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Weird behavior of audio interface at 20KHZ

Discussion in 'Audio' started by dietermoreno, May 4, 2013.

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  1. dietermoreno


    Dec 30, 2012
    Weird behavior of audio interface at 20 KHZ.

    I was experimenting with recording a 20 KHZ tone from YouTube to Audacity using an audio interface. Unfortunately, the interface does not record what it is playing (possibly anti-piracy motivated), so I over come that by the analog loop hole of...literally a loop...a 6 inch instrument cable connected from headphone jack to guitar jack.

    I can not hear the tone on YouTube, but the strange results it causes to my interface and recorded in Audacity allows me to detect the presence of the tone.

    All that appears to be recorded in Audacity is crackles occuring about every 2 seconds. The crackles stop existing when the 20 HZ tone is not played.

    bizzare. its almost like the transistors in the audio interface are treating the 20KHZ tone as a radio signal and demodulating it, with spotty results in the demodulation.

    Funny thing is, the higher I turn up the gain on my interface, the faster the crackles occur, untill at full gain the crackles begin oscillating infinitely faster and infintely louder from -40 db at a lower gain to -5 db at the higher gain, they are oscillating faster and louder without me even turning the gain up any more!

    What I was trying to do was prepare an analog loop hole recorded track for uploading to YouTube with ultra sonic random noise to attempt to confuse YouTube's piracy algorithms and also attempt to even crash YouTube's piracy algorithms in hopes that it can't even handle 20KHZ and will go up in smoke.

    Does this mean that my interface even though it says it has a sample rate of 48 KHZ (24 KHZ audio signal), it actually can't handle 20KHZ and demodulates 20KHZ like some sort of regen radio detector?

    So is this deceptive marketing that although its ICs can sample frequencies up to 24 KHZ, its audio amplification can not handle 20 KHZ?

    Either that or Audacity is what is broken. No I couldn't hear the 20KHZ tone, but that is intentional that no one can hear it, hence the phrase "ultra sonic random noise to attempt to confuse YouTube's piracy algorithms".
  2. GreenGiant


    Feb 9, 2012
    thats just normal noise, when going through the output jack, to a cable, to the input jack then recording that you get all sorts of fun noises, depending on cable length, quality of the sound card, cleanliness of the interface (headphone jacks) as well as weather and ambient temperature.

    I have done exactly as you have in the past, it will almost never result in a very clear recording. I would recommend using one of the sites that allows you to download the video, or download the audio (mp3) of a youtube video then import that into audacity.

    20kHz is perfectly audible, I can hear it (though faintly, and uncomfortably) higher than that is not audible (something like 21-25kHz depending on the person is the cutoff.
  3. docb


    Feb 11, 2010
    You are going from headphone, basically line level, to guitar input - much lower level.

    Probably level distortion.

    Also google "Downloadhelper". Great plugin.
    Last edited: May 25, 2013
  4. exo_man


    May 26, 2013

    It sounds to me like a 20khz tone is really a bit beyond the edge of what the ADC can handle, regardless of the product specs. Its sampler catches the occasional blip of sound, but not enough to reconstruct any real signal. (There's really a lot of fuzziness in Nyquist). Turning up the volume just produces more random junk.

    Also, like Green Giant & docb said, there could also be impedance issues happening too.

    The lesson to be learned here: don't depend on youtube for real music :)
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    20 kHz is normally beyond the frequency limit of human hearing except for young children. It's also at or beyond the frequency limit of the audio compression typically used with videos. Why would you want to do anything with a 20 kHz tone?

    Try a 10 kHz tone. You should get more sensible behaviour with that.
  6. docb


    Feb 11, 2010
    I downloaded a file from Youtube claiming to have 20khz tone in it. Looking at it in an editor, it was basically an empty file.

    Youtube's compression algorithm may destroy a 20k signal.
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