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-   -   nos (http://www.diyaudio.com/forums/digital-source/181826-nos.html)

Richiebuoy 25th January 2011 04:33 PM

nos
 
I have just read the NOS threads and frankly dont undertand it.:(

I am considering buying an already modded CD player with NOS.

Clearly manufacturers put in oversampling for a reason, for the same reason there must be a down side to not having it, so in laymens terms what is that down side ?

(my concern is buying a NOS player and finding it wont play all my CD's)

poynton 25th January 2011 05:15 PM

There's lots of stuff on the net re: oversampling.

kusunoki


A NOS modified player should play all CDs - provided it has not been otherwise messed up.


Andy


.

DF96 25th January 2011 06:09 PM

Oversampling is in theory the best way to do it. Most of the criticism of it is based on misunderstanding (e.g. about the effect of the anti-alias filter on a square wave input to the ADC). Oversampling allows better filters to be used after the DAC.

NOS (with little or no filtering) is theoretically wrong, but some people prefer it. I suspect that the reason (apart from the above misunderstanding) is that NOS allows ultrasonic images above 22.05kHz to mimic the effect of transients which were necessarily removed by the anti-alias filter before the ADC.

macboy 25th January 2011 06:24 PM

DF96 has it right, I agree 100% with that assessment. I would just add that when he says "Oversampling is the best way to do it", I assume he means, "Oversampling is the best way to reconstruct the original waveform from the samples".

martinbls 25th January 2011 06:42 PM

I'm a laymen when it comes to the technical background of oversampling, but from my understanding it's quite simple: No oversampling means one stage less that potentially alters the signal. (Again: In laymens terms!)

Oversampling, done right, can solve certain issues in the digital route of the signal. But without it, you might get a signal that is perhaps "less perfect" (from a technical viewpoint), but "more real" (from a musical viewpoint). I've tested both, and since I got hooked on NOS-DACs there is no way back for me. Every DAC with oversampling I heard in the last few years just sounds less musical but more clinical than a well made NOS-DAC.

One idea: Ask the seller of the CD-player how the nos-mod is done. Pehaps it's just a wire bridge that can be desoldered to have oversampling again. This would give you the opportunity to listen to both versions and decide by yourself what you prefer.

Martin

SoNic_real_one 25th January 2011 10:31 PM

NO. NOS menas one stage less that you need to REALLY reconstruct the signal corectly. The signal is NOT real, is distorted by lack of a clean brickwall analog filter. OS is the only way you can filter corectly that signal with a resonable analog filter.
All NOS modded players (by any DYI) have at the output tons of alias/image components that are NOT present in original recoding. Some people consider that an "improved" sound, call it "musical" - but it is just pain old distortion.

Of course, not all OS is equal. Some is done poorly, but that doesn't mean that OS is "bad". Just some people know how to do it right and some don't.
On the other side - NOS is bad always.

DF96 26th January 2011 03:14 PM

As soon as someone says that he prefers "musical" to "clinical" I assume that he is not interested in reconstructing the signal immediately after the anti-alias filter, but merely hearing a pleasing sound.

Oversampling, done right, solves the problem of brick-wall filters which are difficult to do in analogue. OS lets you have a digital filter (which can be much more complex) followed by a simple analogue filter. The result is a closer approach to the original sound.

You will see NOS fans show pictures of ringing on a square-wave output from an OS system. What they usually don't tell you is that this ringing has not been added to the signal by the OS, but is the result of subtraction (of higher frequencies) before the ADC in the recording system- exactly what the anti-alias filter is supposed to do! This signal is actually on the CD, and OS reconstructs it faithfully. NOS smears the edges, because is has a sinc-type HF frequency droop and at the same time adds images above 22.05kHz which were never there in the original signal.

Richiebuoy 26th January 2011 05:26 PM

Thanks, particularly to Martin (laymen's terms), I was open minded really, until the last 2 posts that is.....

oshifis 27th January 2011 09:49 AM

DF96:
Could a linear phase (Bessel-type) analog filter be applied before the A/D conversion at the recording side? Then there were no ringing square wave signal on the CD, just a band limited signal with rounded edges.
Also, I edited a test CD wav file in a hex editor, that contains alternating 0000s and FFFFs, that is a perfect square wave signal. Playing back the test CD on a NOS DAC I get nice square wave, bot on an OS DAC I get ringing. Obviously, perfect square wave can not go to a CD through ADC, because it contains harmonics beyond 22.05 kHz.

philpoole 27th January 2011 10:25 AM

I think putting a square wave through a DAC is an unfair test - or at least points out that NOS DAC in question is not outputting the correct signal (iow - if its a perfect square wave then it's probably not being filtered).

Anyway, my take on it. I think NOS is easier to get good results - especially for a DIYer (and skimp on the output filter). Because the clock frequencies used are lower, the circuit is somewhat less susceptible to jitter - and the digital filter/oversampling chip is missing (which can add jitter and inject PS noise).
If done properly, I think OS is much better. In theory, and from experience.


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