DAC frequency spectrum using computer

pkane

New Member
Jan 6, 2017
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Let's say that I wanted to measure the frequency spectrum of my DAC when playing back a sinewave.

To do this, let's say that I feed the output of my DAC into a 24bit ADC of average quality.

Let's further say that I take the optical toslink output from the ADC, and then feed it into my Macbook Pro that has a toslink audio input. And then, let's say I use REW software to generate a 24-bit dithered sinewave to feed the DAC over USB, and measure the resulting spectrum using REW RTA function with an averaged FFT display.

I've done this test and it seems to work. The result appears interesting but I can't tell if I can trust it or not. How would I go about validating it? How would I go about separating the extra harmonics produced by the (high quality) DAC from those produced by the (average) ADC?
 

opus112

Well-Known Member
Feb 24, 2016
462
4
148
Zhejiang
You would need to do what Audio Precision does and include a notch filter prior to the ADC. With the fundamental frequency of your sinewave notched out the ADC is only 'seeing' all the harmonics and hence its relatively poor performance matters far less.

If you want to know what such a notch filter looks like in practice you could do a search on 'Bob Cordell's distortion analyzer', he provides schematics for one.
 

Bruce B

WBF Founding Member, Pro Audio Production Member
Apr 25, 2010
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512
1,740
Snohomish, WA
www.pugetsoundstudios.com
Yeah, who knows what the ADC measures. You'd just be shooting in the dark. Until you isolate the deficiencies of the ADC, it's a waste of time.
 

pkane

New Member
Jan 6, 2017
72
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A notch filter is a great idea, and one I would not have thought of myself! Thanks a lot for the suggestion and the pointer, @opus112!
 

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