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Weird reflections when playing 48Khz tone (@192kHz sample rate)
Weird reflections when playing 48Khz tone (@192kHz sample rate)
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Old 22nd October 2018, 02:53 PM   #1
smiba is offline smiba  Netherlands
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Join Date: Nov 2016
Location: Utrecht
Default Weird reflections when playing 48Khz tone (@192kHz sample rate)

Hey everyone,

Sorry if this is the wrong sub-forum for this question, if so please move it.

I've been working on a design for quite a while now and I've created my second prototype of my design, everything is working great (some hotfixes/wires aside) but one issue I've not fully been able to resolve.

At 172 or 192kHz audio with noise at around 45kHz or above will start to really mess with the audio within the audible frequency.

I've created a recording (although, with the microphone input of my computer. The background noise is because of that. With headphones the signal does not have this and neither does the oscilloscope show any noise above -100dB).

You can listen to the recording of the reflections here: https://screenshots.bartstuff.eu/Dac...48Khz_wave.wav

What's noticeable is that the tone changes after a few seconds and at around 12 seconds in the recording makes some kind of a twist. The playback has run over USB with 24-bit/192kHz output and was a sine wave output from audacity at exactly 48000Hz. Running this same file through audacity but at 24-bit/96kHz no reflections could be heard, meaning that the problem is not in the file but in my circuit.

Since a large part of the audio circuit is under NDA or a general "don't share unless needed" kinda rule I can't post my circuit here, but I'd love to know at what area to look and if this phenomenon has a technical name.

Cheers and thanks!
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Old 22nd October 2018, 11:07 PM   #2
MarcelvdG is online now MarcelvdG  Netherlands
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Join Date: Mar 2003
Location: Haarlem, the Netherlands
I don't hear anything, but your description makes me think of some weird aliasing issue, or some other mixing effect (in the RF sense of the word: two signals ending up in a nonlinear circuit that generates sum and difference frequencies). Maybe some kind of aliasing product due to Windows decimating the signal to its default sample rate? Or does your circuit have a clock frequency that can mix to something audible with 48 kHz? Does the pitch change when you touch a pin of a quartz crystal?
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