Building the ultimate NOS DAC using TDA1541A - Page 12

-ecdesigns- · 17th May 2006, 07:07 PM

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)

poobah · 17th May 2006, 07:14 PM

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!

-ecdesigns- · 17th May 2006, 07:23 PM

Hi, poohbah

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

Lower trace: NOS-DAC with 8th order filter

poobah · 17th May 2006, 07:32 PM

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...

-ecdesigns- · 17th May 2006, 07:38 PM

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

poobah · 17th May 2006, 07:56 PM

hmmm...

In both examples there is a ~23 kHz wave beating against the 21 kHz.

poobah · 17th May 2006, 08:01 PM

Anyway you can digitize these signals? Or spectral analysis?

rfbrw · 17th May 2006, 10:47 PM

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.

-ecdesigns- · 17th May 2006, 11:15 PM

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

poobah · 17th May 2006, 11:21 PM

Cool,

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

17th May 2006, 07:07 PM	#111
-ecdesigns- diyAudio Member Join Date: May 2006	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)

17th May 2006, 07:38 PM	#115
-ecdesigns- diyAudio Member Join Date: May 2006	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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17th May 2006, 07:14 PM	#112
poobah diyAudio Member Join Date: Nov 2005	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!

17th May 2006, 07:32 PM	#114
poobah diyAudio Member Join Date: Nov 2005	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...

17th May 2006, 07:56 PM	#116
poobah diyAudio Member Join Date: Nov 2005	hmmm... In both examples there is a ~23 kHz wave beating against the 21 kHz.

17th May 2006, 08:01 PM	#117
poobah diyAudio Member Join Date: Nov 2005	Anyway you can digitize these signals? Or spectral analysis?

17th May 2006, 11:21 PM	#120
poobah diyAudio Member Join Date: Nov 2005	Cool, I can't make it out real well... dont see any markers... but you can.