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Thread: The sound of Jitter

  1. #1
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    The sound of Jitter

    In another discussion, I was challenged on why someone might use a high frequency tone such as 24 KHz (for 48 KHz sampling) to test for jitter. I explained that due to the way jitter works in that it creates sidebands that are proportional to source frequency +- jitter frequency, it is indeed possible to have audible jitter distortion even though the source tone/music itself is ultrasonic.

    To demonstrate this, I used the Adobe Audition software's FM synthesis to create a scenario around this. I used a 24 Khz tone at -2dbfs (same level Paul Miller uses in his jitter tests) . I then added a jitter component at yet another ultrasonic frequency of 21 Khz. Here is the result: (you may want to right click and save the file locally and then playing it, If you don't do this, you may want to wait a few seconds for it to load before hitting the play button) http://www.whatsbestforum.com/tmp/jitter-62-2.wav

    The distortion you are hearing is one million times lower than the 24 Khz tone at -66db yet it is quite audible. For grins, we can lower the music/excitation tone down by 7db, for distortion products at -70db: http://www.whatsbestforum.com/tmp/jitter-70-9.wav. Still readily audible on my laptopís tiny speakers.

    Letís go down to -80db, with the primary tone now at -20db: http://www.whatsbestforum.com/tmp/jitter-80-20.wav. What you are hearing now is at the levels of measured jitter of about 2.5ns. I can still hear them without resorting to headphones.

    Note that since jitter can have many components, frequencies and amplitude, there is no one sound that represents jitter. So this is just a sample, albeit an interesting test case.

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    [WBF Founding Member] Addicted to Best! JackD201's Avatar
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    Amir, in your experience, what analog noise resembles jitter in terms of behavior (sidebands)?

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    I think it has no direct parallel mainly because jitter creates distortions that before the waveform itself. As an example, if you have a 10 KHz tone and a 3 KHz jitter, you get side bands at 7 KHz and 13 KHz. Neither is a harmonic distortion as is typical of analog non-linearity. And at any rate, harmonics are always at higher frequencies, they never occur before the frequency itself.

    When I describe the sound of jitter, I describe as effectively losing resolution. In typical equipment in use, the levels are low enough as to not be heard as direct tones. But rather, they dynamically step on the low order bits that are equal to lower than them in amplitude. The effect is most audible when the music is quiet enough to hear the low order bits (in a decay for example).

    The other artifact is slight increase in high frequencies. The right hand jitter sidebands all combine to make the music sound like it has a bit more highs than it already has.

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