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Old 27th January 2011, 06:40 PM   #21
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Who says that NOS omits the reconstruction filter ?
People who just bypass the os filter in a CD player do it maybe, not NOS.
NOS does nothing, people do. Or better: don't.
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Old 27th January 2011, 06:51 PM   #22
DF96 is offline DF96  England
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Yes, I was assuming that NOS means no reconstruction filter, as this is what DIY NOS usually means. I don't think I have seen any NOS with a reconstruction filter (i.e. brick-wall above 22.05kHz) However, I accept that NOS can use a reconstruction filter and can compensate for the sinc response. Once you do this, you probably lose the claimed benefits of NOS. It is much harder to do a good reconstruction filter for NOS - this is why OS was invented!
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Old 27th January 2011, 07:12 PM   #23
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One difference will be that os has preringing which does not exist in nature.
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Old 27th January 2011, 07:23 PM   #24
DF96 is offline DF96  England
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Some of the pre-ringing may come from the anti-alias filter in the recorder. OS preserves this. Most NOS smears it out due to the sinc filter. Sinc filters don't occur in nature either. So it seems we have to choose which unnatural effect we prefer.
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Old 27th January 2011, 08:16 PM   #25
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Quote:
Originally Posted by DF96 View Post
Some of the pre-ringing may come from the anti-alias filter in the recorder. OS preserves this. Most NOS smears it out due to the sinc filter. Sinc filters don't occur in nature either. So it seems we have to choose which unnatural effect we prefer.
A sinc filter is no effect.
It is just a bandpass filter that brings back the lost 3 dB @ 20 kHz.
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Old 27th January 2011, 08:24 PM   #26
DF96 is offline DF96  England
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Not quite. The sinc filter (an automatic side-effect of sample-and-hold DAC) reduces HF, yet without adding any phase shift. A sinc-compensation filter can boost the HF back to a flat spectrum, but it is difficult to do this without adding phase shift. I suspect that is why many NOS DACs don't bother to include a sinc-compensation filter.
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Old 27th January 2011, 10:19 PM   #27
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Quote:
Originally Posted by DF96 View Post
Not quite. The sinc filter (an automatic side-effect of sample-and-hold DAC) reduces HF, yet without adding any phase shift. A sinc-compensation filter can boost the HF back to a flat spectrum, but it is difficult to do this without adding phase shift. I suspect that is why many NOS DACs don't bother to include a sinc-compensation filter.
The sinc filter reduces HF
A sinc-compensation filter can boost the HF back to a flat spectrum
--------------------------------------------------------
So does it boost or reduce in your opinion ?
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Old 27th January 2011, 11:09 PM   #28
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Can do both... depends of how you design it.

And if you can show me an analog filter that can output 20kHz from a 44.1kHz sample rate without phase shift or 6dB attenuation or image freq up to -20dB... you are right.
All the "NOS" that I see aroud DYI is not filtered garbage presented with a "godlike" image...
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Old 28th January 2011, 12:59 PM   #29
DF96 is offline DF96  England
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Quote:
So does it boost or reduce in your opinion ?
To repeat: the sinc filter (inherent in sample-and-hold DAC, so always present) cuts HF. The sinc-compensation filter, if present, boosts HF. These are two different things - I think you are confusing them. Perhaps you have seen lazy people misname the sinc-compensation filter, by calling it a sinc filter?

Last edited by DF96; 28th January 2011 at 12:59 PM. Reason: typo
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Old 28th January 2011, 01:08 PM   #30
macboy is offline macboy  Canada
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Quote:
Originally Posted by oshifis View Post
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.
Yes, and in this case, that analog signal (the Bessel filtered one) can be faithfully reproduced by the OS DAC, without "ringing". But you must realise that due to the slow roll-off of a Bessel filter, it is a compromise. The Fc must be quite low in order to get sufficient blocking of aliases (signal above 22.05 kHz) You either live with a lot of high frequency roll-off in the audio band, or use a much higher sampling frequency in order to push that roll-off to above 20 kHz.

Quote:
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.
That is exactly the type of digital signal that NOS DAC believers like to use. It is impossible to obtain with an ADC having a proper analog front-end though. Put the NOS output into a spectrum analyzer and just look at the mess you've made. The low-pass filter on the DAC (whether analog or digital or both) is called a "reconstruction filter" for a reason. It is necessary to reconstruct the signal, because without it, the DAC produces high frequency garbage that wasn't there in the original signal that had been sampled. For accurate reproduction at 44.1 kHz sampling, there shall be nothing above the Nyquist frequency (22.05 kHz), but with NOS DACs, there it is.

If you want to hear just how bad of an idea NOS really is, try it at a lower frequency, so that your ears and brain can directly hear the high frequency crud coming out of it.
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