2
abraxalito is offline abraxalito  United Kingdom
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I have a NOS DAC or two that put out a nice 15kHz sinewave on the scope with no visible stairsteps. Takes a fairly steep filter to get this though, but also sounds better than without a filter.
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Old 4th November 2017, 10:33 AM   #3
Extreme_Boky is offline Extreme_Boky
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Quote:
Originally Posted by abraxalito View Post
I have a NOS DAC or two that put out a nice 15kHz sinewave on the scope with no visible stairsteps. Takes a fairly steep filter to get this though, but also sounds better than without a filter.
You may be interested in AES paper by Rob Stuart (2014).
The sharper the filter slope the worse the distortion of transient timing. It's known that the attack phase of sounds is the most important phase of any sound.

I included the pdf. It is a must-read.
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File Type: pdf A Hierarchical Approach to Archiving and Distribution.pdf (887.0 KB, 145 views)

Last edited by Extreme_Boky; 4th November 2017 at 10:44 AM.
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Old 4th November 2017, 11:15 AM   #4
abraxalito is offline abraxalito  United Kingdom
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Thanks, I was aware that JRS was a fan of 'leaky' filters which allow substantial aliasing. Which then violates the Nyquist criterion and that, according to Bruno Putzeys means the timing accuracy is compromised. So which to choose - Nyquist violation on a grand scale or 'time smearing' ?

Just scanning the paper - why would he give an example of 8 30kHz 2nd order filters in sequence? The frequency response being shot at 20kHz from such would mean no self-respecting engineer would introduce that degree of HF attenuation in a line-level audio product. Completely unrealistic in my estimation.
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Last edited by abraxalito; 4th November 2017 at 11:25 AM.
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Old 5th November 2017, 10:01 AM   #5
MarcelvdG is offline MarcelvdG  Netherlands
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Quote:
Originally Posted by Svitjod View Post
My own personal impressions are that a NOS DAC sounds both more round and soft and the treble is like flaky cotton. Summed together, a NOS-DAC sounds a bit like a juicy fruit cake.
The oversampling version sounds more “correct” and also a bit thin and sharp. There is the usual problem with “s” sounds and strings.
The waveforms of your NOS DAC look staircase-like, which means that there is a zeroth order hold somewhere. Maybe you just like the slight treble roll-off caused by this zeroth order hold.
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Old 6th November 2017, 02:43 AM   #6
Extreme_Boky is offline Extreme_Boky
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With a true NOS design, the DAC chip will output the exact same sample rate as it sees it at the input of the whole DAC unit. There are no filters; digital nor analogue. Many people discard the true NOS design because it sounds different from other DAC’s, and it looks “ugly”.

I like the true NOS design because it is a minimalist approach to a digital to analogue conversion. The current-out (if sufficiently high in value) can be converted by the simplest means of a resistor, which keeps the minimalist approach intact.

The sample-and-hold you are referring is a normal behaviour of pure NOS design. The staircase sinewave should stay a true staircase waveform even at 20khz.

The treble roll-off is an inherent characteristic of all DAC’s (not only true NOS ones), because the 22kHz Nyquist frequency drops within an audible range.

The above issue is easily fixed by oversampling (all commercial DAC’s oversample).

Some people like to send an oversampled signal to a true NOS DAC.
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Old 6th November 2017, 02:46 AM   #7
abraxalito is offline abraxalito  United Kingdom
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I was under the impression that 'NOS' meant 'no oversampling' - hence I see no reason why a DAC which uses no oversampling but does use a reconstruction filter shouldn't be called 'NOS'. All the NOS DACs I've ever seen do use some form of filtering, normally just a 2n2 in parallel with the I/V resistor.
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Old 6th November 2017, 04:44 AM   #8
MarcelvdG is offline MarcelvdG  Netherlands
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Indeed. Of course the reconstruction filter of a non-oversampling DAC should peak a bit to compensate for the zeroth order hold filter if you want a flat response; with a simple RC parallel network as filter the roll-off only gets worse.

Anyway, smooth treble roll-off can easily explain the thread starter's subjective impression, can't it?

Quote:
Originally Posted by Svitjod View Post
My own personal impressions are that a NOS DAC sounds both more round and soft and the treble is like flaky cotton. Summed together, a NOS-DAC sounds a bit like a juicy fruit cake.
The oversampling version sounds more “correct” and also a bit thin and sharp. There is the usual problem with “s” sounds and strings.

Last edited by MarcelvdG; 6th November 2017 at 04:48 AM.
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Old 6th November 2017, 05:55 AM   #9
Extreme_Boky is offline Extreme_Boky
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The capacitance is always there. Not necessarily at the I/V resistor, but embedded in the interconnects, or at the input stage of an amp. This could be expanded further to include the speaker cable capacitance, tweeters' roll-off at high frequency, and ultimately the human ear natural roll-off characteristics.

(Now that I mentioned few above, it does make sense not to include any capacitance at I/V resistor... at least to me)

The NOS does indeed mean no oversampling, but we need to come up with the name for a NOS DAC that does not use digital nor analog filtering ...

Is the traditional NOS design (that could include a filter at some point) still a NOS design if you feed it oversampled material?
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Old 6th November 2017, 06:08 AM   #10
Extreme_Boky is offline Extreme_Boky
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Quote:
Originally Posted by MarcelvdG View Post
Indeed. Of course the reconstruction filter of a non-oversampling DAC should peak a bit to compensate for the zeroth order hold filter if you want a flat response; with a simple RC parallel network as filter the roll-off only gets worse.

Anyway, smooth treble roll-off can easily explain the thread starter's subjective impression, can't it?
People who want a flat response (that is just a representation of the frequency response on an oscilloscope, by the way, with absolutely no time-domain correlation info of any kind..) should look at oversampling DAC's.

After many years of listening to various DAC's, I have firmly settled on a "true" NOS design as my favorite. I also have other DAC's that I listen to and tend to enjoy, but the one that lets me immerse myself into music for extended periods of time and experience pure joy, is the "true" (as I call it) NOS DAC.

Have you listened to NOS DAC's and maybe compared them to other types?
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