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Old 17th May 2006, 07:07 PM   #111
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Default 8th order filter

Hi, poobah

Thanks for your reply,

We must have had the same thoughts.. I just quickly connected the 8th order filter after the D-I DAC to see what would happen, guess what, hardly any change. Only the triangle waves look much more like sinewaves. I scanned the 8th order filter with a sweep generator during design. It starts rolling off at 20KHz, the pass band has constant amplitude (butterworth characteristic)
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Old 17th May 2006, 07:14 PM   #112
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Now connect it to the REF DAC note the differences.

Show a photo of the DI @ 21 kHz with the 8th order

and the REF @ 21 kHz with the 8th order.

You might be surprised!

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Old 17th May 2006, 07:23 PM   #113
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Default comparison

Hi, poohbah

Upper trace: D-I DAC with 8th order filter

Lower trace: NOS-DAC with 8th order filter
Attached Images
File Type: jpg compare.jpg (44.8 KB, 4303 views)
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Old 17th May 2006, 07:32 PM   #114
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See what's happening?

The REF has better fidelity...

see the nasty in the middle of the DI waveform?

A spectral analysis would tell more...
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Old 17th May 2006, 07:38 PM   #115
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Default mistake

Hi poobah,

I swapped the descripton of the channels (''), I hurried to much, I doublechecked again and corrected it.

Upper channel (channel 1) is the DI-DAC

Lower channel (channel 2) is the NOS-DAC

sorry my misstake

> compare with post 100
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Old 17th May 2006, 07:56 PM   #116
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hmmm...

In both examples there is a ~23 kHz wave beating against the 21 kHz.
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Old 17th May 2006, 08:01 PM   #117
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Anyway you can digitize these signals? Or spectral analysis?

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Old 17th May 2006, 10:47 PM   #118
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Quote:
Originally posted by poynton


Not so !

As rfbrw rightly says, the problem is the sampling rate... it was set too low but we have to live with it and all it's failings.

Take a 20kHZ waveform..
Only 2 samples are available to reproduce the original.
No method of NOS, interpolation, over-, up-sampling can determine the original waveform. There is not enough information.

But we do not want to reproduce 20kHz sine or square waves.
we want to listen to music.

The most important piece of measuring equipment is your ears.


Standard digital techniques will recover the 20Khz sinewave from a 44k1 sample rate through the very ringing that seems so objectionable to some. Combined with the correct analogue filter, pre and post ringing recreates the sinewave.
Linear interpolation simply joins the sample points with straight line. At 2K you have 20+ samples, enough for a fair representation of the original waveform. At 20k, with only two samples, simply joining the sample points misrepresents the original waveform.
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Old 17th May 2006, 11:15 PM   #119
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Default FFT analysis

Hi, all

Thanks poobah for the good advice!

I ran FFT analysis to see if I could spot some differences,

First FFT picture: NOS-DAC
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File Type: jpg nosdac.jpg (58.4 KB, 4218 views)
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Old 17th May 2006, 11:21 PM   #120
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Cool,

I can't make it out real well... dont see any markers... but you can.
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