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Old 15th July 2009, 02:31 AM   #1
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Default NOS DAC with oversampling - anybody done it?

I know this is an oxymoron statement. But the basic arguments against oversampling itself is the introduction of the digital filter. Does anyone out there know of any DAC which oversamples but does not introduce digital filtering?

I think this would probably be the best implenmentation for this case.

Oon
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Old 15th July 2009, 10:52 AM   #2
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Oversampling is a form of digital filtering.
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Old 15th July 2009, 12:44 PM   #3
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Hardware os / linear imterpolation which does not work properly.
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Old 16th July 2009, 01:38 AM   #4
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Dear Philpoole,

I agree that oversampling is a form of digital filtering, but its not the one that supposedly cause the stuff that is really bad which is the brickwall 20KHz filter which I believe uses some form of DSP to recreate the original signal.

Dear Bernhard,

Is there any reason, why hardware/linear interpolation doesn't work well? Could you point me to the specific article or thread or review.

Oon
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Old 16th July 2009, 06:37 AM   #5
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Hi Oon,

The brickwall filter is an analogue filter, not a digital filter which technically should be used with a NOS DAC, but it was discovered that this could be omitted and it wouldn't sound too bad. It was omitted because it could often be a 5th, 6th or 7th order filter, with lost of phase shifts going on which can alter the sound. Hence the introduction of the use oversampling to simplify the requirements of the analogue filter. My 8xOS can get away with a 3rd order filter, maybe even 2nd order- although I haven't tried.

Phil
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Old 16th July 2009, 02:07 PM   #6
Thor66 is offline Thor66  Denmark
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Borebely Audio has a DAC-module where they try to avoid the drawbacks of traditional oversampling filters. It might be worth having a look.

http://www.borbelyaudio.com/whats_new.asp
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Old 16th July 2009, 03:25 PM   #7
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All I can see is a DAC that has a digital filter that you can disable. So its either OS or NOS. Maybe I've missed something important?
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Old 16th July 2009, 06:47 PM   #8
Thor66 is offline Thor66  Denmark
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Borebely claim that their way of doing oversampling does not add pre- and post ringing to the signal. It is not a complete DAC, and the end result will depend a lot on the chosen I/V-converter and output/filter stage.
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Old 16th July 2009, 09:26 PM   #9
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Quote:
But these filters (ed. oversamplers) add pre- and post ringing to signal. The sound is not so precise and clear. It is slushier. In the
HB-DAC1704 no traditional oversampling filters are used.The upsampling method is done by a very precise interpolation in the SRC and no downsampling filtering is needed when using the direct downsampling feature of the new high end chip (SRC4392)
They're using a sample rate converter, which some argue is worse than an oversampling digital filter. According to some, being more versatile (can convert to a range of sample frequencies and bit depths as opposed to just one) can compromise on performance.
Either way, its still a digital filter.

I don't think there's such a thing as a free lunch.

Have you actually tried oversampling?

I started with a NOS DAC, and progressed to oversampling and haven't looked back.
I think the benefits outweigh the concerns - if implemented well.
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Old 17th July 2009, 01:33 AM   #10
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Dear Philpoole,

I believe the brickwall filter is a digital filter. That is why it couldn't be fixed in the DAC part per se, it's in the digital receiver section. But quite often you couldn't disable the digital filter without disabling the OS feature. I have some experience in this matter looking at how the different DAC respond o this. On an oscilloscope, if you were to feet in a sinewave at very high frequency, say 15KHz, you can actually see and hear the beating of the frequency with the sampling frequency. But with a normal DAC you would actually get a full sine wave. In other words, the receiver part actually reconstruct the missing signal. But when you try fitting in a normal music signal, it could confuse the reconstucting digital circuitry.

I think if look up TNT, you can find the interview with the original NOS DAC from 47 labs I think, he mentions it.

I'll try and look it up for you.

BTW Philpoole, I am a bit confused on your last statement, are you using a you using NOS DAC or an oversampling DAC now, when you say you haven't looked back.

Oon
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