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Old 4th December 2012, 03:14 PM   #241
Charles is offline Charles  Germany
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Okay, I stop the torture now and present the solution.

The question was: What do you have to connect to a filterless SAR ADC, in order to get really nasty alias distortion, even if you sample at 192 kHz ?

Answer: A sigma-delta DAC
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Old 4th December 2012, 03:16 PM   #242
Julf is offline Julf  Europe
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Charles,

Quote:
Originally Posted by Charles View Post
then I have at least shown that really gross distortion does appear with a sigma-delta ADC
I don't think you have. Looking at your oscilloscope pictures (I really suggest using a spectrum analyzer), it's hard to see any difference between the two pictures. Yes, the third picture might have some extremely small HF ringing, but as it is above Nyquist for a 44.1 kHz signal, and above human hearing range, it won't be audible anyway. So so far you have shown that the output of 2 different ADCs might possibly *look* minimally different on an oscilloscope screen. What does that show about either the sound quality or the measurements of your ADC?
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Old 4th December 2012, 03:18 PM   #243
Julf is offline Julf  Europe
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Quote:
Originally Posted by Charles View Post
The question was: What do you have to connect to a filterless SAR ADC, in order to get really nasty alias distortion, even if you sample at 192 kHz ?

Answer: A sigma-delta DAC
Well, if you say so... But you have definitely not shown to us that that is the case.
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Old 4th December 2012, 03:52 PM   #244
DF96 is offline DF96  England
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Original question (post 220):
Quote:
Originally Posted by Charles
Now my question to the experts: What did I have to connect to my ADC's input in order to get really nasty alias-distortion ?
I answered the question originally posed.

Revised question (post 225):
Quote:
Originally Posted by Charles
Can I at least expect from you to read my question correctly ?

Here it is again:

What do I have to connect to my SAR ADC's input in order to get really nasty alias-distortion, even if I sample at 192kHz ?
The answer to the revised question is any source with signal components above 96kHz.

Quote:
Originally Posted by Charles
thank you for your suggestion, but I think I have shown with the simple scopeshots on the Creation ADC website The Altmann Creation ADC, that a click, containing high bandwidth does indeed cause distotion when recorded with a sigma-delta ADC, but does not cause distortion when recorded with my ADC.
No. You have shown that an anti-aliasing filter causes Gibb's phenomena (pre and post ringing) on a sharp transition, whatever ADC technology is used. The lack of such a filter with your ADC means that the ringing is absent, but instead you will get aliasing. This might not be obvious in the time domain (oscilloscope) but a spectrum analyser will show it. If you don't like anti-aliasing filters then you don't have to use them; just don't do any sampling!

I would advise you to stop digging, as the hole you are in is getting deeper. Someone who appears to be a stranger to Shannon and Fourier really should not be 'designing' commercial ADCs.
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Old 4th December 2012, 04:05 PM   #245
Charles is offline Charles  Germany
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As stated earlier, I made this filterless SAR ADC, because I wanted to find out how alias distortion would sound, when it would appear.

After I had finished the ADC, I recorded vinyl, recorded with microphones, recorded my old Philips CD-player, recorded with my Attraction DAC as input, and I never got any alias distortion, all I got was great sound.

But one day, I connected my Squeezebox (this has a sigma-delta DAC inside) to my SAR ADC, and then I got a really noisy hiss on the recording, because all the high frequency garbage of the sigma delta DAC's output folds down right into the audible range if you do not filter it before it enters the ADC.

So the only instance when I was able to get alias distortion with my filterless ADC, is when a sigma-delta DAC pollutes the signal.
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Old 4th December 2012, 04:10 PM   #246
Julf is offline Julf  Europe
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Quote:
Originally Posted by Charles View Post
But one day, I connected my Squeezebox (this has a sigma-delta DAC inside) to my SAR ADC, and then I got a really noisy hiss on the recording, because all the high frequency garbage of the sigma delta DAC's output folds down right into the audible range if you do not filter it before it enters the ADC.
Welcome to the world of the Nyquist–Shannon sampling theorem.
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Old 4th December 2012, 04:13 PM   #247
DF96 is offline DF96  England
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Quote:
Originally Posted by Charles
After I had finished the ADC, I recorded vinyl, recorded with microphones, recorded my old Philips CD-player, recorded with my Attraction DAC as input, and I never got any alias distortion, all I got was great sound.
No, you never noticed any alias distortion. That is the problem with experience; when unconstrained by hard theoretical facts it can mislead people.

Try recording an FM tuner output at 44.1kHz sampling rate. You will probably be able to hear some mush around 6.1kHz - the alias of the stereo mux signal at 38kHz which has made it through the tuner's own filtering. With a good tuner this might be 30-50dB down; a bad tuner could be much greater. Shannon really was right!
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Old 4th December 2012, 10:38 PM   #248
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Quote:
Originally Posted by Charles View Post
Hi Trevor,

I have shown that a sigma-delta ADC causes distortion when fed with a HF-click and that a zero-filter SAR ADC (i.e. my Creation ADC) does not create distortion, btw. the test wav I used is available for download on my website. So this is something everybody can try out himself and I need not add anything to pure facts, as they stand for themselves

In the examples on my websites I have chosen this mixed signal which contains a square step added to a sine wave, so that you can clearly see, that the step as well as the sine contains no distortion when recorded with my Creation ADC, but does get corrupted when recorded with any sigma-delta ADC

But your idea of sampling a 20kHz square wave is good. I have the DAC that can play back a true square-wave and I have the ADC that can sample a true square wave, and the output will again be a pure square wave.

Try this with a sigma-delta ADC/DAC and you will get a huge amount of distortion. To say that this distortion can be explained by the theory of Fourier transformation will not remove the distortion.

The definition of distortion is that the output is different than the input.

Charles
Nonsense !!

With your ADC try and sample a 20KHz squarewave at 96KHz and I can assure you there will be alias components at 36KHz and 4KHz as a result of the 3rd and 5th harmonics of the square wave and you cannot get rid of these artefacts once they have been sampled that way.

And btw the sigma delta ADC's and DAC's are not adding any non linear distortion. The so called linear distortion you are seeing is coming from the anti aliasing filters used which is the correct way to do it. There is no free lunch when it comes to sampling theory. What you are trying to do is to rewrite the laws which are well established.
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Old 5th December 2012, 12:03 AM   #249
qusp is offline qusp  Australia
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yeah Charles, you arent doing yourself any favors here. I know you like to think of yourself as a visionary, but...
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Old 5th December 2012, 06:01 AM   #250
TNT is offline TNT  Sweden
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But, but .... what if the products sound good? The implementations might contain filters functions that the designer is not aware of. But I have to agree, it's hard to disregard math and physics....
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