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Old 17th May 2006, 11:21 PM   #121
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Default FFT analysis

Second FFT analysis:

D-I DAC, now you can see why this DAC sounded better (detail and clarity)
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Old 17th May 2006, 11:27 PM   #122
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Default FFT analysis

Third FFT picture: Sony CD-player with high density linear converter
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Old 17th May 2006, 11:31 PM   #123
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Default FFT analysis

FFT picture 4

NOS-DAC with 8th order filter
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Old 17th May 2006, 11:47 PM   #124
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somthing here is not right...

linear interpolation distorts the signal...

what are the stop and start frequencies on the FFT?
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Old 17th May 2006, 11:53 PM   #125
poobah is offline poobah  United States
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your aliasing the input to the FFT causing errors...

need to see the NOS DAC with the 8th order filter...
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Old 18th May 2006, 12:21 AM   #126
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Default FFT analysis

FFT picture 5 (last one)

D-I DAC with 8th order filter, note the slight drop in the rightmost peak.

The filtered NOS-DAC and D-I DAC have much in common on the FFT scan, exept the slight drop in the rightmost peak. The FFT analysis shows that the spectrum is much clearer than the NOS-DAC without filter. It also shows that the unfiltered D-I DAC has a cleaner spectrum than the unfiltered NOS-DAC. So if you don't want to use a 8th order filter, the D-I DAC seems a good alternative.

But why does a NOS-DAC / D-I DAC sound more natural than a CD player with oversampling and digital interpolation filters?

Personally I think it has to do with both, a clean spectrum (to bring out detail) and phase linearity (to make it sound more natural, realistic just as real live sounds). Small tolerances in phase linearity can make a big difference. By avoiding filters that have non linear phase response, sound quality should improve.
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Old 18th May 2006, 06:54 AM   #127
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Default FFT setting

Hi, poobah,

Thanks for your reply. The screen indicates 2.5KHz/div (bottom-left corner on the oscilloscope screen). So the scan is DC-25KHz. The peaks on the right are the 21KHz and the mirror image on 23.1 KHz (21KHz mirrored to Fs/2). So basically we are looking at the entire audiorange.
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Old 18th May 2006, 07:24 AM   #128
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Default FFT explanation

Hi, all

Perhaps for some of you these FFT analysis are confusing, what do they indicate?
What the FFT analysis shows us what frequencies are present in the audio signal when we apply a single 21KHz (44.1 KHz sampled) sinewave to the DAC. The first peak at the right is the 21KHz sinewave we applied, the second peak next to it is the 23.1KHz mirrored image around half the sample frequency (Fs/2). When brickwall filtering is used the mirror image peak amplitude drops. When all is ok this is all what you should see, if there are more peaks (as with the unfiltered NOS-DAC's) this means addittional (undesired) harmonics are added to the audio spectrum, these tend to cover up "detail". But the FFT scans also show that these can be reduced by a non-brickwall phaselinear filter (8th order filter as used in this example). The amplitude modulation that occurs with the NOS-DAC's is simply a side effect of the low sampling frequency as rfbrk pointed out.

So what can we do to improve the D-I DAC or NOS-DAC? just add a phaselinear filter (Bessel) that filters out most of the unwanted harmonics (see FFT analysis). In this way we can still preserve phase linearity ensuring realistic sound reproduction.
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Old 18th May 2006, 07:39 AM   #129
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Hello. where can I buy TDA1541 double crowns? does anyone have a reliable source?

Thanks.
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Old 18th May 2006, 07:49 AM   #130
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Why use the TDA1541A? With a 20 bit dac and the appropriate hardware one can build the equivalent of 16 dacs.Even if one must remain with the TDA1541A, one can cut the 8 dac version to 2 dacs and still retain the data rate.
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