NOS DAC with oversampling - anybody done it? - Page 2 - diyAudio
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Old 17th July 2009, 02:35 AM   #11
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Dear Thor 66,

thanks for the link, sure wish it wasn't so expensive.... willl mull over it...

Did you try it?

Oon
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Old 17th July 2009, 05:25 AM   #12
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Hello

ecdesigns do a thread about a nos dac using an analog interpolation, but he made some big changes recently. Analog interpolation do have its own problems, possibly more jitter cause by using many dac to do the interpolation.

Here is the thread;

Building the ultimate NOS DAC using TDA1541A


Bye

Gaetan
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Old 17th July 2009, 07:17 AM   #13
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Hi,

I vaguely recall some comment about a brickwall filter and a digital filter. But I still feel the most obvious brickwall filter is in the analogue stage.
Oversampling effectively keeps the transfer function the same (audio up to 22kHz, then nothing), perhaps that is interpreted as a brickwall? The point of oversampling is that you raise the sampling frequency, so it is much easier to filter in the analogue domain.

I'll investigate what you suggest.

With DSP, at the other side of the sampling frequency, you get an aliased signal. In NOS, this starts at 66kHz, so you ideally need to pass 22kHz, but have cutoff completely at 60kHz - that's tricky, and is often called a brick wall.

I have 8xOS now, and I much prefer it to NOS. That's what I meant. I've never looked back, I wouldn't return to NOS.

Cheers,
Phil
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Old 17th July 2009, 07:25 AM   #14
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Quote:
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.
What DACs were you comparing? Were you seeing the output of the DAC unfiltered? Maybe you were seeing IMI caused by aliasing that would be filtered out by the analogue filter?

I guess it was filtered. With an unfiltered NOS DAC, 15kHz would be attenuated.
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Old 17th July 2009, 11:46 AM   #15
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I can't quite recall excatly where I was probing but would have been the analogue output. But when you put it so close together, frequency so close to the nyquist frequency, your sampling point in a sine wave would progress a little bit down the phase with every new cycle and hence it looks like a beating.

Oon
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Old 17th July 2009, 01:31 PM   #16
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Analogue output of the DAC? If so, that's an unfair test. Testing after the filter would be better.
Quote:
But when you put it so close together, frequency so close to the nyquist frequency, your sampling point in a sine wave would progress a little bit down the phase with every new cycle and hence it looks like a beating.
- Are you referring to NOS then? The idea of oversampling is to effectively increase the nyquist frequency, so wont have frequencies so close to the nyquist frequency.
I thought you were reporting this odd beating with oversampling.
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Old 20th July 2009, 02:39 AM   #17
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Analogue output of the after the low pass filter of a NOS DAC. I downloaded some .wav file at 15kHz or so then send it through the DAC. Measured at the output of DAC. the point is the sampling point taken on each of the waveform is a bit behind the phase. So without the digital filter it won't get corrected.

For example if the first sample is taken at positive peak at 15kHZ. The next positive sample will be taken at positive peak minus 21us (66us -44us). And hence it gives the illusion the waveform is decreasing in amplitude and creates a beat.

The OS sampling DAC recreates the sine wave quite beautifully.

Oon
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Old 20th July 2009, 02:55 AM   #18
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I neglected to mention you will actually wind up with a 22KHz signal which has an amplitude modulation (beat) with a NOS DAC.

Here is some write up from one of the first guy to do the NOS DAC. The theory is explained by him much better

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

Oon
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Old 20th July 2009, 01:05 PM   #19
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I think I'm confused by this whole discussion
I now realise you saw issues with the NOS DAC (e.g. beating) rather than the OS DAC. Initially, I thought you were looking out for an improvement to NOS, because of problems like this beating (which, confusingly for me, I assumed you saw with OS).
I now understand that's all false.
You've seen the fundamental flaw of NOS. So, what's the matter with OS?
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Old 22nd July 2009, 11:13 AM   #20
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Hi Philpoole,

Yup, I was looking on a compromise between NOS and OS, maybe interpolation as was mentioned by thor66. The standard OS actually recreates the wave again from the signal input. That is why you get a nice sinewave instead of a beating signal. However, when you push in a non sinewave, the behaviour becomes unpredictable. For example as shown by the interview at sakura systems, if you put in a square wave, you get a pulsed sine wave.

So since music waveform is rather random, there could be issue in the accuracy of the reconstruction.

For one, i notices, that in a NOS, the ssss sound in a singer's voice tent to be more muted, and to me that sounds more natural, since I don't hear sss all the time during a conversation with some one.

Oon
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